Insect Science (2014) 21, 342–351, DOI 10.1111/1744-7917.12099
ORIGINAL ARTICLE
Seasonal occurrence of Aphis glycines and physiological responses of soybean plants to its feeding Xing-Ya Wang1 , Li-Hong Zhou2 , Biao Xu3 , Xing Xing3 and Guo-Qing Xu1 1 Institute
of Plant Protection, 2 Institute of Flower Research, Liaoning Academy of Agricultural Sciences, Shenyang, and
3 Agricultural
Technology Extension Center of Xiuyan Manchu Autonomous County, Anshan, Liaoning, China
Abstract The soybean aphid Aphis glycines Matsumura (Hemiptera: Aphididae) is an important pest of soybean in China. To monitor and manage this pest effectively it is necessary to understand its population dynamics and demographics, as well as the physiological responses of soybean plants to its feeding. In this study, using field surveying and suctiontrap monitoring, we investigated the population dynamics of the soybean aphid in Xiuyan County, Liaoning Province in northeastern China during 2009–2012. The results indicated that the population dynamics of the soybean aphid followed a unimodal curve distribution, with the insect generally colonizing soybean fields from the middle of June to early July and the population reaching a peak between early July and early August. On the whole, soybean aphids occurred in suction-traps at least 2 weeks earlier than they were found in field surveys. A total of 72 alates were collected by suction-trapping over the 4 years, with the earliest alate captures occurring on 28 May in 2009, 2011, 2012 and 4 June in 2010. The life table parameters clearly showed that this aphid had a short doubling time (4.73 ± 0.21 days), and 7.36 ± 0.98 nymphs were produced by a soybean aphid adult during its lifetime (13.57 ± 0.30 days). Finally, biochemical assays indicated that the amount of malondialdehyde and the activities of four defense-related enzymes in soybean leaves significantly changed between 0 day and 7 days of aphid infestation. Polyphenol oxidase (PPO) and catalase (CAT) activities increased more dramatically after 1 day of aphid feeding. In addition, significantly higher levels of superoxide dismutase and CAT were found after aphid feeding for 7 days, whereas there was no significant change in the activities of peroxidase and PPO. Consequently, this study will be beneficial in determining the seasonal occurrence of the soybean aphid and selecting insect-resistant soybean varieties, and thus in developing a theoretical framework for appropriate management strategies. Key words Aphis glycines, defense-related enzymes, life table, population dynamics, suction trap
Introduction The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an important agricultural pest Correspondence: Guo-Qing Xu, Institute of Plant Protection, Liaoning Academy of Agricultural Sciences, No. 84 Dongling Road, Shenhe District, Shenyang 110161, China. Tel: +86 24 31021234; fax: +86 24 31025677; email: [email protected]
in China, which mainly attacks soybean plants (Glycine max) (Hill et al., 2004; Wang et al., 2005). In general, A. glycines not only decreases soybean yields through direct plant damage, such as stunting, leaf distortion and reduced pod sets, but also can transmit soybean mosaic virus and alfalfa mosaic virus to soybeans (Hill et al., 2001; Clark & Perry, 2002). This species is native to eastern Asia and is widely distributed in mainland and Taiwan of China, Japan, Korea, Indonesia, Malaysia, Philippines, and Thailand (Wang et al., 1994; van den Berg et al., 342
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Zoology, Chinese Academy of Sciences
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1997). Nevertheless, A. glycines is an invasive species that has quickly spread over the northern United States and some provinces of southern Canada in recent years (Venette & Ragsdale, 2004). Thus, it seriously threatens the quality and quantity of soybeans in some of the main production areas worldwide. For example, in China, over 1.39 million ha of soybean in Heilongjiang Province were damaged by A. glycines in 2004 (Wang et al., 2005; Ragsdale et al., 2007; Hodgson et al., 2012). Moreover, its economic impact on soybean production annually has been estimated to range from US$3.6 to $4.9 billion in North America (Kim et al., 2008). Due to its short generation time, overlapping generations, high growth rate and lack of natural enemies, it is very difficult to effectively control the soybean aphid (Ragsdale et al., 2004; Tierney et al., 2007). A. glycines has a typical heteroecious holocyclic life cycle (Wang et al., 1962; Ragsdale et al., 2004). Rhamnus davurica and R. japonica are the main primary hosts in China and Japan (Takahashi et al., 1993), and R. cathartica and R. alnifolia in North America (Ragsdale et al., 2004). In general, overwintering eggs are deposited at the interface between the bud and twig of the primary host, and the fundatrices hatch in early spring and continue to breed