Effects of Host Plant and Larval Density on Intraspecific Competition in Larvae of the Emerald Ash Borer (Coleoptera: Buprestidae) Author(s): Jian J. Duan , Kristi Larson , Tim Watt , Juli Gould , and Jonathan P. Lelito Source: Environmental Entomology, 42(6):1193-1200. 2013. Published By: Entomological Society of America URL: http://www.bioone.org/doi/full/10.1603/EN13209
BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use. Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder.
BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research.
POPULATION ECOLOGY
Effects of Host Plant and Larval Density on Intraspecific Competition in Larvae of the Emerald Ash Borer (Coleoptera: Buprestidae) JIAN J. DUAN,1,2 KRISTI LARSON,3 TIM WATT,3 JULI GOULD,4
AND
JONATHAN P. LELITO5
Environ. Entomol. 42(6): 1193Ð1200 (2013); DOI: http://dx.doi.org/10.1603/EN13209
ABSTRACT Competition for food, mates, and space among different individuals of the same insect species can affect density-dependent regulation of insect abundance or population dynamics. The emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), is a serious invasive pest of North American ash (Fraxinus spp.) trees, with its larvae feeding in serpentine galleries between the interface of sapwood and phloem tissues of ash trees. Using artiÞcial infestation of freshly cut logs of green ash (Fraxinus pennsylvanica Marshall) and tropical ash (Fraxinus uhdei [Wenzig] Lingelsh) with a series of egg densities, we evaluated the mechanism and outcome of intraspeciÞc competition in larvae of A. planipennis in relation to larval density and host plant species. Results from our study showed that as the egg densities on each log (1.5Ð 6.5 cm in diameter and 22Ð25 cm in length) increased from 200 to 1,600 eggs per square meter of surface area, larval survivorship declined from ⬇68 to 10% for the green ash logs, and 86 to 55% for tropical ash logs. Accordingly, larval mortality resulting from cannibalism, starvation, or both, signiÞcantly increased as egg density increased, and the biomass of surviving larvae signiÞcantly decreased on both ash species. When larval density was adjusted to the same level, however, larval mortality from intraspeciÞc competition was signiÞcantly higher and mean biomasses of surviving larvae was signiÞcantly lower in green ash than in tropical ash. The role of intraspeciÞc competition of A. planipennis larvae in density-dependent regulation of its natural population dynamics is discussed. KEY WORDS Agrilus planipennis, larval density, competition, Þtness, density dependence
For many insect species, competition for food, mates, and space among different individuals is a pervasive phenomenon with ecological consequences such as density-dependent regulation of insect abundance or population dynamics (Klomp 1964, Bultman and Faeth 1986, Dukas et al. 2001, Gibbs et al. 2004, Lieske and Zwick 2008, Legros et al. 2009, Boulay et al. 2010). Understanding the intensity and mechanisms of intraspeciÞc competition in insect pests can foster the development of sound pest management strategies (Klomp 1964, Denno et al. 1995, Kaplan and Denno 2007). The emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), is a serious invasive forest pest in North America, where it has caused large-scale ash (Fraxinus spp.) mortality and has spread to 21 U.S. states (Kovacs et al. 2010, Cooperative Emerald Ash Borer Project 2013) and several Canadian provinces (Canadian Food Inspection Agency 2013) since it was Þrst discovered in Michigan 1 BeneÞcial Insects Introduction Research Unit, USDAÐARS, 501 S. Chapel St., Newark, DE 19713. 2 Corresponding author, e-mail: [email protected]. 3 Department of Entomology and Wildlife Ecology, University of Delaware, Newark, DE 19713. 4 USDA APHIS PPQ-CPHST, Buzzards Bay, MA 02542. 5 USDA APHIS PPQ Emerald Ash Borer Biocontrol Laboratory, Brighton, MI 48116.
(Haack et al. 2002). Unlike many other Agrilus beetles that primarily attack host trees weakened by biotic or abiotic stresses such as diseases and drought (Carlson and Knight 1969), A. planipennis attacks live healthy ash trees (Haack et al. 2002, Cappaert et al. 2005). Adults of A. planipennis lay individual eggs, either singly or in small clutches (⬍10 eggs) in crevices and under bark ßakes on limbs and trunks of ash trees (Wei et al. 2007, Wang et al. 2010, Rigsby et al. 2013). Larvae of A. planipennis bore into the interface between phloem and sap wood tissue, feed, and develop for several months primarily on phloem tissues, creating extensive galleries under the bark. When populations of A. planipennis are high, larval consumption of tree phloem is substantial, resulting in tree girdling and death (Poland and McCullough 2006, McCullough et al. 2009). In both newly invaded and native ranges, densities of A. planipennis larvae in infested trees can be extremely high. For example, in Michigan (the epicenter of the U.S. invasion), green ash (Fraxinus pennsylvanica Marshall) and white ash (Fraxinus americana L.) trees killed by A. planipennis on average produced 89 adult beetles per square meter of surface areas of phloem tissue (McCullough and Siegert 2007). When considering the high rate (50 Ð90%) of mortality caused by woodpeckers and other biotic factors (as reported in Cappaert at al. 2005 and Duan et al. 2012a,
0046-225X/13/1193Ð1200$04.00/0 䉷 2013 Entomological Society of America
1194
ENVIRONMENTAL ENTOMOLOGY
2013a), however, larval densities in those North American ash trees killed by A. planipennis would have been several-fold higher than the density of emerged adults. Most recently, a Þeld study conducted in the Russian Far East showed that living F. pennsylvanica trees infested with A. planipennis sustained densities as high as 200 A. planipennis larvae per square meter of phloem (Duan et al. 2012b). Previous Þeld studies showed that several biotic factors including woodpeckers, hymenopteran parasitoids, pathogens, and putative host-plant resistance inßicted between 10 and 90% mortality of larval stages of this pest in both the newly invaded region (Cappaert et al. 2005; Duan et al. 2010, 2012a, 2013a) and the native range (Liu et al. 2007, Wang et al. 2008, Duan et al. 2012b). To date, however, few studies have explicitly examined the mechanism and outcome of intraspeciÞc competition for this invasive pest. The lack of studies on intraspeciÞc competition in A. planipennis is probably because of the lack of feasible techniques to manipulate densities of A. planipennis on living trees in the Þeld. With the recent advancements in rearing in the laboratory (Duan et al. 2011), methods for manipulation of larval density on freshly cut host tree logs have become available. For example, Duan et al. (2011, 2013b) and Yang et al. (2012) successfully infested freshly cut green ash and tropical ash (Fraxinus uhdei [Wenzig] Lingelsh) logs with emerald ash borer eggs laid on coffee Þlters in the laboratory. Larvae hatching from those eggs successfully bored into the logs and developed into mature (J-shaped) larvae in the ash logs; the mature larvae successfully developed into adults after 3 mo of chill treatment (at 12⬚C) to break diapause (Duan et al. 2011; J.J.D., unpublished data). This method is now routinely used to rear larvae of A. planipennis for mass production of biological agents in the United States (Duan et al. 2011, 2013b; J.P.L., personal communication). In the current study, using the laboratory rearing technique developed by Duan et al. (2011, 2013b), we examine the mechanism and intensity of intraspeciÞc competition in larvae of A. planipennis in relation to resource-adjusted larval density on freshly cut logs of two different species of ash: the natural host F. pennsylvanica (green ash) and the alternative (rearing) host F. uhdei (tropical ash). SpeciÞcally, we documented the rate of mortality of A. planipennis larvae owing to direct interference (cannibalism), resource competition (starvation), or both, and we also measured the biomass of surviving mature larvae reared at different larval densities on logs of both green and tropical ash trees. This information will not only contribute to our understanding of the mechanisms and ecological consequences of intraspeciÞc competition in A. planipennis, but also help our selection of optimal egg or larval density for rearing A. planipennis with different type of ash logs.
