Journal of Chemical Ecology, Vol. 18, No. 4, 1992

INFLUENCE OF DIFFERENT HABITATS AND MATING ON OLFACTORY BEHAVIOR OF ONION FLIES SEEKING OVIPOSITIONAL HOSTS

G.J.R.

JUDD l'* and J.H.

BORDEN

Centre for Pest Management Department of Biological Sciences Simon Fraser University Burnaby, British Columbia, Canada V5A 1S6 (Received November 26, 1990; accepted December 11, 1991)

Abstract--Using traps baited with natural and synthetic onion volatiles, we examined the effects of different habitats and mating on the olfactory behavior of laboratory-reared and wild onion flies. Rankings of olfactory treatments as host-finding stimuli for females were dependent on their mating status and the habitat in which they were foraging. In habitats devoid of hosts, traps baited with individual alkyl sulfides were as effective as 4-day-old chopped onions and more effective than 1-day-old onions in elieiting host-finding behavior in laboratory-reared unmated females (LUF) and laboratory-reared mated females (LMF). However, upwind dispersal and percent recapture were always significantly greater in LUF. In one experiment, Pr2S2 was 19 times more attractive to LMF in a fallow field, as than it was in an onion field. Reduced effectiveness of alkyl sulfides as host-finding stimuli in onion fields probably results in part because they are less findable, but more importantly because of a change in searching behavior after females have mated. Evidence to support the latter contention is that traps baited with alkyl sulfides and onions were equally findable by unmated females in both habitats. The behavior of LMF was identical to that of wild females, whereas the behavior of LUF was identical to wild males. The hypothesis that olfactory host-finding behavior in onion flies is modified by the resource level was upheld. Alkyl sulfides appear to be the primary, and possibly the only, chemical effeetors of hostfinding at the patch level of resource distribution, whereas the complex blend emitted by aged, chopped, or damaged onions appears to be acting at the final level of host-finding, while egg-laying females are moving between adjacent hosts in search of an optimal oviposition site. *To whom correspondence should be addressed. ~Present address: Agriculture Canada, Research Station, Summerland, British Columbia, Canada V0H IZO. 605 009843331/92/0400-0605506.50/09 1992PlenumPublishingCorporation

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Key Words--Delia antiqua, Diptera, Anthomyiidae, onion fly, onion maggot, host finding, olfactory behavior, resource distribution.

INTRODUCTION

While foraging for oviposition sites, herbivorous insects may encounter hostplant resources in a hierarchical sequence: habitat, patch, and resource item (Hassel and Southwood, 1978). This hierarchy of resource levels probably selects for a hierarchical host-selection response pattern, especially with regard to the sorts of stimuli and sensory systems insects use to recognize resources (Prokopy, 1986; Harris and Miller, 1988). Insects that use olfaction to locate host plants will likely encounter variations in the quantity and quality of chemical information as a function of resource level. For example, plant odor concentrations may range from minute when the insect is several meters downwind of the source, to high when the insect alights on the plant. Likewise, while plant odors are usually released as complex blends, the number of constituents that an insect might detect simultaneously should decrease as a function of downwind distance from the source (Visser, 1986), especially if the chemical mixture is composed of different classes of compounds with different molecular weights. Thus, perceived plant odor composition might be expected to vary at different levels of resource distribution. The anthomyiid fly, Delia antiqua (Meigen), is an oligophagous herbivore that infests a small number of domesticated AUium spp. (Perron et al., 1958, 1960). The olfactory basis for host recognition and oviposition by female onion flies after alighting on or near a plant was shown to depend on the presence of propylthiol chemicals specific to plants in the AUium genus (Bernhard, 1970; Vernon et al., 1978). However, whether they were tested at optimal release rates (Dindonis and Miller, 1981a) or as simple mixtures (Vernon et al., 1981), alkyl sulfides such as n-dipropyl disulfide (Pr2S2) have never proven to be potent baits for onion flies in onion fields (Miller et al., 1984). Consequently, the most effective host-finding stimulus for the female onion fly is thought to be a "complex blend" of volatiles commonly emitted by aged, microbially and/or insectinfested onions (Ikeshoji et al., 1980; Dindonis and Miller, 1981b; Vernon et al., 1981; Hausmann and Miller, 1989). To date, the hypothesis that this complex blend of volatiles is the most effective host-finding stimulus for onion flies at all levels of resource distribution has not been tested experimentally. Contrary to the above findings, synthetic Pr2S2 has proved to be a potent host-finding stimulant for laboratory-reared onion flies foraging in habitats devoid of host plants (Judd and Borden, 1988, 1989). Pr2S2 induces significant upwind dispersal in unmated, but not mated female onion flies located at least 100 m downwind, and on average ca. 65 % of the released insects were recaptured with

OLFACTORY BEHAVIOR OF ONION FLIES

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Pr2S2 baits. Apparent differences in the response of onion flies to Pr2S 2 while foraging in and outside onion fields may be evidence that resource level, mating, and/or laboratory rearing influence olfactory behavior of onion flies seeking ovipositional hosts, either individually or collectively. We tested four hypotheses with regard to olfactory behavior of female onion flies: (1) individual propylthiol compounds are as effective host-finding stimuli as aged, microbially infested onions; (2) olfactory responses are independent of mating status; (3) olfactory responses are independent of habitat; and (4) the olfactory behavior of laboratory-reared onion flies is equivalent to wild onion flies.