for two or three generations before migrating to soybean. Many overlapping generations can occur on soybean, and alatae and apterae are produced throughout the growing season. In autumn, under the influence of reduced photoperiod and temperature, gynoparae and males migrate to Rhamnus, where the gynoparae produce nymphs that develop into oviparae (Ragsdale et al., 2004; Wu et al., 2004; Hodgson et al., 2005). Males emigrate from soybean plants in search of oviparae and mate with them. Finally, mated oviparae lay fertilized eggs at the base of the dormant Rhamnus shoot buds and in twig crevices (Ragsdale et al., 2004). Increasing our understanding of the population dynamics of A. glycines may improve predictions of its outbreaks and enhance management efforts. In general, population scouting is an important component of soybean aphid management and the standard sampling method for A. glycines is in situ field scouting. However, this method is labor-intensive and costly to growers. Fortunately, a network of more than 70 suction-traps, has been established in the western United States and Canada over the past few decades and has been effective in assessing aphid flights (Allison & Pike, 1988; Schmidt et al., 2012). Currently, in China, with the support of the Chinese National Special Fund for Agro-scientific Research in the Public Interest, a new aphid monitoring network system, including more than 20 suction-traps, has been constructed around the major soybean- and wheat-producing areas to monitor the C 2013
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movements of soybean aphids and cereal aphids (Qiao et al., 2011). Several studies have successfully monitored the migration of other small insects and determined the diversity of the arthropod community in agro-ecosystems (Harrington & Woiwod, 2007; Jiang et al., 2011; Zheng et al., 2011; Jiang et al., 2012). Temperature is a key abiotic factor that regulates insect population dynamics, developmental rates and seasonal occurrence (Campbell et al., 1974; McCornack et al., 2004). Several studies have demonstrated that knowledge of the intrinsic rate of increase, fecundity and survivorship schedules are essential for describing the effects of temperature on aphid population dynamics (Walgenbach et al., 1988; Aldyhim & Khalil, 1993; Asin & Pons, 2001). Although life tables for the soybean aphid under constant temperature conditions have been investigated (McCornack et al., 2004), the age-specific life table of A. glycines under fluctuating temperatures is still not well understood. To better understand the mechanisms of soybean resistance to aphids and develop aphid-resistant soybean varieties, it is necessary to investigate the physiological responses of soybeans to soybean aphid feeding. A limited number of studies has indicated that plants have different mechanisms of aphid resistance because different enzymes respond differently to aphid infestation (Chaman et al., 2001; Ni et al., 2001). For example, changes in the amount of malondialdehyde (MDA) and the activities of the defense-related enzymes superoxide dismutase (SOD), peroxidase (POD), phenylalanine ammonia-lyase (PAL) and polyphenol oxidase (PPO) in susceptible and resistant varieties are closely related to aphid resistance of alfalfa under the stress of A. medicaginis infestation, and these biochemical markers may be used as a physiological index for aphid resistance appraisal (Huang et al., 2007). The activities of SOD, POD and PPO in the leaves of three varieties of alfalfa seedlings increased significantly with increasing Therioaphis trifolii densities (Liu & Lan, 2009). The herbivory of cabbage leaves by Brevicoryne brassicae increased the activities of PPO and POD while reducing the activities of SOD and catalase (CAT) in the leaves. The enhanced PPO activity increased the capacity for scavenging free oxygen species (Khattab, 2007). However, to date, little is known about the changes in these enzymes under the stresses of different feeding times and infestation densities of A. glycines. The objectives of this study were: (i) to investigate the population dynamics of A. glycines in North China using field surveys and suction-trap monitoring; (ii) to construct age-specific life tables under fluctuating temperature conditions in the greenhouse; and (iii) to determine the responses of soybeans to this aphid
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feeding for different periods and at different infestation densities. Materials and methods Field surveys to investigate the population dynamics of A. glycines
fresh plants for rearing aphids. The experimental plants were grown individually in a pot and were watered every 3 days. Soybean aphids were collected from the soybean field in Xiuyan Manchu Autonomous County, Liaoning Province and reared in a growth chamber at 27 ± 1°C, with 75% ± 5% RH and with a photoperiod of 14 : 10 h L : D on soybean plants.