Vol. 42, no. 6 Materials and Methods
Host Plant. Green ash (F. pennsylvanica) logs were used within 48 h after being cut from the main trunk of ash trees (4 Ð 6 cm DBHÑ diameter at the breast height) in Newark, DE (free of A. planipennis infestations), while tropical ash (F. uhdei) logs were freshly cut from potted trees (2Ð 4 cm DBH) grown in greenhouses at the U.S. Department of AgricultureÐBeneÞcial Insects Introduction Research Unit (Newark, DE). Logs from each species of host plants were 22Ð25 cm in length, but with diameters varying from 3 to 6.5 cm for green ash and 1.5Ð 4 cm for tropical ash. The exact diameter and length of each log were measured and then divided into two categories for each of the two ash species: large logs (diameter range from 5 to 6.5 cm for green ash and 3Ð 4 cm for tropical ash) and small logs (diameter range from 3 to 4.5 cm for green ash and 1.5Ð2.5 cm for tropical ash). Emerald Ash Borer Eggs. All eggs of A. planipennis were laid on unbleached coffee Þlter papers by mated gravid females according to method described by Duan et al. (2011) and Yang et al. (2012) and used in the study within a week before hatching. All adults used for egg production originated from emerald ash borer-infested ash trees collected in Maryland (Yang et al. 2012). Egg Infestation. To create a series of larval densities of A. planipennis, logs of each size category (large or small) from each ash species were infested with four densities of eggs: 200, 400, 800, and 1,600 eggs per square meter of surface areas. For infestation of logs, coffee Þlter papers with A. planipennis eggs were cut into small individual pieces (⬇ 3 mm in length by 3 mm in width) and were then placed on the surface of each log with the bottom side of eggs (attached to Þlter paper) against the bark. ParaÞlm strips (BEMIS, BEMIS Flexible Packaging, Neenah, WI) were wrapped around each log to hold the eggs in place. Following placement of eggs, ash sticks were placed upright in ßoral foam bricks (OASIS, SmithersÑOasis Company, Kent, OH) in storage boxes (58.4 by 41.3 by 31.4 cm, Sterilite, Sterilite Corporation, Townsend, MA) saturated with distilled water and a 0.1% solution of methylparaben to prevent mold. The storage boxes containing egg-infested ash sticks were then incubated in an environmental chamber at 27⬚C (⫾1), 65 (⫾10)% relative humidity (RH), and a photoperiod of 16:8 (L:D) h for larval growth and development. Observation of Emerald Ash Borer Larvae. Eight weeks after incubation at the aforementioned environmental condition, the bark was peeled off each log with utility knives and pruners and the fate of each A. planipennis larva was observed. All larvae of A. planipennis, if alive, would normally have reached mature stage (J-larvae) 8 wk after oviposition at 27⬚C (Duan et al. 2013b). Mature fourth-instar larvae normally stop feeding, make chambers inside the sapwood, and then fold into the J-larval stage for overwintering (via obligatory diapause). On removal of the bark layer, we recorded the stadium (instar) of all exposed larvae based on their body size, width of their gallery, or both
December 2013
DUAN ET AL.: INTRASPECIFIC COMPETITION OF EMERALD ASH BORER LARVAE
1195
Table 1. Target density of emerald ash borer eggs (per square meter of phloem area) and mean number of larvae actually observed for each category (large or small) of both green and tropical ash logs Host tree speciesa Green ash
Tropical ash
a
Target egg density (no. eggs per m2)a
Log category (diam range in cm)a
Mean ⫾ SE no. eggs infested per log
Mean ⫾ SE no. larvae produced per log
Mean ⫾ SE no. larvae per m2 phloem area
200 400 800 1,600 200 400 800 1,600 200 400 800 1,600 200 400 800 1,600
Large (5Ð6.5) Large (5Ð6.5) Large (5Ð6.5) Large (5Ð6.5) Small (3Ð4.5) Small (3Ð4.5) Small (3Ð4.5) Small (3Ð4.5) Large (3Ð4) Large (3Ð4) Large (3Ð4) Large (3Ð4) Small (1.5Ð2.5) Small (1.5Ð2.5) Small (1.5Ð2.5) Small (1.5Ð2.5)
8.7 ⫾ 0.7 18.0 ⫾ 1.0 37.7 ⫾ 1.8 74.3 ⫾ 1.9 6.0 ⫾ 0.6 12.3 ⫾ 0.3 26.0 ⫾ 1.1 54.0 ⫾ 3.0 6.0 ⫾ 1.0 10.3 ⫾ 0.3 22.3 ⫾ 0.7 42.0 ⫾ 0.0 2.7 ⫾ 0.3 6.0 ⫾ 0.6 13.0 ⫾ 0.6 22.7 ⫾ 2.3
8.0 ⫾ 0.0 13.0 ⫾ 1.5 24.0 ⫾ 2.0 46.3 ⫾ 1.8 4.7 ⫾ 0.9 10.0 ⫾ 1.0 21.7 ⫾ 0.7 32.7 ⫾ 2.3 5.0 ⫾ 1.0 7.3 ⫾ 1.8 12.0 ⫾ 1.0 25.0 ⫾ 1.5 2.3 ⫾ 0.3 4.3 ⫾ 0.3 9.0 ⫾ 2.8 13.0 ⫾ 2.5
175 ⫾ 11 292 ⫾ 42 516 ⫾ 12 960 ⫾ 35 155 ⫾ 29 324 ⫾ 30 673 ⫾ 10 976 ⫾ 20 183 ⫾ 29 275 ⫾ 66 426 ⫾ 43 941 ⫾ 55 188 ⫾ 33 301 ⫾ 51 571 ⫾ 51 929 ⫾ 56
Three logs (replicates) for each combination of host tree species, log size, and egg density.
(Wang et al. 2010), and their fate (alive or dead). We distinguished two types of mortality: 1) dead larvae with part of the body cut through or damaged by other larvae, and 2) dead larvae with no apparent evidence of body damage by other larvae. We classiÞed the Þrst type of larval mortality as a result of cannibalism and the second type as a result of starvation (or indirect interference). All surviving larvae were removed from their galleries or pupation chambers and then weighed with an electronic balance (AB135-S/FACT, Mettler Toledo AG Laboratory Weighing and Technology, Greifensee, Switzerland) to determine their biomass (weight). Target Egg Density and Density of Observed Larvae. To reach the target egg density, the number of eggs infested on each log was determined by the surface area of the log, which was calculated based on the log diameter and length. Table 1 summarizes the mean number of eggs infested per log for each size (small and large) category of green and tropical ash logs in corresponding to each target egg density (200, 400, 800, or 1,600 eggs per square meter of surface area) and the mean number of emerald ash borer larvae (alive and dead) observed at the termination of experiment (after 8-wk incubation). The Þnal density of emerald ash borer larvae observed on each log was ⬇10 Ð58% lower than the corresponding targeted egg density because of losses from handling and natural egg mortality. The observed larval density for each log was used in Þnal data analyses. Data Analysis. The experimental design constituted a factorial design with three treatment factors: host tree species (green vs. tropical ash), log size (large vs. small), and egg density (four levels). Three replicates (logs) were tested for each combination of the treatments. However, we used the number of the larvae observed per unit surface area (or the size) of the log as a continuous dependent variable in ANOVA based on linear regression analysis. Likelihood chi-square tests based on multinomial logistic regression model were used to evaluate the effect of the targeted egg
density and host tree species on the fate (alive, dead of starvation, and cannibalization) response of observed emerald ash borer larvae. Multiple or simple linear regression analyses, or both, were used to evaluate the inßuence of host tree species and larval density on mortalities (either from cannibalism or starvation) and biomass of surviving larvae. In addition, ANCOVA (analysis of covariance) was used to analyze the effect of host tree species on the relationship (i.e., line slope) between the larval density and mortality rate or biomass of surviving larvae. All data on mortality rate (proportion) were transformed with the arcsine square root function to normalize the distribution before linear regression analyses. All statistical analyses were carried out with JMP 10.01 (SAS Institute 2012). Results Larval Mortality. As egg densities increased from 200 to 1,600 eggs per square meter of surface area, larval survivorship decreased from ⬇68 to 10% for the green ash (Fig. 1A), and from 86 to 55% for tropical ash (Fig. 1B). Accordingly, larval mortality resulting from cannibalism, starvation, or both, signiÞcantly increased as egg density increased (Fig. 1A and B). Likelihood ratio 2 tests showed that there were signiÞcant differences in larval mortality (combined from both cannibalism and starvation) between green and tropical ash (2 ⫽ 149.549; df ⫽ 2; P ⬍ 0.0001) as well as among different egg density treatments (2 ⫽ 110.383; df ⫽ 6; P ⬍ 0.0001). The log-size (small vs. large) had no effect on larval mortality or survivorship (2 ⫽ 4.044; df ⫽ 2; P ⫽ 0.1324) when adjusted for egg density based on the unit surface area (square meter) of phloem and host plant species (in the multinomial logistic regression model). ANOVA (based on a multiple linear regression model) detected highly signiÞcant effects of host plant species (F ⫽ 17.3177; df ⫽ 1, 44; P ⬍ 0.0001) and observed larval density (F ⫽ 46.7536; df ⫽ 44, 1; P ⬍
1196
ENVIRONMENTAL ENTOMOLOGY
Fig. 1.