METHODS AND MATERIALS

Experimental Insects. Natural populations of onion flies are very low or at least very transient outside commercial onion fields (Martinson et al., 1989), therefore it is very difficult to examine the behavior of wild onion flies in different habitats. For that reason, and in order to control the sex, age, mating status, and hunger of onion flies responding to host-plant volatiles, all studies were conducted with laboratory-reared and released flies. When these studies were conducted, our culture of flies had been reared on onion in the laboratory for three years. However, simultaneous comparisons of the behavior of laborato.ry-reared and wild onion flies allowed us to test for effects of laboratory culturing on behavior. Rearing and release of onion flies were as described by Judd and Borden (1988). Larvae were reared on onions and stored as pupae until required for experiments. Emergent adults were maintained at 22~ under a 16 : 8 hr lightdark photoperiod, in an.environmental chamber where they fed ad libitum on Ticheler's (1971) diet. At least 24 hr before being released flies were dusted lightly with one of several colored fluorescent powders (DayGlo Color Corp., Cleveland, Ohio). Experiment 1: Trap Catches with Different Host-Plant Olfactory Stimuli. Trap catches provide a relative measure of the effectiveness of different olfactory baits as host-finding stimuli (Dindonis and Miller, 198 la). Therefore, five Alliumrelated baits were compared as host-finding stimuli: 1- and 4-day-old, chopped onion tissue and three synthetic, but naturally occurring, alkyl sulfides (Pierce et al., 1978), PrzS2, methyl-propyl-disulfide (MPrS2) and propenyl-propyl-disulfide (PPrS2). Onions of an unknown cultivar were chopped (mean particle weight ca. 100 mg) and 100 g was placed 2 cm deep into plastic Petri dishes (2 cm high • 9 cm diameter). Lids were placed over the Petri dishes to reduce desiccation, and they were held at 22~ and 65% relative humidity in an environmental

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chamber for either one or four days until required for testing. Pr2S2 was purchased (Eastman Kodak, Rochester, New York, 98% pure by gas chromatography) and MPrS2 and PPrS2 were synthesized as described by Pierce et al. (1978). Each alkyl sulfide was suspended in paraflin oil (1% by volume) and dispensed individually into plastic Petri dishes (25 mm high x 60 mm diameter) in 5-ml aliquots. Dishes containing sulfides were set inside 9-cm-diameter Petri dishes. All 9-cm Petri dishes holding baits were placed in the center of 20 • 20-cm white, horizontal, sticky traps to capture alighting flies (Judd and Borden, 1988). In order to allow volatile emission and prevent entry of flies into baits, 9-cm Petri dish lids were supported ca. 5 mm above all baits on three nails, which also held the sticky square in position. An unbaited 9-cm Petri dish and lid on a white, sticky trap served as a control to measure visual responses to traps. To ensure that released insects were distributed homogeneously, equal numbers of flies were released from 49 equally spaced, intersecting points of a 60 • 60 m grid. Six replicates of the six treatments (unbaited control, Pr2S2, MPrS2, PPrS2, and 1- and 4-day-old chopped onion) were arranged within the 60 • 60-m release area using a 6 • 6 Latin-square randomization (Cochran and Cox, 1957), with rows and traps within rows 10 m apart. This experimental design ensured that each trap was surrounded by four groups of released flies, each at a distance of ca. 7 m. Baits were placed in traps ca. 1 hr before release of flies at 2000-2200 hr Pacific daylight time (PDT). This experiment was concluded after 24 hr.

Experiment 2: Upwind Dispersal in Response to Host-Plant Olfactory Stimuli. In the previous experiment, trap catches were employed as a measure of the "effectiveness" of olfactory stimuli in eliciting host-finding behavior. However, trap catches reveal very little about the underlying behavioral mechanisms that the captured insects used to find the volatile source (Kennedy, 1978). For example, trap catches do not reveal whether onion flies are stimulated to fly towards the trap from some distance or are simply arrested after randomly encountering the trap. A circular trapping design (Judd and Borden, 1988, 1989), in which onion flies are released from the center of a circular array of traps, provides information on the direction of movement before capture. Used in conjunction with continuous wind measurement, this circular design can be used to assess the effectiveness of different olfactory stimuli in eliciting upwind dispersal and longrange host finding in released onion flies. Two treatments, 4-day-old chopped onion tissue and Pr2S2, both as previously described,were compared for their ability to elicit upwind dispersal in female onion flies. A third, unbaited control treatment served to measure dis-