A 0.5-ha field of conventional soybeans (Dandou – 14), located at Xiuyan Manchu Autonomous County, Liaoning Province in northeastern China (40°19 N, 123°19 E) was used for this study. Soybeans were seeded on 8 May during 2009–2012, planted in 0.6-m rows to achieve final stands of approximately 15 000 soybean plants per ha. The field was hand-weeded periodically during the soybean growing season, and no chemical pesticides were applied. In this experimental field, 20 locations were selected as fixed sampling sites, each one containing five plants. Samples were collected every 5 days from 25 June to 31 August each year. Each plant was visually examined and the total number of A. glycines counted.
Experimental design The research was conducted in the greenhouse in which the temperature fluctuated between 18°C and 28°C. A clip-cage was used to permit uniform infestation, which provided good ventilation and protection against predators. When parthenogenetic adults were transferred into the cage, they settled rapidly in the confined area. Nymphs were observed the day after infestation, when adults were removed using a camel-hair brush. In each repetition, a total of 60 first-instar nymphs of A. glycines was transferred individually onto soybean plants when the plants reached the V3 stage (three unfolded trifoliolate leaves). Adult survival rate and fecundity were recorded daily and the experiment was continued until all females died. The experiments were replicated three times.
Monitoring the population dynamics of alate soybean aphids by suction-trapping
Age-specific life table parameters Age-specific life tables were constructed based on the age (x), age-specific survival rate (lx ) and average progeny produced in x age class (mx ). The most important parameter of an agespecific life table is the intrinsic rate of natural increase (r), which was estimated using the following equation (Birch, 1948):
An 8.8-m-tall suction traps (Keyun ST – 1B fan power: 0.12 kW; air volume: 2 000 m3 /h; fan velocity: 1450 r/m; inner diameter of the pipe: 244 mm) developed by the Institute of Zoology, Chinese Academy of Sciences (Miao et al., 2011), was built near the soybean field in Xiuyan County. A. glycines samples were collected weekly from 8 April to 19 November during 2009–2012. The main weather parameters, such as mean daily temperature, precipitation, wind direction and speed, and rainfall were noted. All of the aphid samples were stored in 95% ethanol at −20°C and deposited in the Institute of Zoology of the Chinese Academy of Sciences in Beijing, China. Greenhouse trial to determine age-specific life tables of A. glycines Plants and insects Soybeans (cultivar ‘Dandou – 14’) were grown in 10-cm diameter plastic pots in a growth chamber (MGC350HP, Shanghai Yiheng Technical Co., Ltd, Shanghai City, China) from early May to late September in 2012. The chamber was maintained at 27 ± 1°C, with 75% ± 5% RH and a photoperiod of 14 : 10 h L : D. Soybeans were planted every week to provide C 2013
e−r x l x m x = 1,
where r = intrinsic rate of natural increase, lx = age-specific survival, mx = age-specific number of female offspring, x = age in days. Other parameters of the age-specific life tables were calculated (Birch, 1948), including: net reproduction rate (Ro = ࢣlx mx ), mean generation time (T = lnRo /r) and doubling time (DT = ln2/r), as well as finite rate of increase (λ = er ). Physiological responses of soybean plants to A. glycines feeding Insect feeding treatments Two experiments were conducted to investigate the physiological responses of soybean plants to soybean aphid feeding. In the first Institute of Zoology, Chinese Academy of Sciences, 21, 342–351
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Fig. 1 Population dynamics of Aphis glycines in a soybean field in Xiuyan County during 2009–2012.
experiment, aphids were released on soybean leaves at three densities (0, 10 and 30 adult aphids per leaf) when the soybean plants reached the V2 stage (two sets of unfolded trifoliolate leaves). In every aphid density treatment, after introducing the aphids, the soybean leaf was covered with a clip-cage to prevent the aphids from escaping. After 7 days, the aphids were carefully removed from the damaged leaves, which were individually excised and flash frozen using liquid nitrogen, and then stored at −80°C for later processing. Each treatment was replicated three times. In the second experiment, 30 soybean aphids were released on a soybean leaf which was then covered with a clip-cage, and samples were taken at five harvest dates (0, 1, 3, 5 and 7 days) after the initiation of feeding damage by the aphids. Upon collection, the samples were immediately placed in liquid nitrogen and stored at −80°C for later processing. Each treatment was replicated three times.