Vol. 42, no. 6
Fate of emerald ash borer larvae at different egg densities on green and tropical ash logs.
0.0001) on larval mortality from cannibalism, but no signiÞcant effect of log size (F ⫽ 0.0028; df ⫽ 1, 44; P ⫽ 0.9579) when adjusted for host plant species and larval density (based on the unit surface area [square meter] of phloem). For larval mortality from starvation, ANOVA detected highly signiÞcant effects of host plant species (F ⫽ 15.5903; df ⫽ 1, 44; P ⫽ 0.0003), but only marginally signiÞcant effect of larval density (F ⫽ 3.823; df ⫽ 1, 44; P ⫽ 0.0569), and again no signiÞcant effect of log size (F ⫽ 0.0660; df ⫽ 1, 44; P ⫽ 0.7984). Simple linear regression analysis showed a signiÞcantly positive relationship between larval density (adjusted to per square meter of surface area of phloem) and mortality rate (arcsine square-root transformation) from cannibalism (Fig. 2A), but not be-
tween the larval density and mortality rate owing to starvation (Fig. 2B). Although the positive densitydependent relationship for mortalities from larval cannibalism was numerically stronger (i.e., with a greater line slope) for green ash logs than for tropical ash logs (Fig. 2A), ANCOVA detected no signiÞcant differences in the line slope between the two host plant species (F ⫽ 1.4467; df ⫽ 1, 4; P ⫽ 0.2355). In contrast, the positive density-dependent relationship for mortalities from starvation was approximately parallel for green and tropical ash logs with no statistical signiÞcance (Fig. 2B; ANCOVA for slope difference F ⬍ 0.0001; df ⫽ 1, 44; P ⫽ 0.9974). Biomass of Larvae. ANOVA showed signiÞcant effects of host plant species (F ⫽ 61.6904.3177; df ⫽ 1,
December 2013
DUAN ET AL.: INTRASPECIFIC COMPETITION OF EMERALD ASH BORER LARVAE
1197
Fig. 2. Relationship between mortality from cannibalism (A) or starvation (B) and observed larval density of emerald ash borer on logs of green ash (Þlled circle with solid regression line) and tropical ash (clean circle with dashed regression line). Each data point represents the observation from an individual log.
38; P ⬍ 0.0001) and observed larval density (F ⫽ 29.29175; df ⫽ 38, 1; P ⬍ 0.0001) on the mean biomass of surviving emerald ash borer larvae, but no effect of log size (F ⫽ 0.01076; df ⫽ 1, 38; P ⫽ 0.7447). While the mean biomass of surviving larvae was signiÞcantly higher on tropical ash logs than on green ash logs, the mean larval biomass was signiÞcantly reduced with the increase of larval density for both green ash and tropical ash (Fig. 3). ANCOVA detected no signiÞcant difference in the rate (i.e., line slope) of biomass reduction between the ash species (F ⫽ 0.7440; df ⫽ 1, 38; P ⫽ 0.3938).
Discussion Findings from our study indicate that host plant and larval density strongly affect the outcome of intraspeciÞc competition for larvae of A. planipennis when reared on ash logs. As larval density increased, the rate of cannibalization increased signiÞcantly on both green and tropical ash; at the same time, the biomass of surviving larvae signiÞcantly decreased for logs of both ash species. When larval density was controlled to the same levels, however, larval mortality rates from intraspeciÞc competition (from both cannibalism and
1198
ENVIRONMENTAL ENTOMOLOGY
Vol. 42, no. 6
Fig. 3. Relationship between biomass of surviving emerald ash borer larvae and larval density on logs of green ash (Þlled circle with solid regression line) and tropical ash (clean circle with dashed regression line).
starvation) were signiÞcantly higher in green ash logs than in tropical ash logs. In addition, the mean biomass of surviving larvae was signiÞcantly lower when reared in green ash logs than in tropical ash logs. This is an important consideration for mass rearing A. planipennis parasitoids because the gregarious larval parasitoids can adjust clutch size and produce more progeny on larger hosts (Wang et al. 2008). As the density of eggs infested on each log was resourceadjusted (according to the surface area of phloem tissues of each log) in our study, it is not surprising to us that the size of both green and tropical ash logs had little effects on larval mortality rate and biomasses (because of intraspeciÞc competition). It is well documented that green ash is the most susceptible natural host plant for A. planipennis (Rebek et al. 2008, Duan et al. 2012b). However, tropical ash is not known to be a natural host plant because A. planipennis has not yet infested the range of F. uhdei. One of the reasons that tropical ash was proposed for use in rearing A. planipennis in the laboratory is because it can be easily grown year-round in greenhouses, so adult beetles can feed on its foliage during the winter (Duan et al. 2011). During this study, we used the smaller diameter tropical ash logs (range, from 1.5 to 4.5 cm) mainly for logistical reasons (because of the difÞculty of growing large trees in the greenhouse). It is a bit surprising to us that when the egg-infestation density was adjusted for the unit surface area according to log size (diameter), rearing in tropical ash logs resulted in a lower rate of larval mortality and higher biomasses of surviving A. planipennis larvae than green ash logs. Those unexpected
Þndings may have resulted from differential responses of green and tropical ash logs to cutting and subsequent environmental conditions in the growth chamber for this study. We noted that throughout the experiment, nearly all tropical ash logs regenerated new sprout and leaves in the growth chamber while most of green ash logs did not do so. Higher cannibalism in green ash may indicate that tropical ash logs in general have more stored nutritional resources than green ash per unit surface area of phloem tissues thus reducing the intraspeciÞc competition for resources at a given (same) density of larvae A. planipennis. Emerald ash borer larvae can only cannibalize each other if they encounter each other during feeding. If the nutritional content of green ash is lower, they may need to feed more, creating longer galleries and thus encountering each other more frequently. Future chemical and biochemical analyses of changes in nutritional elements in both tropical and green ash logs after cutting would provide a deÞnitive explanation for differences in larval performances when reared on logs of these two different ash plants. For the purpose of rearing healthy A. planipennis larvae, the optimal emerald ash borer egg density should be adjusted according to host plant species. For green ash logs, the lowest egg density (200 eggs per square meter of phloem areas, Table 1) resulted in the highest larval survival rate (68% of larvae surviving to the mature J-larval stage, Fig. 1A) and largest mean biomass (Fig. 3). This indicates that the optimal egg density for rearing healthy mature emerald ash borer with green ash logs should not be ⬎200 eggs per square meter of phloem areas. In contrast, 200 and 400 eggs
December 2013
DUAN ET AL.: INTRASPECIFIC COMPETITION OF EMERALD ASH BORER LARVAE
per square meter of tropical ash phloem resulted in a similar level of survival (⬇86% of larvae surviving to the mature J-larval stage, Fig. 1B) and mean biomass (Fig. 3). This indicates that the optimal emerald ash borer egg density for rearing mature larvae with tropical ash logs should be 400 eggs per square meter of phloem, much higher than with green ash logs. Understanding the mechanism and outcome of intraspeciÞc competition is important in studying density-dependent process in regulation of the population abundance or dynamics in insects and other organisms (Klomp 1964). To the best of our knowledge, our study is the Þrst to show that cannibalism is the major mechanism that operates in A. planipennis larval competition and eventually leads to strong density-dependent mortality (Figs. 1A and B and 2A and B). This is a bit surprising to us, as competition for food has been a primary mechanism of intraspeciÞc competition in many phytophagous insects that feed inside the plant tissues or organs, which have Þnite resources (Heering 1956, Bultman and Faeth 1986, Dukas et al. 2001, Gibbs et al. 2004). Nevertheless, results from our study show that competition for food (i.e., live phloem tissues) does not result in density-dependent mortality, rather reduction in biomass of surviving (mature) larvae (Fig. 3). However, smaller mature (J-larvae) in general would result in smaller adults, which would in turn have reduced reproduction potentials and thus may result in slower population growth. Whether these two mechanisms of intraspeciÞc competition in larvae of A. planipennis play a role in regulating its natural population dynamics is currently not known, and worthy of further investigation in nature. Acknowledgments We thank Jacqueline Hoban and Susan Barth (all University of Delaware employees) for their assistance in all phases of the study. We appreciate the assistance of Dick Bean, Kim Rice, Charles Pickett, Steven Bell (all at Maryland Department of Agriculture) in producing and shipping adult emerald ash borer to us for this study. We are also grateful to Jonathan Schmude, Roger Fuester, and Doug Luster (USDA Agricultural Research Service, BeneÞcial Insects Introduction Research Units) for helpful comments to the earlier version of the manuscript.