OLFACTORY BEHAVIOR OF ONION FLIES

609

persal in the absence of olfactory stimuli and visual response to traps. Individual treatments were compared in three separate but simultaneous circular releases. Each treatment circle (10 m radius) consisted of eight traps, one in each major and minor cardinal direction. Individual trap circles were set 50-100 m apart. Treatment baits and flies were placed in the field between 2000 and 2200 hr PDT. This experiment was concluded at 1200 hr PDT the following day. Wind direction was measured continuously throughout this experiment with a SIAP mechanical anemometer (Judd and Borden, 1988). Influence of Resource Level on Olfactory Behavior. Searching for onions in a fallow field is considered equivalent to searching at the habitat level of resource distribution, while searching for individual onions in an onion field is probably equivalent to within-patch search (Judd and Borden, 1988; Harris and Miller, 1988). To test the hypothesis (Judd and Borden, 1988, 1989) that response to host-plant olfactory stimuli is dependent on resource level, we conducted experiments 1 and 2 (as described above) in two sites simultaneously: a 5-ha fallow, bare-soil field and a 0.5-ha field of 10- to 25-cm high onions (vat. Autumn Spice). A significant treatment • site interaction would indicate that olfactory behavior is dependent on resource level. Influence of Mating on Olfactory Behavior. Female onion flies reach reproductive maturity at ca. 8-10 days of age and will not usually oviposit until they have mated (Theunisson, 1976). As a result, mated, gravid females are probably more stimulated to oviposit than similarly aged unmated females. To test the hypothesis (Judd and Borden, 1988) that olfactory behavior of female onion flies is dependent of their mating status, we released both mated and unmated, 10-day-old, gravid females simultaneously in each habitat, in both experiments 1 and 2 described above. In experiment 1 (grid-type release) we released 245 unmated and 343 mated females in the fallow field, and 490 females of each type in the onion field. In experiment 2 (circular release) we released 50 females of each mating status from the center of each treatment circle in each habitat. A significant treatment • fly interaction in these experiments would indicate that mating influences olfactory behavior. Statistical Analyses. To make valid comparisons between the response of mated and unmated females or laboratory-reared females and wild flies, we had to convert all trap-catch data to percentages (treatment catch/total catch on all treatments), because actual trap catches reflect differences in population size as well as behavior. Transforming catches to percentages serves to rank treatments within a fly class, which allows us to compare across fly classes independent of differences in the population size of each class of flies. Percentages were transformed (arcsine ~p), and analyzed by an ANOVA appropriate for Latin squares (Cochran and Cox, 1957). Treatment means were compared using the Student-

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Newman-Keuls (SNK) multiple-range test (Zar, 1984). Factorial ANOVAs (treatment, habitat, fly class) were used to test for the effects of olfactory stimuli, resource level, mating, and laboratory-rearing on olfactory behavior. In experiment 2, contingency tables and X 2 tests were used to test the hypotheses that trap catches with different olfactory treatments were independent of resource level and mating, and to test for differences in the behavior of laboratory-reared and wild onion flies. Directional-flight data from experiment 2 (circular release) were analyzed using a Rayleigh test (Batschelet, 1981; Judd and Borden, 1988).

RESULTS

Trap Catches with Different Host-Plant Olfactory Stimuli. The response of laboratory-reared unmated and mated females to olfactory baits in fallow versus onion fields was analyzed with a three-way factorial ANOVA (treatment, fly class, habitat). This analysis revealed a significant treatment effect (Fs, 12o = 16.93, P < 0.001). However, treatment rankings are dependent on mating status and habitat, as the treatment x fly class (Fs, ~2o = 3.75) and treatment x habitat (Fs, L20 = 5.70) interactions were both highly significant (P < 0.005). Therefore, statements about the effectiveness of various olfactory baits as host-finding stimuli must be restricted to a particular fly class or habitat. The ranking of olfactory baits as host-finding stimuli for unmated and mated females in the fallow field was very similar. Traps baited with individual sulfides caught as many unmated and mated flies as did 4-day-old onion and more than 1-day-old onion (Table 1). However, there was a marked change in the behavior of mated females across habitats. For example, unmated females were equally responsive to Pr2S 2, MPrS2, and 4-day-old onion in both habitats, whereas mated females exhibited a significant response to 4-day-old onion over all of the alkyl sulfides tested in the onion field (Table 1). Comparing mean percent catches with Pr2S 2 and 4-day-old onion baits across rows in Table 1 clearly illustrates how the relative responsiveness of mated females to alkyl sulfides is reduced by about threefold in onion fields, while their responsiveness to 4-day-old onion is increased by about twofold. The olfactory response of laboratory-reared unmated and mated females (LUF and LMF, respectively) and wild female (WF) and male (WM) onion flies in an onion field (Table 2) was compared using a two-way ANOVA (treatment, fly class). This analysis revealed a significant treatment effect (Fs, ~10 = 21.06, P < 0.0001), but a nonsignificant (F3,~0 = 0.001, P > 0.05) fly effect. The fly x treatment interaction was also not significant (F15. ~o = 1.62, P = 0.079). However, data in Table 2 indicate some differences and similarities in behavior based on mating status and sex. For example, while LMF and WF

611

OLFACTORY BEHAVIOR OF ONION FLIES TABLE 1. MEAN PERCENTAGE OF LABORATORY-REARED UNMATED AND MATED FEMALE ( L U F , L M F ) ONION FLIES RECAPTURED ON TRAPS BAITED WITH INDIVIDUAL, SYNTHETIC DISULFIDES OR AGED CHOPPED ONION IN DIFFERENT HABITATS (SIx REPLICATES/TREATMENT)

LUF ~

LMF ~

Treatments

Fallow field (N = 245)

Onion field (N = 490)

Fallow field (N = 343)

Onion field (N = 490)

Methyl-propyl-disulfide Propenyl-propyl-disulfide Dipropyl-disulfide 4-day-old onion 1-day-old onion Unbaited control Total catch (n) Recaptured (%, n / N ) b

5.9a 2.8bc 3.5bc~ 3.7ab2 1.3cd 0.2d 171 701

4.0ab 2.2bc 3.3ab~ 5.4a2 1.6bc 0.8c 258 532

3.6a 3.3a 2.8a~ 3.7a2 1.9b 0.8b 254 74E

1.3b 2.2b 0.9b2 8.9al 2.8b 0.5b 137 283

aPercentages within a column = means of number recaptured per treatment and replicate divided by total number recaptured (n). Column means followed by the same letter or row means subscripted by the'same number are not significantly different (SNK test, c~ = 0.05). ~ within this row subscripted by the same letter are not significantly different (cr = 0.05) using a modified SNK test for independent binomial data (Zar, 1984).