The disappearance of H2 O2 was measured by the decrease in the absorbance at 240 nm for 1 min at 25°C. For the PPO activity assay, the protocol of Zhang et al. (2006) was followed. The change in absorbance at 398 nm was measured for 2 min at 25°C.
Soybean biochemical assays Biochemical assays were conducted to test whether there was a significant change in the amount of MDA and the activities of four defense-related enzymes (SOD, POD, CAT and PPO) in soybean leaves under different stresses of soybean aphid feeding time and density. For the MDA assay, the protocol of Wang et al. (2011) was followed. The SOD activity assay was based on the method of Dhindsa et al. (1980), which measured the inhibition of the photochemical reduction of nitroblue tetrazolium (NBT) spectrophotometrically at 560 nm. The POD activity assay was based on the method of Zhang et al. (2002), which measured the increase in absorbance at 470 nm over 3 min. For the CAT assay, the protocol of Beer & Sizer (1952) was followed.
Results
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Statistical analyses All of the data collected in this study were analyzed using SPSS for Windows, Version 12.0 (SPSS, Chicago, IL, USA) statistical software. The data were subjected to a one-way analysis of variance (ANOVA) to determine the differences between the mean values, which were compared using the least significant differences (LSD) test. The test data were transformed prior to conducting the LSD test where appropriate, to satisfy the assumptions of normality. The data are all expressed as the mean values ± SE, and the level of significance was set at 0.05.
Fields surveys to investigate the population dynamics of A. glycines On the whole, only one annual population peak of A. glycines occurred in the soybean field in Xiuyan County during 2009–2012 (Fig. 1). The aphid colonized the soybean plants on 11 June during 2010–2012. Soybean aphid density per leaf did not increase until 2 weeks later in each of the 3 years. In contrast, A. glycines colonized the soybean plants on 6 July in 2009. In general, as the temperature increased in early July, the population density of A. glycines increased rapidly, reaching a peak
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Table 1 Population parameters in the age-specific life table of Aphis glycines fed on soybeans in the greenhouse. Population parameter Ro r T DT λ
Mean values ± SE 7.36 0.15 13.57 4.73 1.16
± ± ± ± ±
0.98 0.01 0.30 0.21 0.01
than in the other 3 years. In contrast, only seven alate soybean aphids were collected in 2012 during the whole year.
Greenhouse trials to determine the age-specific life tables of A. glycines
Ro , net reproductive rate (female/female·generation); r, intrinsic rate of increase (female/female·days); T, mean generation time (days); DT, doubling time (days); λ, finite rate of increase (female/female/day).
of 64.0, 816.3, 46.1 and 112.3 aphids per plant on 16 July in 2009 and 2010, 6 July in 2011 and 1 August in 2012, and then the population declined rapidly so that there were fewer than eight aphids per plant in late August.
As shown in Figure 3, there are irregular reproductive peaks during the A. glycines adult lifetimes. As the aphids aged, their survival rate declined and their fecundity gradually increased. In addition, the results indicated that the population of A. glycines had a high intrinsic rate of natural increase (r = 0.15 ± 0.01 female/female/day), finite rate of increase (λ = 1.16 ± 0.01 female/female/day), and short doubling time (DT = 4.73 ± 0.21 day). These findings mean that 7.36 ± 0.98 nymphs were produced by a soybean aphid adult during its lifetime (13.57 ± 0.30 days), doubling every 4.73 days (Table 1). Physiological responses of soybean plants to soybean aphid feeding
Monitoring the occurrence of alate soybean aphids in the population by suction-traps Over the 4-year period, a total of 72 alate soybean aphids were collected by suction-traps in Xiuyan County (Fig. 2). The earliest alate soybean aphid capture occurred during the week beginning 28 May in three of the years (2009, 2011 and 2012) and during the week beginning 4 June in 2010. In 2011 (between 25 June and 6 August), a larger number of alate aphids was collected in the suction-trap
As shown in Figure 4, aphid infestation increased the amount of MDA and the activities of CAT and PPO in the leaves, whereas it reduced the activities of SOD and POD. The amount of MDA and the activities of four defenserelated enzymes in leaves infested with A. glycines significantly changed between 0 day and 7 days (MDA: F4,10 = 5.79, P = 0.021; CAT: F4,10 = 63.25, P = 0.000; PPO: F4,10 = 127.53, P = 0.020; SOD: F4,10 = 22.12,
Fig. 2 Alate Aphis glycines collected by suction-trap in Xiuyan County during 2009–2012.