References Cited Bultman, T. L., and S. H. Faeth. 1986. Experimental evidence for intraspeciÞc competition in a lepidopteran leaf miner. Ecology 67: 442Ð448. Boulay, R., J. A. Galarza, B. Che´ron, A. Hefetz, A. Lenoir, L. van Oudenhove, and X. Cerda´ . 2010. IntraspeciÞc competition affects population size and resource allocation in an ant dispersing by colony Þssion. Ecology 91: 3312Ð3321. Canadian Food Inspection Agency. 2013. Emerald ash borer Ð Agrilus planipennis. (http://www.inspection.gc.ca/ english/plaveg/pestrava/agrpla/agrplae.shtml) Cappaert, D., D. G. McCullough, T. M. Poland, and N. W. Siegert. 2005. Emerald ash borer in North America: a research and regulatory challenge. Am. Entomol. 51: 152Ð 165.
1199
Carlson, R. W., and F. B. Knight. 1969. Biology, taxonomy, and evolution of four sympatric Agrilus beetles (Coleoptera: Buprestidae). Contrib. Am. Entomol. Inst. 4: 1Ð105. Cooperative Emerald Ash Borer Project. 2013. EAB distribution map published on line. (http://www.aphis.usda. gov/plant_health/plant_pest_info/emerald_ash_b/ downloads/AshRangeMap.pdf) Denno, F. R., M. S. McClure, and J. R. Ott. 1995. InterspeciÞc interactions in phytophagous insects: competition reexamined and resurrected. Annu. Rev. Entomol. 40: 297Ð331. Dukas, R., R. J. Prokopy, and J. J. Duan. 2001. Effects of larval competition on survival and growth in Mediterranean fruit ßies. Ecol. Entomol. 26: 587Ð593. Duan, J. J., M. D. Ulyshen, L. S. Bauer, J. Gould, and R. Van Driesche. 2010. Measuring the impact of biotic factors on populations of immature emerald ash borer (Coleoptera: Buprestidae). Environ. Entomol. 39: 1513Ð1522. Duan, J. J., T. Watt, and C. B. Oppel. 2011. An alternative host plant-based method for laboratory rearing of emerald ash borer to produce larval parasitoids for biological control, pp. 3. In G. Parra, D. Lance, V. Mastro, R Reardon, and C. Benedict (eds.), Proceedings of Emerald Ash Borer Research and Technology Meeting, 10 Ð23 October 2011, Wooster, OH. USDA Forest Service/APHIS/ FHTET-2011Ð 06. Duan, J. J., L. S. Bauer, K. J. Abell, and R. G. Van Driesche. 2012a. Population responses of hymenopteran parasitoids to the emerald ash borer (Coleoptera: Buprestidae) in recently invaded areas in north central United States. BioControl 57: 199 Ð209. Duan, J. J., G. Yurchenko, and R. W. Fuester. 2012b. Occurrence of emerald ash borer (Coleoptera: Buprestidae) and biotic factors affecting its immature stages in the Russian Far East. Environ. Entomol. 41: 245Ð254. Duan, J. J., L. S. Bauer, K. J. Abell, R. G. Van Driesche, and J. P. Lelito. 2013a. Establishment and abundance of Tetrastichus planipennisi (Hymenoptera: Eulophidae) in Michigan: potential for success in classical biocontrol of the invasive emerald ash borer (Coleoptera: Buprestidae). J. Econ. Entomol. 106: 1145Ð1154. Duan, J. J., T. Watt, K. Larson, P. Taylor, and J. P. Lelito. 2013b. Effects of ambient temperature on egg and larval development of the invasive emerald ash borer (Coleoptera: Buprestidae): implications for laboratory-rearing. J. Econ. Entomol. 106: 2101Ð2108. Gibbs M, L. A. Lace, M. J. Jones, and A. J. Moore. 2004. IntraspeciÞc competition in the speckled wood butterßy Parargeaegeria: effect of rearing density and gender on larval life history. J. Insect Sci. 4: 16. (http://www.insectscience.org/4.16) Haack, R. A., J. E. Jendek, H. Liu, K. R. Marchant, T. R. Petrice, R. M. Poland, and H. Ye. 2002. The emerald ash borer: a new exotic pest in North America. Newsl. Mich. Entomol. Soc. 47: 1Ð5. Heering, R. 1956. Zur Biologie, Okologie und zum Massenwechsel des Buchenpracht-Kafers (Agrilus viridis L.). II. Teil Zeitschrift fu¨ r Angewandte Entomologie 39: 76 Ð114. Klomp, H. 1964. IntraspeciÞc competition and regulation of insect numbers. Annu. Rev. Entomol. 9: 17Ð 40. Kaplan, I., and R. F. Denno. 2007. InterspeciÞc interactions in phytophagous insects revisited: a quantitative assessment of theory. Ecol. Lett. 10: 977Ð994. Kovacs, F. K., R. G. Haight, D. G. McCullough, R. J. Mercader, N. W. Siegert, and A. M. Leibhold. 2010. Cost of potential emerald ash borer damage in U.S. communities, 2009 Ð2019. Ecol. Econ. 69: 569 Ð578.
1200
ENVIRONMENTAL ENTOMOLOGY
Legros, M., A. L. Lloyd, Y. Huang, and F. Gould. 2009. Density-dependent intraspeciÞc competition in the larval stage of Aedes aegypti (Diptera: Culicidae): Revisiting the current paradigm. J. Med. Entomol. 46: 409 Ð 419. Lieske, R., and P. Zwick. 2008. Effects of intraspeciÞc competition on the life cycle of the stoneßy, Nemurella pictetii (Plecoptera: Nemouridae). BMC Ecol. 8: 5. (doi:10.1186/ 1472Ð 6785-8 Ð5) Liu, H. P., L. S. Bauer, D. L. Miller, T. H. Zhao, R. T. Gao, L. Song, Q. Luan, R. Jin, and C. Gao. 2007. Seasonal abundance of Agrilus planipennis (Coleoptera: Buprestidae) and its natural enemies Oobius agrili (Hymenoptera: Encyrtidae) and Tetrastichus planipennisi (Hymenoptera: Eulophidae) in China. Biol. Control 42: 61Ð71. McCullough, D. G., and N. W. Siegert. 2007. Estimating potential emerald ash borer (Coleoptera: Buprestidae) populations using ash inventory data. J. Econ. Entomol. 100: 1577Ð1586. McCullough, D. G., T. M. Poland, and D. Cappaert. 2009. Attraction of the emerald ash borer to ash trees stressed by girdling, herbicide treatment, or wounding. Can. J. Forest Res. 39: 1331Ð1345. Poland, T. M., and D. G. McCullough. 2006. Emerald ash borer: invasion of the urban forest and the threat to North AmericaÕs ash resource. J. Forestry 104: 118 Ð124. Rebek, E. J., D. A. Herms, and D. R. Smitley. 2008. InterspeciÞc variation in resistance to emerald ash borer (Co-
Vol. 42, no. 6
leoptera: Buprestidae) among North American and Asian ash (Fraxinus spp.). Environ. Entomol. 37: 242Ð246. Rigsby, C. M., D. Cipollini, E. M. Amstutz, T. J. Smith, and Y. A. Loder. 2013. Water conservation features of ova of Agrilus planipennis (Coleoptera: Buprestidae). Environ. Entomol. 42: 363Ð369. SAS Institute. 2012. JMP PRO 10 modeling and multivariate methods. SAS Institute, Cary NC. Wei, X., W. Yun, R. Reardon, T.-H. Sun, M. Lu, and J.-H. Sun. 2007. Biology and damage traits of emerald ash borer (Agrilus planipennis Fairmaire) in China. Insect Sci. 14: 367Ð373. Wang, X. Y., Z. Q. Yang, H. Wu, and J. R. Gould. 2008. Effects of host size on the sex ratio, clutch size, and size of adult Spathius agrili, and ectoparasitoid of emerald ash borer. Biol. Control 44: 7Ð12. Wang X. Y., Z. Q. Yang, J. R. Gould, Y. N. Zhang, G. J. Lin, and E. S. Liu. 2010. The biology and ecology of the emerald ash borer, Agrilus planipennis, in China. J. Insect Sci. 10: 128. (http://www.insectscience.org/10.128). Yang, S., J. J. Duan, T. Watt, K. J. Abell, and R. Van Driesche. 2012. Responses of an idiobiont ectoparasitoid Spathius galinae to host larvae parasitized by the koinobiont endoparasitoid Tetrastichus planipennisi: implications for biological control of the emerald ash borer (Coleoptera: Buprestidae). Environ. Entomol. 41: 925Ð932. Received 25 July 2013; accepted 15 October 2013.
BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use. Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder.
BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research.
POPULATION ECOLOGY
Effects of Host Plant and Larval Density on Intraspecific Competition in Larvae of the Emerald Ash Borer (Coleoptera: Buprestidae) JIAN J. DUAN,1,2 KRISTI LARSON,3 TIM WATT,3 JULI GOULD,4
AND
JONATHAN P. LELITO5
Environ. Entomol. 42(6): 1193Ð1200 (2013); DOI: http://dx.doi.org/10.1603/EN13209
ABSTRACT Competition for food, mates, and space among different individuals of the same insect species can affect density-dependent regulation of insect abundance or population dynamics. The emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), is a serious invasive pest of North American ash (Fraxinus spp.) trees, with its larvae feeding in serpentine galleries between the interface of sapwood and phloem tissues of ash trees. Using artiÞcial infestation of freshly cut logs of green ash (Fraxinus pennsylvanica Marshall) and tropical ash (Fraxinus uhdei [Wenzig] Lingelsh) with a series of egg densities, we evaluated the mechanism and outcome of intraspeciÞc competition in larvae of A. planipennis in relation to larval density and host plant species. Results from our study showed that as the egg densities on each log (1.5Ð 6.5 cm in diameter and 22Ð25 cm in length) increased from 200 to 1,600 eggs per square meter of surface area, larval survivorship declined from ⬇68 to 10% for the green ash logs, and 86 to 55% for tropical ash logs. Accordingly, larval mortality resulting from cannibalism, starvation, or both, signiÞcantly increased as egg density increased, and the biomass of surviving larvae signiÞcantly decreased on both ash species. When larval density was adjusted to the same level, however, larval mortality from intraspeciÞc competition was signiÞcantly higher and mean biomasses of surviving larvae was signiÞcantly lower in green ash than in tropical ash. The role of intraspeciÞc competition of A. planipennis larvae in density-dependent regulation of its natural population dynamics is discussed. KEY WORDS Agrilus planipennis, larval density, competition, Þtness, density dependence
For many insect species, competition for food, mates, and space among different individuals is a pervasive phenomenon with ecological consequences such as density-dependent regulation of insect abundance or population dynamics (Klomp 1964, Bultman and Faeth 1986, Dukas et al. 2001, Gibbs et al. 2004, Lieske and Zwick 2008, Legros et al. 2009, Boulay et al. 2010). Understanding the intensity and mechanisms of intraspeciÞc competition in insect pests can foster the development of sound pest management strategies (Klomp 1964, Denno et al. 1995, Kaplan and Denno 2007). The emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), is a serious invasive forest pest in North America, where it has caused large-scale ash (Fraxinus spp.) mortality and has spread to 21 U.S. states (Kovacs et al. 2010, Cooperative Emerald Ash Borer Project 2013) and several Canadian provinces (Canadian Food Inspection Agency 2013) since it was Þrst discovered in Michigan 1 BeneÞcial Insects Introduction Research Unit, USDAÐARS, 501 S. Chapel St., Newark, DE 19713. 2 Corresponding author, e-mail: [email protected]. 3 Department of Entomology and Wildlife Ecology, University of Delaware, Newark, DE 19713. 4 USDA APHIS PPQ-CPHST, Buzzards Bay, MA 02542. 5 USDA APHIS PPQ Emerald Ash Borer Biocontrol Laboratory, Brighton, MI 48116.
(Haack et al. 2002). Unlike many other Agrilus beetles that primarily attack host trees weakened by biotic or abiotic stresses such as diseases and drought (Carlson and Knight 1969), A. planipennis attacks live healthy ash trees (Haack et al. 2002, Cappaert et al. 2005). Adults of A. planipennis lay individual eggs, either singly or in small clutches (⬍10 eggs) in crevices and under bark ßakes on limbs and trunks of ash trees (Wei et al. 2007, Wang et al. 2010, Rigsby et al. 2013). Larvae of A. planipennis bore into the interface between phloem and sap wood tissue, feed, and develop for several months primarily on phloem tissues, creating extensive galleries under the bark. When populations of A. planipennis are high, larval consumption of tree phloem is substantial, resulting in tree girdling and death (Poland and McCullough 2006, McCullough et al. 2009). In both newly invaded and native ranges, densities of A. planipennis larvae in infested trees can be extremely high. For example, in Michigan (the epicenter of the U.S. invasion), green ash (Fraxinus pennsylvanica Marshall) and white ash (Fraxinus americana L.) trees killed by A. planipennis on average produced 89 adult beetles per square meter of surface areas of phloem tissue (McCullough and Siegert 2007). When considering the high rate (50 Ð90%) of mortality caused by woodpeckers and other biotic factors (as reported in Cappaert at al. 2005 and Duan et al. 2012a,
0046-225X/13/1193Ð1200$04.00/0 䉷 2013 Entomological Society of America
1194
ENVIRONMENTAL ENTOMOLOGY
2013a), however, larval densities in those North American ash trees killed by A. planipennis would have been several-fold higher than the density of emerged adults. Most recently, a Þeld study conducted in the Russian Far East showed that living F. pennsylvanica trees infested with A. planipennis sustained densities as high as 200 A. planipennis larvae per square meter of phloem (Duan et al. 2012b). Previous Þeld studies showed that several biotic factors including woodpeckers, hymenopteran parasitoids, pathogens, and putative host-plant resistance inßicted between 10 and 90% mortality of larval stages of this pest in both the newly invaded region (Cappaert et al. 2005; Duan et al. 2010, 2012a, 2013a) and the native range (Liu et al. 2007, Wang et al. 2008, Duan et al. 2012b). To date, however, few studies have explicitly examined the mechanism and outcome of intraspeciÞc competition for this invasive pest. The lack of studies on intraspeciÞc competition in A. planipennis is probably because of the lack of feasible techniques to manipulate densities of A. planipennis on living trees in the Þeld. With the recent advancements in rearing in the laboratory (Duan et al. 2011), methods for manipulation of larval density on freshly cut host tree logs have become available. For example, Duan et al. (2011, 2013b) and Yang et al. (2012) successfully infested freshly cut green ash and tropical ash (Fraxinus uhdei [Wenzig] Lingelsh) logs with emerald ash borer eggs laid on coffee Þlters in the laboratory. Larvae hatching from those eggs successfully bored into the logs and developed into mature (J-shaped) larvae in the ash logs; the mature larvae successfully developed into adults after 3 mo of chill treatment (at 12⬚C) to break diapause (Duan et al. 2011; J.J.D., unpublished data). This method is now routinely used to rear larvae of A. planipennis for mass production of biological agents in the United States (Duan et al. 2011, 2013b; J.P.L., personal communication). In the current study, using the laboratory rearing technique developed by Duan et al. (2011, 2013b), we examine the mechanism and intensity of intraspeciÞc competition in larvae of A. planipennis in relation to resource-adjusted larval density on freshly cut logs of two different species of ash: the natural host F. pennsylvanica (green ash) and the alternative (rearing) host F. uhdei (tropical ash). SpeciÞcally, we documented the rate of mortality of A. planipennis larvae owing to direct interference (cannibalism), resource competition (starvation), or both, and we also measured the biomass of surviving mature larvae reared at different larval densities on logs of both green and tropical ash trees. This information will not only contribute to our understanding of the mechanisms and ecological consequences of intraspeciÞc competition in A. planipennis, but also help our selection of optimal egg or larval density for rearing A. planipennis with different type of ash logs.