TABLE 2. MEAN PERCENTAGE OF LABORATORY-REARED AND WILD ONION FLIES CAPTURED ON TRAPS BAITED WITH INDIVIDUAL, SYNTHETIC DISULFIDES OR AGED CHOPPED ONION IN AN ONION FIELD (SIx REPLICATES/TREATMENT)

Mean percentage capture a Treatment

LUF

LMF

WF

WM

Methyl-propyl-disulfide Propenyl-propyl-disulfide Dipropyl-disulfide 4-day-old onion 1-day-old onion Unbaited control Total catch (n)

4.0ab 2.2bc 3.3ab~ 5.4az 1.6bc 0.8c 258

1.3b 2.2b 0.9b 2 8.9al 2.8b 0.5b 137

1.6b 1.8b 1.8b2 7. lal 3.5b 0.8b 140

2.7ab 2.4ab 2.9ab 5.3a2 1.7b 1.5b 118

aLaboratory-reared unmated females (LUF), laboratory-reared mated females (LMF), wild females (WF), and wild males (WM). Percentages within a column = means of number caught per treatment and replicate divided by total number caught (n). Column means followed by the same letter or row means subscripted by the same number are not significantly different (SNK test, c~ = 0.05).

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r a n k olfactory stimuli identically, both behave differently than L U F or W M (Table 2). L M F and W F were most responsive to 4-day-old o n i o n , while L U F and W M discriminated very little b e t w e e n 4-day-old o n i o n and alkyl sulfides (Table 2). These differences and similarities are seen most clearly w h e n m e a n responses to 4-day-old o n i o n and Pr2S z are compared within rows across colu m n s o f T a b l e 2. Trap Catches with Host-Plant Olfactory Stimuli in Circular Arrays. As in the previous e x p e r i m e n t (Table 1), the olfactory behavior of L U F and L M F was similar in a fallow field b u t significantly different in the onion field when the circular trapping array was e m p l o y e d (Table 3). L M F were equally responsive to o n i o n and Pr2S2 baits in the fallow field (38 vs 38%), but aged-onion baits were 18-fold m o r e attractive than Pr2S2 baits in the o n i o n field (36 vs 2%). C o m p a r i n g female recapture within a habitat clearly indicates that L U F are more likely to be caught o n traps baited with real and artificial host odors than are L M F , especially w h e n a circular trapping a r r a n g e m e n t is used, thus corroborating findings o f Judd a n d B o r d e n (1988). Upwind Dispersal in Response to Host-Plant Olfactory Stimuli. As in all previous experiments of this type (Judd and Borden, 1988, 1989), dispersal o f L U F in fallow fields was significantly directional in the presence o f host-plant odor and nondirectional in its absence (Table 3). Pr2S2 and 4-day-old onion

TABLE 3. COMPARISONOF PERCENT RECAPTURE, MEAN FLIGHT DIRECTION, AND FLIGHT DIRECTEDNESSOF LABORATORY-REAREDFEMALE ONION FLIES EXPOSED TO DIFFERENTOLFACTORY STIMULIIN FALLOW FIELD (FF) AND ONION FIELD (OF) Percent recapture~

Directedness of flight (~)b

Mean flight direction (~)r

Mating status

FF

OF

FF

OF

FF

OF

4-day-old onion

unmated mated

68a~ 38bl

64al 36bl

0.33* 0.29 NS

0.58** 0.36 NS

22 ~ 38 ~

97 ~ 167~

Dipropyl-disulfide

unmated mated

60al 38b~

32a2 2b2

0.32* 0.25 NS

0.74** NC

22 ~ 12~

80 ~ NC

Unbaited control

unmated mated

8az 14al

22a~ 2bl

0.12 NS 0.14 NS

0.71"* NC

167 ~ 182 ~

86 ~ NC

Treatment

a Paired percentages within a column followed by different letters or within a row subscripted by different numbers are significantly different (P < 0.05) by a X2 test on actual counts. bStatistical significance of R (Rayleigh test lBatschelet, 19811) indicated by: NS = P > 0.05; * = P < 0.05; ** = P < 0.005. NC indicates values not calculated due to small sample size. CMean wind direction was 37 ~

OLFACTORY BEHAVIOR OF ONION FLIES

613

treatments elicited similar mean dispersal directions (~) and directedness (R), indicating similar effectiveness as host-finding stimuli. Random dispersal of LMF in the presence of Pr2S2 (Table 3) is consistent with our previous studies (Judd and Borden, 1988); however, lack of directed dispersal by LMF exposed to 4-day-old onion baits is demonstrated for the first time (Table 3). Although dispersal of onion flies within onion fields was directional, it appeared to be independent of trap-released volatiles because dispersal by LUF was significantly directional even in the unbaked control treatment (Table 3). The mean dispersal directions for LUF tended to be crosswind to upwind (Table 3). Conversely, dispersal by LMF in the circle of 4-day-old onion baits was not significantly directional and the mean direction of dispersal was slightly downwind (Table 3). Unfortunately, directional statistics could not be calculated for dispersal of LMF in the Pr2S2 and unbaited control treatment circles, as recapture rates were too low. Catches of Laboratory-Reared and WiM Onion Flies in Circular Assays. The total relative catch of LUF, LMF, WF, and WM on 4-day-old onion, Pr2S2, and unbaited traps for the circular experiment conducted in the onion field were compared using contingency tables and X 2 tests. A 3 • 4 contingency table (three treatments, four fly classes) was highly significant (X 2 = 37.79, df = 6, P < 0.001), indicating that responses to olfactory treatments were dependent on fly class. Table 4 summarizes the results of all possible pairwise comparisons using 2 • 3 contingency tables. The experimentwise error rate for these cornTABLE 4. NUMBER AND PERCENTAGE OF LABORATORY-REARED AND WILD ONION FLIES CAUGHT ON TRAPS ARRANGED IN CIRCULAR ARRAYS IN ONION FIELD