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Fig. 3 Survival rate (lx ) and fecundity (mx ) of Aphis glycines under greenhouse conditions. Table 2 Changes in the amount of MDA and the activities of four defense-related enzymes in soybean leaves caused by Aphis glycines feeding at different densities. Aphid density per leaf
0 10 30
Contents of MDA and activities of enzyme activity MDA (μmol/g)
SOD (U/g)
POD (U/g·min)
CAT (U/g·min)
PPO (U/g·min)
50.27 ± 3.42 a 20.33 ± 0.57 c 29.80 ± 4.45 b
198.00 ± 24.07 b 176.57 ± 18.60 b 436.63 ± 58.81 a
4.90 ± 0.95 a 5.80 ± 0.78 a 5.40 ± 1.11 a
128.43 ± 11.05 b 388.13 ± 145.20 a 458.43 ± 133.87 a
10.6 ± 0.66 a 10.9 ± 1.77 a 12.53 ± 1.37 a
Mean values in a column indicated by different lowercase letters are significantly different (LSD test, P < 0.05). MDA, malondialdehyde; SOD, superoxide dismutase; POD, peroxidase; CAT, catalase; PPO, polyphenol oxidase.
P = 0.003; and POD: F4,10 = 48.80, P = 0.000). The PPO and CAT activities in the leaves increased more dramatically after 1 day of feeding by A. glycines. Furthermore, little change in the CAT activity occurred after 3 days. Significantly increased SOD and CAT activities in the leaves were found in the soybean leaves after feeding by A. glycines for 7 days (F2,6 = 42.646, P = 0.000; F2,6 = 6.950, P = 0.027), whereas there was no significant change in the activities of POD and PPO (F2,6 = 0.663, P = 0.549; F2,6 = 2.269, P = 0.185). In contrast, the amount of MDA in the soybean leaves was reduced by A. glycines infestation (Table 2).
Discussion In the present study, the population dynamics of A. glycines followed a unimodal curve distribution. However, the peak soybean aphid densities in Xiuyan County be C 2013
tween the 4 years were highly variable, ranging from early July to early August. In addition, A. glycines colonization of soybean plants was delayed until early July in 2009, approximately 1 month later than in the other 3 years. We suggest that this delay was possibly due to the low temperature in the early spring and the limited success of aphid overwintering. Due to the suitable climate conditions in this region (22.7–24.5°C daily mean temperature during June to August), the population densities of A. glycines increase quickly. In particular, the optimum climate for soybean aphid development was reported in 2010, with a relatively high daily mean temperature (20.6–23.9°C) and low daily mean rainfall and wind speed (1.6–15.2 mm, 1.5–1.8 m/s) during June to August; Thus, the number of soybean aphids was the largest among the 4 years – more than 800 aphids per plant in mid-July, which surpassed the suggested economic threshold (273 ± 38 aphids per plant) (Ragsdale et al., 2007). The mean number of aphids in the other 3 years did not reach more than 120 aphids per plant.
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Fig. 4 Changes in the amount of malondialdehyde (MDA) and the activities of four defense-related enzymes caused by the stress of Aphis glycines feeding for different times. A: amount of MDA (μmol/g); B: activity of superoxide dismutase (SOD) (U/g); C: activity of peroxidase (POD) (U/g·min); D: activity of catalase (CAT) (U/g·min); E: the activity of polyphenol oxidase (PPO) (U/g·min).