Vol. 42, no. 6 Materials and Methods
Host Plant. Green ash (F. pennsylvanica) logs were used within 48 h after being cut from the main trunk of ash trees (4 Ð 6 cm DBHÑ diameter at the breast height) in Newark, DE (free of A. planipennis infestations), while tropical ash (F. uhdei) logs were freshly cut from potted trees (2Ð 4 cm DBH) grown in greenhouses at the U.S. Department of AgricultureÐBeneÞcial Insects Introduction Research Unit (Newark, DE). Logs from each species of host plants were 22Ð25 cm in length, but with diameters varying from 3 to 6.5 cm for green ash and 1.5Ð 4 cm for tropical ash. The exact diameter and length of each log were measured and then divided into two categories for each of the two ash species: large logs (diameter range from 5 to 6.5 cm for green ash and 3Ð 4 cm for tropical ash) and small logs (diameter range from 3 to 4.5 cm for green ash and 1.5Ð2.5 cm for tropical ash). Emerald Ash Borer Eggs. All eggs of A. planipennis were laid on unbleached coffee Þlter papers by mated gravid females according to method described by Duan et al. (2011) and Yang et al. (2012) and used in the study within a week before hatching. All adults used for egg production originated from emerald ash borer-infested ash trees collected in Maryland (Yang et al. 2012). Egg Infestation. To create a series of larval densities of A. planipennis, logs of each size category (large or small) from each ash species were infested with four densities of eggs: 200, 400, 800, and 1,600 eggs per square meter of surface areas. For infestation of logs, coffee Þlter papers with A. planipennis eggs were cut into small individual pieces (⬇ 3 mm in length by 3 mm in width) and were then placed on the surface of each log with the bottom side of eggs (attached to Þlter paper) against the bark. ParaÞlm strips (BEMIS, BEMIS Flexible Packaging, Neenah, WI) were wrapped around each log to hold the eggs in place. Following placement of eggs, ash sticks were placed upright in ßoral foam bricks (OASIS, SmithersÑOasis Company, Kent, OH) in storage boxes (58.4 by 41.3 by 31.4 cm, Sterilite, Sterilite Corporation, Townsend, MA) saturated with distilled water and a 0.1% solution of methylparaben to prevent mold. The storage boxes containing egg-infested ash sticks were then incubated in an environmental chamber at 27⬚C (⫾1), 65 (⫾10)% relative humidity (RH), and a photoperiod of 16:8 (L:D) h for larval growth and development. Observation of Emerald Ash Borer Larvae. Eight weeks after incubation at the aforementioned environmental condition, the bark was peeled off each log with utility knives and pruners and the fate of each A. planipennis larva was observed. All larvae of A. planipennis, if alive, would normally have reached mature stage (J-larvae) 8 wk after oviposition at 27⬚C (Duan et al. 2013b). Mature fourth-instar larvae normally stop feeding, make chambers inside the sapwood, and then fold into the J-larval stage for overwintering (via obligatory diapause). On removal of the bark layer, we recorded the stadium (instar) of all exposed larvae based on their body size, width of their gallery, or both
December 2013
DUAN ET AL.: INTRASPECIFIC COMPETITION OF EMERALD ASH BORER LARVAE
1195
Table 1. Target density of emerald ash borer eggs (per square meter of phloem area) and mean number of larvae actually observed for each category (large or small) of both green and tropical ash logs Host tree speciesa Green ash
Tropical ash
a
Target egg density (no. eggs per m2)a
Log category (diam range in cm)a
Mean ⫾ SE no. eggs infested per log
Mean ⫾ SE no. larvae produced per log
Mean ⫾ SE no. larvae per m2 phloem area
200 400 800 1,600 200 400 800 1,600 200 400 800 1,600 200 400 800 1,600
Large (5Ð6.5) Large (5Ð6.5) Large (5Ð6.5) Large (5Ð6.5) Small (3Ð4.5) Small (3Ð4.5) Small (3Ð4.5) Small (3Ð4.5) Large (3Ð4) Large (3Ð4) Large (3Ð4) Large (3Ð4) Small (1.5Ð2.5) Small (1.5Ð2.5) Small (1.5Ð2.5) Small (1.5Ð2.5)
8.7 ⫾ 0.7 18.0 ⫾ 1.0 37.7 ⫾ 1.8 74.3 ⫾ 1.9 6.0 ⫾ 0.6 12.3 ⫾ 0.3 26.0 ⫾ 1.1 54.0 ⫾ 3.0 6.0 ⫾ 1.0 10.3 ⫾ 0.3 22.3 ⫾ 0.7 42.0 ⫾ 0.0 2.7 ⫾ 0.3 6.0 ⫾ 0.6 13.0 ⫾ 0.6 22.7 ⫾ 2.3
8.0 ⫾ 0.0 13.0 ⫾ 1.5 24.0 ⫾ 2.0 46.3 ⫾ 1.8 4.7 ⫾ 0.9 10.0 ⫾ 1.0 21.7 ⫾ 0.7 32.7 ⫾ 2.3 5.0 ⫾ 1.0 7.3 ⫾ 1.8 12.0 ⫾ 1.0 25.0 ⫾ 1.5 2.3 ⫾ 0.3 4.3 ⫾ 0.3 9.0 ⫾ 2.8 13.0 ⫾ 2.5
175 ⫾ 11 292 ⫾ 42 516 ⫾ 12 960 ⫾ 35 155 ⫾ 29 324 ⫾ 30 673 ⫾ 10 976 ⫾ 20 183 ⫾ 29 275 ⫾ 66 426 ⫾ 43 941 ⫾ 55 188 ⫾ 33 301 ⫾ 51 571 ⫾ 51 929 ⫾ 56
Three logs (replicates) for each combination of host tree species, log size, and egg density.
(Wang et al. 2010), and their fate (alive or dead). We distinguished two types of mortality: 1) dead larvae with part of the body cut through or damaged by other larvae, and 2) dead larvae with no apparent evidence of body damage by other larvae. We classiÞed the Þrst type of larval mortality as a result of cannibalism and the second type as a result of starvation (or indirect interference). All surviving larvae were removed from their galleries or pupation chambers and then weighed with an electronic balance (AB135-S/FACT, Mettler Toledo AG Laboratory Weighing and Technology, Greifensee, Switzerland) to determine their biomass (weight). Target Egg Density and Density of Observed Larvae. To reach the target egg density, the number of eggs infested on each log was determined by the surface area of the log, which was calculated based on the log diameter and length. Table 1 summarizes the mean number of eggs infested per log for each size (small and large) category of green and tropical ash logs in corresponding to each target egg density (200, 400, 800, or 1,600 eggs per square meter of surface area) and the mean number of emerald ash borer larvae (alive and dead) observed at the termination of experiment (after 8-wk incubation). The Þnal density of emerald ash borer larvae observed on each log was ⬇10 Ð58% lower than the corresponding targeted egg density because of losses from handling and natural egg mortality. The observed larval density for each log was used in Þnal data analyses. Data Analysis. The experimental design constituted a factorial design with three treatment factors: host tree species (green vs. tropical ash), log size (large vs. small), and egg density (four levels). Three replicates (logs) were tested for each combination of the treatments. However, we used the number of the larvae observed per unit surface area (or the size) of the log as a continuous dependent variable in ANOVA based on linear regression analysis. Likelihood chi-square tests based on multinomial logistic regression model were used to evaluate the effect of the targeted egg
density and host tree species on the fate (alive, dead of starvation, and cannibalization) response of observed emerald ash borer larvae. Multiple or simple linear regression analyses, or both, were used to evaluate the inßuence of host tree species and larval density on mortalities (either from cannibalism or starvation) and biomass of surviving larvae. In addition, ANCOVA (analysis of covariance) was used to analyze the effect of host tree species on the relationship (i.e., line slope) between the larval density and mortality rate or biomass of surviving larvae. All data on mortality rate (proportion) were transformed with the arcsine square root function to normalize the distribution before linear regression analyses. All statistical analyses were carried out with JMP 10.01 (SAS Institute 2012). Results Larval Mortality. As egg densities increased from 200 to 1,600 eggs per square meter of surface area, larval survivorship decreased from ⬇68 to 10% for the green ash (Fig. 1A), and from 86 to 55% for tropical ash (Fig. 1B). Accordingly, larval mortality resulting from cannibalism, starvation, or both, signiÞcantly increased as egg density increased (Fig. 1A and B). Likelihood ratio 2 tests showed that there were signiÞcant differences in larval mortality (combined from both cannibalism and starvation) between green and tropical ash (2 ⫽ 149.549; df ⫽ 2; P ⬍ 0.0001) as well as among different egg density treatments (2 ⫽ 110.383; df ⫽ 6; P ⬍ 0.0001). The log-size (small vs. large) had no effect on larval mortality or survivorship (2 ⫽ 4.044; df ⫽ 2; P ⫽ 0.1324) when adjusted for egg density based on the unit surface area (square meter) of phloem and host plant species (in the multinomial logistic regression model). ANOVA (based on a multiple linear regression model) detected highly signiÞcant effects of host plant species (F ⫽ 17.3177; df ⫽ 1, 44; P ⬍ 0.0001) and observed larval density (F ⫽ 46.7536; df ⫽ 44, 1; P ⬍
1196
ENVIRONMENTAL ENTOMOLOGY
Fig. 1.
Vol. 42, no. 6
Fate of emerald ash borer larvae at different egg densities on green and tropical ash logs.