Number (and percentage) of flies of each type caught/treatment~

Statistical significance level"

LUF

LMF

WF

WM

Contingency tables tested h

4-day-old onion

32 (54.2)

18 (90)

172 (75.1)

39 (46.9)

LUF vs LMF LUF vs WF

0.025 0.005

Dipropyl-disnlfide

16 (27.1)

1 (5)

43 (18.8)

23 (27.7)

LUF vs WM LMF vs WF

NS NS

Unbaited control

11 (18.7)

1 (5)

14 (6.1)

21 (25.4)

LMF vs WM WF vs WM

0.005 0.001

Treatment

aPercentages in a column = numbers caught per treatment divided by total number caught on all treatments. LUF = laboratory-reared unmated females; LMF = laboratory-reared mated females; WF = wild females; WM = wild males. b2 • 3 contingency tables (two fly classes • three treatments) tested the hypothesis that response to treatments was independent of fly classification. cSignificance of X2 statistic for contingency tables: NS = P > 0.05.

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parisons was held below 5% by adjusting alpha of individual comparisons. These analyses corroborated previous observations (Table 2) that LMF behave the same as WF, but different from LUF and WM, while LUF behave the same as WM.

DISCUSSION

Despite ample evidence that plant-odor blends mediate host selection, most available data are related to behaviors that occur after an insect has found the resource, e.g., amounts of foliage eaten or numbers of eggs laid. Very little direct evidence bears on the role of odor mixtures in distant host-plant location (May and Ahmad, 1983), and even less about the effects of resource level on insects' response to host-plant olfactory stimuli (Prokopy, 1986; Bell, 1990). The view that a complex odor blend associated with damaged onions is the most effective olfactory stimulus for onion flies seeking ovipositional hosts has arisen out of research conducted in agricultural crops (Dindonis and Miller, 1980a, 1981b; Ishikawa et al., 1981; Vernon et al., 1981; Miller et al., 1984). To a host-seeking onion fly, onion fields probably represent the patch level of resource distribution (Judd and Borden, 1988; Harris and Miller, 1988). Our results indicate that olfactory behavior of onion flies, like visual behavior (Judd and Borden, 1991), is quite different when they are foraging for patches of onions as opposed to individual host plants in a patch of onions. Thus, discussion of host-finding behavior of onion flies should consider resource levels. Whether the "effectiveness" of olfactory treatments as host-finding stimuli is based on rates of recapture or stimulation of upwind dispersal, when tested in a fallow field, individual alkyl sulfides were as effective as the complex blend of alkyl sulfides, aldehydes, and alcohols emitted by 4-day-old onion (Ikeshoji et al., 1980; Vernon et al., 1981). In contrast, the complex odor blend from damaged onions was clearly the most effective host-finding stimulus in onion fields, but only for mated, gravid, and presumably ovipositing females. These results do not necessarily disprove the hypothesis that a complex blend of volatiles is the most effective host-finding stimulus for onion flies foraging outside onion fields, because a valid comparison of various volatiles as host-finding stimuli requires that each be tested at an optimal concentration and release rate. As the concentrations and release rates of volatiles from the 4-day-old onion tissue are unknown and variable, we cannot say categorically that a complex, aged-onion blend would not be a superior host-finding stimulus to Pr2S2 if it were released at higher concentrations. In fact, Miller et al. (1984) showed that catches of onion flies increased with increasing amounts of aged onion. However, the latter results should be interpreted with caution as increased catches with larger baits may simply reflect traps that are sampling a larger area

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615

and population. Hence, increased catches with a higher release-rate bait does not identify a bait that is inherently more attractive unless the populations and active spaces are controlled to the extent they were in our experiments. A tree test of the complex blend hypothesis (Dindonis and Miller, 1981b) must await identification of the active constituents of the aged-onion odor blend, as previously identified chemicals (Ikeshoji et al., 1980; Ishikawa et al., 1983) have not proven themselves to be as effective as aged onion (Miller et al., 1984; Hausmann and Miller, 1989). In spite of the caveat above, what remains important from the perspective of this paper is that an amount of aged onion that was more effective than alkyl sulfides in an onion field was not more effective in a fallow field. Our results indicate that a simple, but distinctive chemical signal such as Pr2S 2, even when released at a relatively low rate (6-9 #g/min), acted as effectively as a complex odor signal when onion flies were foraging outside an onion patch. Additionally, our finding that each synthetic disulfide bait caught more onion flies than freshly chopped onion outside an onion field, combined with the observation that freshly cut onion emits almost exclusively alkyl disulfides (Vernon et al., 1981), suggests that the release rate of alkyl disulfides may be more important than the complexity of the signal when onion flies are foraging at the habitat level of resource distribution. One might infer from these results that the probability of an onion fly locating a new patch of onions is dependent on patch size or density, rather than the quality of individual host plants within the patch. In contrast, assessment of host-plant quality after finding a patch likely depends on more complex chemical signals, hence, the strong response of ovipositing females to 4-day-old onion in an onion field. The question of why mated females are more responsive to aged-onion baits than alkyl sulfides at the patch level of foraging has both proximate and ultimate answers. One proximate explanation may be that alkyl sulfide baits are simply less findable in an onion field. Note that the proportion of mated females recaptured with Pr2S 2 decreased substantially in the onion field. It was this reduction in response to Pr2S2 by mated females, more than an increase in response to aged onion that produced the significant treatment differences in nearly all statistical tests performed. The low release rate from sulfide baits, relative to the concentration of natural sulfides surrounding many thousand of plants, may make sulfide baits indiscernible, i.e., their plumes may be masked in the same way insects' pheromones may be masked in a crop permeated with pheromone. Likewise, onion flies somewhat habituated by natural sulfides in an onion field may be less likely to respond to a point source of sulfides. Yet, if these were the only explanations, why did LUF appear capable of finding the alkyl sulfides in both habitats? An alternative proximate explanation for the different olfactory response pattern in onion fields may be that compounds from aged-onion baits are inter-