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A suction-trap is an effective tool for monitoring the aerial population dynamics of small insects, particularly aphids. There are many advantages to monitoring using a suction-trap. For example, sampling is not restricted by time, season or weather. In this study, although a small number of alate soybean aphids were collected from 2009 to 2012, our results strongly indicated that this method allows prediction of the occurrence of soybean aphids at least 2 weeks earlier than field surveying in early spring, when alates began to migrate to soybean fields. Thus, these results were consistent with those of other studies (Qiao et al., 2011). In contrast to the large number of A. glycines collected by suction-traps in North America (Rhainds et al., 2010; Schmidt et al., 2012), the number caught in Xiuyan County was exceedingly low. Over 4 years, a total of 72 alates were collected, and 15, 17, 33 and seven alates were collected in 2009, 2010, 2011 and 2012, respectively. Moreover, it is worth mentioning that soybean aphid catches form a very small proportion (< 0.4%) of the total aphid catch from the Xiuyan suction-trap. The reason for this is unclear, but may be related to the site of the suction-trap (Xiuyan is located in a mountainous region of Liaoning Province), the speed and direction of airflow, or the amount of seasonally migrating soybean aphids in Liaoning Province compared to elsewhere. Understanding the demography of A. glycines could advance the development of a successful integrated pest management program. In general, temperature is the key abiotic factor that regulates insect population growth (Campbell et al., 1974, Logan et al., 1976). Our data indicate that A. glycines tend to break out and cause serious damage to soybeans in suitable temperatures and when predators are excluded. Natural enemies (such as predators, parasitoids and pathogens) are thought to be the most significant biotic factor in regulating A. glycines populations (Claire et al., 2004; Rutledge et al., 2004; Brosius et al., 2007; Li et al., 2011) and populations often considered to be primarily controlled via top-down influences of generalist predators under a wide range of agricultural management systems (Costamagna & Landis, 2006). Future research efforts should focus on the fecundity parameters of A. glycines on resistant and susceptible soybean cultivars to detect their resistance modalities and the sources of resistance. Numerous studies indicate that the host plant’s resistance to herbivores can suppress herbivore population densities, which offers a promising approach to managing pests in a sustainable, economical and environmentally safe manner (Heng-Moss et al., 2004). Defense-related enzymes, including SOD, POD, PPO and CAT, are considered the most important enzymes involved in defensive Institute of Zoology, Chinese Academy of Sciences, 21, 342–351
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responses of plants to insects and pathogens (Weckx & Clisters, 1997; Nandwal et al., 2000). In general, SOD and POD exhibit simultaneous induction and decline, which may be due to their co-regulation (Shigeoka et al., 2002). Our results show that herbivory of soybean leaves by soybean aphids increased the activities of MDA, PPO and CAT, while reducing the activities of SOD and POD. Similar results have been reported for different plant species (Stout et al., 1999; Chaman et al., 2001; Ni et al., 2001; Khattab, 2007). Our study also demonstrated that the activities of these four defense-related enzymes increased under high densities of aphid-infestation stress, particularly those of SOD and CAT, which work to eliminate lipid peroxide and protect against cell membrane damage. Consistent with our results, previous studies reported that the activities of SOD, POD and PPO in the seedling leaves of three alfalfa varieties increased significantly along with increasing aphid densities (Liu & Lan, 2009). The enhanced activities of defense-related enzymes may increase the capacity for scavenging free oxygen species, allowing the infested plants to maintain their photosynthetic activity. These results can be used as practical biochemical parameters for the selection of aphid-tolerant soybean varieties. To date, by using field surveying and suction-trapping, we have investigated the population dynamics of A. glycines in Shenyang, Xiuyan, Xinmin and Changtu counties in Liaoning Province in northern China during 2009–2012, and herein, this work has been continued. Therefore, future research will focus on the relationship between the weather and soybean aphid abundance, forecasts of the timing and size of the spring migration, population sources, migration routes and landing areas. Furthermore, future research efforts should also focus on the constitutive and induced activities of the defense-related enzymes in aphid-resistant and aphidsusceptible cultivars of soybean. All of these efforts will help to determine appropriate management strategies and reduce the aphid’s negative impact on soybean production.
Acknowledgments We are grateful to Dr. X. L. Huang of the Key Laboratory of Zoological Systematics and Evolution, Institute of Zoology, Chinese Academy of Sciences, Beijing, China for providing valuable comments and suggestions. This study was financially supported by the Chinese National Special Fund for Agro-scientific Research in the Public Interest (No. 200803002, 201103002) and the Natural Science Foundation of China (No. 31101626). C 2013
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Disclosure The authors of this manuscript are not involved in any potential conflicts of interest, including financial interests, relationships and affiliations.