0.0001) on larval mortality from cannibalism, but no signiÞcant effect of log size (F ⫽ 0.0028; df ⫽ 1, 44; P ⫽ 0.9579) when adjusted for host plant species and larval density (based on the unit surface area [square meter] of phloem). For larval mortality from starvation, ANOVA detected highly signiÞcant effects of host plant species (F ⫽ 15.5903; df ⫽ 1, 44; P ⫽ 0.0003), but only marginally signiÞcant effect of larval density (F ⫽ 3.823; df ⫽ 1, 44; P ⫽ 0.0569), and again no signiÞcant effect of log size (F ⫽ 0.0660; df ⫽ 1, 44; P ⫽ 0.7984). Simple linear regression analysis showed a signiÞcantly positive relationship between larval density (adjusted to per square meter of surface area of phloem) and mortality rate (arcsine square-root transformation) from cannibalism (Fig. 2A), but not be-
tween the larval density and mortality rate owing to starvation (Fig. 2B). Although the positive densitydependent relationship for mortalities from larval cannibalism was numerically stronger (i.e., with a greater line slope) for green ash logs than for tropical ash logs (Fig. 2A), ANCOVA detected no signiÞcant differences in the line slope between the two host plant species (F ⫽ 1.4467; df ⫽ 1, 4; P ⫽ 0.2355). In contrast, the positive density-dependent relationship for mortalities from starvation was approximately parallel for green and tropical ash logs with no statistical signiÞcance (Fig. 2B; ANCOVA for slope difference F ⬍ 0.0001; df ⫽ 1, 44; P ⫽ 0.9974). Biomass of Larvae. ANOVA showed signiÞcant effects of host plant species (F ⫽ 61.6904.3177; df ⫽ 1,
December 2013
DUAN ET AL.: INTRASPECIFIC COMPETITION OF EMERALD ASH BORER LARVAE
1197
Fig. 2. Relationship between mortality from cannibalism (A) or starvation (B) and observed larval density of emerald ash borer on logs of green ash (Þlled circle with solid regression line) and tropical ash (clean circle with dashed regression line). Each data point represents the observation from an individual log.
38; P ⬍ 0.0001) and observed larval density (F ⫽ 29.29175; df ⫽ 38, 1; P ⬍ 0.0001) on the mean biomass of surviving emerald ash borer larvae, but no effect of log size (F ⫽ 0.01076; df ⫽ 1, 38; P ⫽ 0.7447). While the mean biomass of surviving larvae was signiÞcantly higher on tropical ash logs than on green ash logs, the mean larval biomass was signiÞcantly reduced with the increase of larval density for both green ash and tropical ash (Fig. 3). ANCOVA detected no signiÞcant difference in the rate (i.e., line slope) of biomass reduction between the ash species (F ⫽ 0.7440; df ⫽ 1, 38; P ⫽ 0.3938).
Discussion Findings from our study indicate that host plant and larval density strongly affect the outcome of intraspeciÞc competition for larvae of A. planipennis when reared on ash logs. As larval density increased, the rate of cannibalization increased signiÞcantly on both green and tropical ash; at the same time, the biomass of surviving larvae signiÞcantly decreased for logs of both ash species. When larval density was controlled to the same levels, however, larval mortality rates from intraspeciÞc competition (from both cannibalism and
1198
ENVIRONMENTAL ENTOMOLOGY
Vol. 42, no. 6
Fig. 3. Relationship between biomass of surviving emerald ash borer larvae and larval density on logs of green ash (Þlled circle with solid regression line) and tropical ash (clean circle with dashed regression line).
starvation) were signiÞcantly higher in green ash logs than in tropical ash logs. In addition, the mean biomass of surviving larvae was signiÞcantly lower when reared in green ash logs than in tropical ash logs. This is an important consideration for mass rearing A. planipennis parasitoids because the gregarious larval parasitoids can adjust clutch size and produce more progeny on larger hosts (Wang et al. 2008). As the density of eggs infested on each log was resourceadjusted (according to the surface area of phloem tissues of each log) in our study, it is not surprising to us that the size of both green and tropical ash logs had little effects on larval mortality rate and biomasses (because of intraspeciÞc competition). It is well documented that green ash is the most susceptible natural host plant for A. planipennis (Rebek et al. 2008, Duan et al. 2012b). However, tropical ash is not known to be a natural host plant because A. planipennis has not yet infested the range of F. uhdei. One of the reasons that tropical ash was proposed for use in rearing A. planipennis in the laboratory is because it can be easily grown year-round in greenhouses, so adult beetles can feed on its foliage during the winter (Duan et al. 2011). During this study, we used the smaller diameter tropical ash logs (range, from 1.5 to 4.5 cm) mainly for logistical reasons (because of the difÞculty of growing large trees in the greenhouse). It is a bit surprising to us that when the egg-infestation density was adjusted for the unit surface area according to log size (diameter), rearing in tropical ash logs resulted in a lower rate of larval mortality and higher biomasses of surviving A. planipennis larvae than green ash logs. Those unexpected
Þndings may have resulted from differential responses of green and tropical ash logs to cutting and subsequent environmental conditions in the growth chamber for this study. We noted that throughout the experiment, nearly all tropical ash logs regenerated new sprout and leaves in the growth chamber while most of green ash logs did not do so. Higher cannibalism in green ash may indicate that tropical ash logs in general have more stored nutritional resources than green ash per unit surface area of phloem tissues thus reducing the intraspeciÞc competition for resources at a given (same) density of larvae A. planipennis. Emerald ash borer larvae can only cannibalize each other if they encounter each other during feeding. If the nutritional content of green ash is lower, they may need to feed more, creating longer galleries and thus encountering each other more frequently. Future chemical and biochemical analyses of changes in nutritional elements in both tropical and green ash logs after cutting would provide a deÞnitive explanation for differences in larval performances when reared on logs of these two different ash plants. For the purpose of rearing healthy A. planipennis larvae, the optimal emerald ash borer egg density should be adjusted according to host plant species. For green ash logs, the lowest egg density (200 eggs per square meter of phloem areas, Table 1) resulted in the highest larval survival rate (68% of larvae surviving to the mature J-larval stage, Fig. 1A) and largest mean biomass (Fig. 3). This indicates that the optimal egg density for rearing healthy mature emerald ash borer with green ash logs should not be ⬎200 eggs per square meter of phloem areas. In contrast, 200 and 400 eggs
December 2013
DUAN ET AL.: INTRASPECIFIC COMPETITION OF EMERALD ASH BORER LARVAE
per square meter of tropical ash phloem resulted in a similar level of survival (⬇86% of larvae surviving to the mature J-larval stage, Fig. 1B) and mean biomass (Fig. 3). This indicates that the optimal emerald ash borer egg density for rearing mature larvae with tropical ash logs should be 400 eggs per square meter of phloem, much higher than with green ash logs. Understanding the mechanism and outcome of intraspeciÞc competition is important in studying density-dependent process in regulation of the population abundance or dynamics in insects and other organisms (Klomp 1964). To the best of our knowledge, our study is the Þrst to show that cannibalism is the major mechanism that operates in A. planipennis larval competition and eventually leads to strong density-dependent mortality (Figs. 1A and B and 2A and B). This is a bit surprising to us, as competition for food has been a primary mechanism of intraspeciÞc competition in many phytophagous insects that feed inside the plant tissues or organs, which have Þnite resources (Heering 1956, Bultman and Faeth 1986, Dukas et al. 2001, Gibbs et al. 2004). Nevertheless, results from our study show that competition for food (i.e., live phloem tissues) does not result in density-dependent mortality, rather reduction in biomass of surviving (mature) larvae (Fig. 3). However, smaller mature (J-larvae) in general would result in smaller adults, which would in turn have reduced reproduction potentials and thus may result in slower population growth. Whether these two mechanisms of intraspeciÞc competition in larvae of A. planipennis play a role in regulating its natural population dynamics is currently not known, and worthy of further investigation in nature. Acknowledgments We thank Jacqueline Hoban and Susan Barth (all University of Delaware employees) for their assistance in all phases of the study. We appreciate the assistance of Dick Bean, Kim Rice, Charles Pickett, Steven Bell (all at Maryland Department of Agriculture) in producing and shipping adult emerald ash borer to us for this study. We are also grateful to Jonathan Schmude, Roger Fuester, and Doug Luster (USDA Agricultural Research Service, BeneÞcial Insects Introduction Research Units) for helpful comments to the earlier version of the manuscript.