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acting with volatiles released from onions growing in the field in a way that enhances mated female response. The fact that traps releasing specific host-plant volatiles often capture more insects when placed in a crop of host plants, as opposed to a fallow field, is sometimes cited as evidence that mixtures of volatiles are the most effective host-finding stimuli for phytophagous insects (Geurin et al., 1983). The inference is that volatiles tested in the host crop are synergyzed by crop-released volatiles to provide a superior stimulus (Finch, 1980). If chemical interaction of this type were responsible for enhancing mated female response to aged-onion odor in an onion field, we would expect the ratio of female recaptures with onion-baited traps in the onion field and the fallow field to be greater than 1 : 1. In fact, the ratios for both unmated and mated females' response to aged onion in different habitats were 1 : 1. Therefore, chemical synergysm of this type does not seem adequate to explain our results. The most probable ultimate cause for the response of mated females to odor from damaged onions in an onion field is that this odor identifies host plants that may maximize a female's reproductive success and decrease developmental time of her offspring (Gorlenko et al., 1956; Zuflini and Robinson, 1978; Schneider et al., 1983). Such behavior would be consistent with predictions from optimal foraging models (Pyke et al., 1977; Jaenike, 1978) that suggest the most suitable resources should always be consumed when encountered and, hence, responded to when detected. Likewise, the fact that unmated females do not oviposit may explain in part why they are less discriminating than ovipositing females. Our results support the hypothesis (Judd and Borden, 1991) that the olfactory behavior of females, in particular their ranking of host stimuli, changes as a function of mating status and resource level. How ovipositing females might locate these optimal resources while foraging among individual onion plants is not entirely clear from our studies. It has been suggested that onion flies respond anemotactically to damaged onions from several meters downwind (Dindonis and Miller, 1980b), independent of reproductive development (Miller and Strickler, 1984). While unmated females clearly disperse upwind in response to simple synthetic and complex natural host-plant chemical signals, we were unable to find any evidence of anemotactic responses by mated females in fallow areas or onion fields, in this or previous studies (Judd and Borden, 1988). Another interesting observation from these studies was that recapture of mated females was always significantly less than that of unmated females regardless of the type of olfactory stimulus involved. These differences were most apparent in the circular trapping arrays. Perhaps the mode of search used by females changes with mating status. Movement of unmated females may be more free ranging and directional than that of mated females, allowing them to locate upwind baits more easily in the experimental setting we have employed. As movement by mated females appears to be random with respect to the mean wind direction, perhaps odor from decomposing onions

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influences behavior at short-range, by arresting movement through increased turning or stimulating landing. Although complex blends can increase the host-finding responses in several species, including the Colorado potato beetle, Leptinotarsa decemlineata L., apple maggot, Rhagoletis pomonella (Walsh), and carrot rest fly, Psila rosae (F.), to our knowledge the role of, or necessity for, complex blends in distant host-plant patch-finding has not been demonstrated for any oligophagous species (Visser and Av6, 1978; Fein et al., 1982; Geurin et al., 1983). It seems predictable, however, that when olfaction is an important component of distant host-plant recognition, insects will utilize a simple, but distinctive chemical message. In some cases this chemical message may also be the predominant host volatile (Miller and Borden, 1990). This prediction seems likely in light of the fact that the integrity of a complex chemical signal may not be maintained several meters downwind in a mixed plant community (Wilson, 1968; Visser, 1986). Of course the validity of this hypothesis will depend on the chemical class, structure, and molecular weight of those chemicals making up the blend. The more diverse these chemicals are, the more likely this hypothesis will hold tree. In the case of onion fly, it remains to be shown exactly what chemicals are responsible for the response to damaged onions. Our results support the hypothesis that the predominant propylthiol volatiles from onions, such as Pr2S2, are probably the primary chemical effectors of longrange host finding at the patch level of resource distribution for onion flies. The complex odor blend associated with damaged onions appears to be acting at the final level of the host-finding sequence, when egg-laying females are moving between adjacent host plants in search of an optimal oviposition site. Hypothetical Optimal Host-Finding Strategy for Onion Flies. It is now possible to construct a plausible sequence of events for an optimal host-finding strategy in D. antiqua (Judd, 1986). For onion flies that do not locate onions by chance during random dispersal after emergence (Martinson et al., 1989), long-range host finding of new patches (fields) of onions probably occurs through the detection and upwind response to volatile host alkyl sulfides (Judd and Borden, 1988, 1989). Visual stimuli come into play at short range and are probably used to delineate patch boundaries when flies approach the source of olfactory stimuli (Judd and Borden, 1991). Once a female onion fly crosses the boundary into a patch of onions, as she would on the border of an onion field, she must locate individual plants on which to lay her eggs. Healthy onions are often the site of oviposition early in the season; however, if some plants are already mechanically damaged, diseased, or infested with insects, location of these plants would be enhanced by the release of metabolites resulting from microbial degradation of onion tissue (Miller et al., 1984). Within a patch, damaged or decomposing onions are probably located through odor-mediated

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arrestment or landing responses rather than long-range directed movements. N e a r this o d o r s o u r c e (ca. 1 m ) , t h e d e c i s i o n to a l i g h t o n a p o t e n t i a l h o s t will b e d e t e r m i n e d b y t h e size, s h a p e , a n d spectral r e f l e c t a n c e o f the s t e m (Harris a n d M i l l e r , 1988; J u d d a n d B o r d e n , 1991). Acknowledgments--We thank the B.C. Ministry of Forestry and G. Sprangers for making field sites available to us, A.D. Wynne for his technical assistance, and H.D. Pierce, Department of Chemistry, Simon Fraser University, for synthesizing the sulfide compounds. This research was completed while the senior author was on educational leave from Agriculture Canada, Harrow, Ontario, and supported in part by Natural Sciences and Engineering Research Council, Canada, Operating Grant A3881, and Undergraduate Scholarship to A.D. Wynne.

REFERENCES BATSCHELET, E. 1981. Circular Statistics in Biology. Academic Press, New York. BELL, W.J. 1990. Searching behavior patterns in insects. Annu. Rev. Entomol. 35:447-468. BERNHARD, R.A. 1970. Chemotaxonomy: Distribution studies of sulfur compounds in Allium. Phytochemistry 9:2019-2027. COCHRAN,W.G., and Cox, G.M. 1957. Experimental Designs, 2nd ed. John Wiley & Sons, New York. DINDON1S, L.L., and MILLER, J.R. 1980a. Host-finding responses of onion and seedcom flies to healthy and decomposing onions and several synthetic constituents of onion. Environ. Entomol. 9:467472. DINDONIS, L.L., and MILLER, J.R. 1980b. Host-finding behavior of the onion fly, Hylemya antiqua. Environ. Entomol. 9:769-772. DINDONIS, L.L., and MILLER, J.R. 1981a. Onion fly trap catch as affected by release rates of n-dipropyl disulfide from polyethylene enclosures. J. Chem. Ecol. 7:411-418. DINDONIS, L.L., and MILLER, J.R. 1981b. Onion fly and little house fly host-finding selectively mediated by decomposing onion and microbial volatiles. J. Chem. Ecol. 7:419426. FERN, B.L., REISSIG, W.H., and ROELOFS, W.L. 1982. Identification of apple volatiles attractive to the apple maggot Rhagoletis pomonella. J. Chem. Ecol. 8:1473-1487. FINCH, S. 1980. Chemical attraction of plant-feeding insects to plants, pp. 67-143, in T.H. Coaker (ed.). Applied Biology, Vol. 5. Academic Press, New York. GEURIN, P.M., ST,/~DLER,E., and BUSER, H.R. 1983. Identification of host plant attractants for the carrot fly, Psila rosae. J. Chem. Ecol. 9:843-861. GORLENKO, M.V., VORONKEVICH,I.V., and MAXIMOVA,Z.S. 1956. The interrelation of Hylemyia antiqua Meig. and Eumerus strigatus Fall. with bacteria causing damp rot in plants. Zool. Zh. 35:16-20 (in Russian; English summary, supplement, p. 4). HARRIS, M.O., and MILLER, J.R. 1988. Host-acceptance behaviour in an herbivorous fly, Delia antiqua. J. Insect Physiol. 34:179-190. HASSLE, M.P., and SOUTHWOOD, T.R.E. 1978. Foraging strategies of insects. Annu. Rev. Ecol. Syst. 9:75-98. HAUSMANN, S.M., and MILLER, J.R. 1989. Production of onion fly attractants and ovipositional stimulants by bacterial isolates cultured on onion. J. Chem. Ecol. 15:905-916. IKESnOJI, T., ISHIKAWA,Y., and MATSUMOTO,Y. 1980. Attractants against the onion maggots and flies Hylemya antiqua, in onions inoculated with bacteria. J. Pestic. Sci. 5:343-350. ISHIKAWA,Y., IKESHOJI,T., and MATSUMOTO,Y. 1981. Field trapping of onion and seed-corn flies with baits of fresh onion pulp. Appl. EntomoL Zool. 16:490-493.