References Cited Bultman, T. L., and S. H. Faeth. 1986. Experimental evidence for intraspeciÞc competition in a lepidopteran leaf miner. Ecology 67: 442Ð448. Boulay, R., J. A. Galarza, B. Che´ron, A. Hefetz, A. Lenoir, L. van Oudenhove, and X. Cerda´ . 2010. IntraspeciÞc competition affects population size and resource allocation in an ant dispersing by colony Þssion. Ecology 91: 3312Ð3321. Canadian Food Inspection Agency. 2013. Emerald ash borer Ð Agrilus planipennis. (http://www.inspection.gc.ca/ english/plaveg/pestrava/agrpla/agrplae.shtml) Cappaert, D., D. G. McCullough, T. M. Poland, and N. W. Siegert. 2005. Emerald ash borer in North America: a research and regulatory challenge. Am. Entomol. 51: 152Ð 165.
1199
Carlson, R. W., and F. B. Knight. 1969. Biology, taxonomy, and evolution of four sympatric Agrilus beetles (Coleoptera: Buprestidae). Contrib. Am. Entomol. Inst. 4: 1Ð105. Cooperative Emerald Ash Borer Project. 2013. EAB distribution map published on line. (http://www.aphis.usda. gov/plant_health/plant_pest_info/emerald_ash_b/ downloads/AshRangeMap.pdf) Denno, F. R., M. S. McClure, and J. R. Ott. 1995. InterspeciÞc interactions in phytophagous insects: competition reexamined and resurrected. Annu. Rev. Entomol. 40: 297Ð331. Dukas, R., R. J. Prokopy, and J. J. Duan. 2001. Effects of larval competition on survival and growth in Mediterranean fruit ßies. Ecol. Entomol. 26: 587Ð593. Duan, J. J., M. D. Ulyshen, L. S. Bauer, J. Gould, and R. Van Driesche. 2010. Measuring the impact of biotic factors on populations of immature emerald ash borer (Coleoptera: Buprestidae). Environ. Entomol. 39: 1513Ð1522. Duan, J. J., T. Watt, and C. B. Oppel. 2011. An alternative host plant-based method for laboratory rearing of emerald ash borer to produce larval parasitoids for biological control, pp. 3. In G. Parra, D. Lance, V. Mastro, R Reardon, and C. Benedict (eds.), Proceedings of Emerald Ash Borer Research and Technology Meeting, 10 Ð23 October 2011, Wooster, OH. USDA Forest Service/APHIS/ FHTET-2011Ð 06. Duan, J. J., L. S. Bauer, K. J. Abell, and R. G. Van Driesche. 2012a. Population responses of hymenopteran parasitoids to the emerald ash borer (Coleoptera: Buprestidae) in recently invaded areas in north central United States. BioControl 57: 199 Ð209. Duan, J. J., G. Yurchenko, and R. W. Fuester. 2012b. Occurrence of emerald ash borer (Coleoptera: Buprestidae) and biotic factors affecting its immature stages in the Russian Far East. Environ. Entomol. 41: 245Ð254. Duan, J. J., L. S. Bauer, K. J. Abell, R. G. Van Driesche, and J. P. Lelito. 2013a. Establishment and abundance of Tetrastichus planipennisi (Hymenoptera: Eulophidae) in Michigan: potential for success in classical biocontrol of the invasive emerald ash borer (Coleoptera: Buprestidae). J. Econ. Entomol. 106: 1145Ð1154. Duan, J. J., T. Watt, K. Larson, P. Taylor, and J. P. Lelito. 2013b. Effects of ambient temperature on egg and larval development of the invasive emerald ash borer (Coleoptera: Buprestidae): implications for laboratory-rearing. J. Econ. Entomol. 106: 2101Ð2108. Gibbs M, L. A. Lace, M. J. Jones, and A. J. Moore. 2004. IntraspeciÞc competition in the speckled wood butterßy Parargeaegeria: effect of rearing density and gender on larval life history. J. Insect Sci. 4: 16. (http://www.insectscience.org/4.16) Haack, R. A., J. E. Jendek, H. Liu, K. R. Marchant, T. R. Petrice, R. M. Poland, and H. Ye. 2002. The emerald ash borer: a new exotic pest in North America. Newsl. Mich. Entomol. Soc. 47: 1Ð5. Heering, R. 1956. Zur Biologie, Okologie und zum Massenwechsel des Buchenpracht-Kafers (Agrilus viridis L.). II. Teil Zeitschrift fu¨ r Angewandte Entomologie 39: 76 Ð114. Klomp, H. 1964. IntraspeciÞc competition and regulation of insect numbers. Annu. Rev. Entomol. 9: 17Ð 40. Kaplan, I., and R. F. Denno. 2007. InterspeciÞc interactions in phytophagous insects revisited: a quantitative assessment of theory. Ecol. Lett. 10: 977Ð994. Kovacs, F. K., R. G. Haight, D. G. McCullough, R. J. Mercader, N. W. Siegert, and A. M. Leibhold. 2010. Cost of potential emerald ash borer damage in U.S. communities, 2009 Ð2019. Ecol. Econ. 69: 569 Ð578.
1200
ENVIRONMENTAL ENTOMOLOGY
Legros, M., A. L. Lloyd, Y. Huang, and F. Gould. 2009. Density-dependent intraspeciÞc competition in the larval stage of Aedes aegypti (Diptera: Culicidae): Revisiting the current paradigm. J. Med. Entomol. 46: 409 Ð 419. Lieske, R., and P. Zwick. 2008. Effects of intraspeciÞc competition on the life cycle of the stoneßy, Nemurella pictetii (Plecoptera: Nemouridae). BMC Ecol. 8: 5. (doi:10.1186/ 1472Ð 6785-8 Ð5) Liu, H. P., L. S. Bauer, D. L. Miller, T. H. Zhao, R. T. Gao, L. Song, Q. Luan, R. Jin, and C. Gao. 2007. Seasonal abundance of Agrilus planipennis (Coleoptera: Buprestidae) and its natural enemies Oobius agrili (Hymenoptera: Encyrtidae) and Tetrastichus planipennisi (Hymenoptera: Eulophidae) in China. Biol. Control 42: 61Ð71. McCullough, D. G., and N. W. Siegert. 2007. Estimating potential emerald ash borer (Coleoptera: Buprestidae) populations using ash inventory data. J. Econ. Entomol. 100: 1577Ð1586. McCullough, D. G., T. M. Poland, and D. Cappaert. 2009. Attraction of the emerald ash borer to ash trees stressed by girdling, herbicide treatment, or wounding. Can. J. Forest Res. 39: 1331Ð1345. Poland, T. M., and D. G. McCullough. 2006. Emerald ash borer: invasion of the urban forest and the threat to North AmericaÕs ash resource. J. Forestry 104: 118 Ð124. Rebek, E. J., D. A. Herms, and D. R. Smitley. 2008. InterspeciÞc variation in resistance to emerald ash borer (Co-
Vol. 42, no. 6
leoptera: Buprestidae) among North American and Asian ash (Fraxinus spp.). Environ. Entomol. 37: 242Ð246. Rigsby, C. M., D. Cipollini, E. M. Amstutz, T. J. Smith, and Y. A. Loder. 2013. Water conservation features of ova of Agrilus planipennis (Coleoptera: Buprestidae). Environ. Entomol. 42: 363Ð369. SAS Institute. 2012. JMP PRO 10 modeling and multivariate methods. SAS Institute, Cary NC. Wei, X., W. Yun, R. Reardon, T.-H. Sun, M. Lu, and J.-H. Sun. 2007. Biology and damage traits of emerald ash borer (Agrilus planipennis Fairmaire) in China. Insect Sci. 14: 367Ð373. Wang, X. Y., Z. Q. Yang, H. Wu, and J. R. Gould. 2008. Effects of host size on the sex ratio, clutch size, and size of adult Spathius agrili, and ectoparasitoid of emerald ash borer. Biol. Control 44: 7Ð12. Wang X. Y., Z. Q. Yang, J. R. Gould, Y. N. Zhang, G. J. Lin, and E. S. Liu. 2010. The biology and ecology of the emerald ash borer, Agrilus planipennis, in China. J. Insect Sci. 10: 128. (http://www.insectscience.org/10.128). Yang, S., J. J. Duan, T. Watt, K. J. Abell, and R. Van Driesche. 2012. Responses of an idiobiont ectoparasitoid Spathius galinae to host larvae parasitized by the koinobiont endoparasitoid Tetrastichus planipennisi: implications for biological control of the emerald ash borer (Coleoptera: Buprestidae). Environ. Entomol. 41: 925Ð932. Received 25 July 2013; accepted 15 October 2013.