OLFACTORY BEHAVIOROF ONION FLIES

619

ISHIKAWA, Y., IKESHOJI, T., and MATSUMOTO, Y. 1983. 2-Phenyl-ethanol: An attractant for the onion and seed-com flies Hylemya antiqua and H. platura (Diptera: Anthomyiidae). Appl. Entomol. Zool. 18:270-277. JAENmE, J. 1978. On optimal oviposition behavior in phytophagous insects. Theor. Pop. Biol. 14:350-356. JUDD,G.J.R. 1986. Integration of visual and olfactory host-finding mechanisms in the onion maggot, Delia antiqua (Meigen) (Diptera: Anthomyiidae). PhD thesis. Simon Fraser University, Burnaby, British Columbia, Canada. JUDD, G.J.R., and BORDEN, J.H. 1988. Long-range host-finding behaviour of the onion fly Delia antiqua (Diptera: Anthomyiidae): Ecological and physiological constraints. J. Appl. Ecol. 25:829-845. JUDD, G.J.R., and BORDEN, J.H. 1989. Distant olfactory response of the onion fly, Delia antiqua, to host-plant odour in the field. Physiol. Entomol. 14:429--444. JUDD, G.J.R., and BORDEN, J.H. 1991. Sensory interaction during trap-finding by female flies: Implications for ovipositional host-plant finding. Entomol. Exp. Appl. 58:234-249. KENNEDY, J.S. 1978. The concepts of olfactory "arrestment" and "attraction." Physiol. Entomol. 3:91-98. MARTINSON, T.E., NYROP, J.P., and ECKENRODE, C.J. 1989. Long-range host-finding behavior and colonization of onion fields by Delia antiqua (Diptera: Anthomyiidae). J. Econ. Entomol. 82:1111-1120. MAY, M.L., and AHMAD, S. 1983. Host location in the Colorado potato beetle: Searching mechanisms in relation to oligophagy, pp. 173-200, in S. Ahmad (ed.). Herbivorous Insects: HostSeeking Behavior and Mechanisms. Academic Press, New York. MILLER, D.R., and BORDEN, J.H. 1990. /3-Phellandrene: Kairomone for pine engraver, lps pini (Say) (Coleoptera: Scolytidae). J. Chem. Ecol. 16:2519-2531. MILLER, J.R., and STRICKLER,K.L. 1984. Finding and accepting host plants, pp. 127-202, in W.J. Bell and R.T. Card6 (eds.). Chemical Ecology of Insects. Chapman and Hall, London. MILLER, J.R., HARRIS, M.O., and BREZNAK,J.A. 1984. Search for potent attractants of onion flies. J. Chem. Ecol. 10:1477-1488. PERRON, J.P., JASMIN, J.J., and LAFRANCE, J. 1958. Varietal resistance of seeded onions to the onion maggot, Hylemya antiqua (Meigen) (Diptera: Anthomyiidae). Can. Entomol. 90:653656. PERRON, J.P., JASMIN, J.J., and LAFRANCE, J. 1960. Attractiveness of some onion varieties grown in muck soil to oviposition by the onion maggot (Hylemya antiqua (Meigen) (Diptera: Anthomyiidae). Can. Entomol. 92:765-767. PIERCE, H.D., JR., VERNON, R.S., BORDEN, J.H., and OEHLSCHLAGER, A.C. 1978. Host selection by Hylemya antiqua (Meigen): Identification of three new attractants and oviposition stimulants. J. Chem. Ecol. 4:65-72. PROKOPY, R.J. 1986. Visual and olfactory stimulus interaction in resource finding by insects, pp. 80-89, in T.L. Payne, M.C. Birch, and C.E.J. Kennedy (eds.). Mechanisms in Insect Olfaction. Clarendon Press, Oxford. PYKE, G.H., PULLIAM, H.R., and CHARNOV, E.L. 1977. Optimal foraging: A selective review of theory and tests. Qt. Rev. Biol. 52:137-154. SCHNEIDER, W.D., MILLER, J.R., BREZNAK, J.A., and FOBES, J.F. 1983. Onion maggot, Delia antiqua, survival and development on onions in the presence and absence of microorganisms. Entomol. Exp. Appl. 33:50-56. THEUNISSON, J.A.B.M. 1976. Aspects of gametogenesis and radiation pathology in the onion fly Hylemya antiqua (Meigen). I. Gametogenesis. Meded. Landbouwhogech. Wageningen 76:1-164. TICHELER, J. 1971. Rearing of the onion fly, Hylemya antiqua (Meigen) with a view to release of

620

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sterilized insects, pp. 341-346, in Sterility Principle for Insect Control or Eradication. International Atomic Energy Agency, Vienna. VERNON, R.S., BORDEN, J.H., PIERCE, H.D., JR., and OEHLSCHLAOER,A.C. 1978. Host selection by Hylemya antiqua: Identification of oviposition stimulants based on proposed active thioalkane moieties. Environ. Entomol. 7:728-731. VERNON, R.S., JUDD, G.J.R., BORDEN, J.H., PIERCE, H.D., JR., and OEHLSCHLAOER,A.C. 1981. Attraction of Hylemya antiqua (Meigen) (Diptera: Anthomyiidae) in the field to host-produced oviposition stimulants and their non-host analogues. Can. J. Zool. 59:872-881. VISSER, J.M. 1986. Host odor perception in phytophagous insects. Annu. Rev. Entomol. 31:121144. VISSER, J.M., and AVE, D.A. 1978. General green leaf volatiles in the orientation of the Colorado beetle, Leptinotarsa decemlineata. Entomol. Exp. Appl. 24:538-549. WILSON, E.O. 1968. Chemical systems, pp. 75-102, in T.A. Sebok (ed.). Animal Communication: Techniques of Study and Results of Research. Indiana University Press, Bloomington. ZAR, J. 1984. Biostatistical Analysis, 2nd ed. Prentice-Hall, Englewood Cliffs, New Jersey. ZURLINL G., and ROBINSON,A.S. 1978. Onion conditioning pertaining to larval preference and rate of development in (=Hylemya) antiqua. Entomol. Exp. Appl. 23:279-286.

Influence of different habitats and mating on olfactory behavior of onion flies seeking ovipositional hosts.

Using traps baited with natural and synthetic onion volatiles, we examined the effects of different habitats and mating on the olfactory behavior of l...
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