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hyde content variation and SSR fingerprint construction for CCRI 35 and CCRI 12. China Cotton, 38, 20–31. (in Chinese) Wang, C.L., Siang, N.I., Chang, G.S. and Chu, H.F. (1962) Studies on the soybean aphid, Aphis glycines Matsumura. Acta Entomologica Sinica, 11, 31–44. (in Chinese) Wang, C.R., Deng, X.C., Yin, L.J., Song, Y.H., Zhang, D.Y. and Shen, H.B. (2005) Analysis of factors on the outbreak of Aphis glycines in Heilongjiang province in 2004. Soybean Bulletin, 3, 19–20. (in Chinese) Wang, X.B., Fang, Y.H., Lin, S.Z., Zhang, L.R. and Wang, H.D. (1994) A study on the damage and economic threshold of the soybean aphid at the seedling stage. Plant Protection, 20, 12–13. (in Chinese) Weckx, J.E.J. and Clijsters, M.M. (1997) Zn phytotoxicity induces oxidative stress in primary leaves of Phaseolus vulgaris. Plant Physiology and Biochemistry, 35, 405–410. Wu, Z.S., Schenk-Hamlin, D., Zhan, W.Y., Ragsdale, D.W. and Heimpel, G.E. (2004) The soybean aphid in China: a historical review. Annals of Entomological Society of America, 97, 209– 218. Zhang, L.J., Yang, Q.K. and Zhang, C.Y. (2002) The changes of POD activity in leaves of soybean varieties infected by Cercospora sojina Hara. Soybean Science, 21, 172–176. (in Chinese) Zhang, L.J., Du, J.Z. and Yang, Q.K. (2006) Study on the changes of polyphenol oxidase in soybean varieties leaves infected by Cercospora sojina Hara. Acta Agriculturae Boreali-Sinica, 21, 91–95. (in Chinese) Zheng, G., Li, X.J., Wand, S.X., Chen, Q., Xu, B. and Xing, X. (2011) Community composition and its characteristics of ballooning spiders in east Liaoning Province of Northeast China. Chinese Journal of Ecology, 30, 40–44. (in Chinese)
Accepted December 16, 2013
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ORIGINAL ARTICLE
Seasonal occurrence of Aphis glycines and physiological responses of soybean plants to its feeding Xing-Ya Wang1 , Li-Hong Zhou2 , Biao Xu3 , Xing Xing3 and Guo-Qing Xu1 1 Institute
of Plant Protection, 2 Institute of Flower Research, Liaoning Academy of Agricultural Sciences, Shenyang, and
3 Agricultural
Technology Extension Center of Xiuyan Manchu Autonomous County, Anshan, Liaoning, China
Abstract The soybean aphid Aphis glycines Matsumura (Hemiptera: Aphididae) is an important pest of soybean in China. To monitor and manage this pest effectively it is necessary to understand its population dynamics and demographics, as well as the physiological responses of soybean plants to its feeding. In this study, using field surveying and suctiontrap monitoring, we investigated the population dynamics of the soybean aphid in Xiuyan County, Liaoning Province in northeastern China during 2009–2012. The results indicated that the population dynamics of the soybean aphid followed a unimodal curve distribution, with the insect generally colonizing soybean fields from the middle of June to early July and the population reaching a peak between early July and early August. On the whole, soybean aphids occurred in suction-traps at least 2 weeks earlier than they were found in field surveys. A total of 72 alates were collected by suction-trapping over the 4 years, with the earliest alate captures occurring on 28 May in 2009, 2011, 2012 and 4 June in 2010. The life table parameters clearly showed that this aphid had a short doubling time (4.73 ± 0.21 days), and 7.36 ± 0.98 nymphs were produced by a soybean aphid adult during its lifetime (13.57 ± 0.30 days). Finally, biochemical assays indicated that the amount of malondialdehyde and the activities of four defense-related enzymes in soybean leaves significantly changed between 0 day and 7 days of aphid infestation. Polyphenol oxidase (PPO) and catalase (CAT) activities increased more dramatically after 1 day of aphid feeding. In addition, significantly higher levels of superoxide dismutase and CAT were found after aphid feeding for 7 days, whereas there was no significant change in the activities of peroxidase and PPO. Consequently, this study will be beneficial in determining the seasonal occurrence of the soybean aphid and selecting insect-resistant soybean varieties, and thus in developing a theoretical framework for appropriate management strategies. Key words Aphis glycines, defense-related enzymes, life table, population dynamics, suction trap
Introduction The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an important agricultural pest Correspondence: Guo-Qing Xu, Institute of Plant Protection, Liaoning Academy of Agricultural Sciences, No. 84 Dongling Road, Shenhe District, Shenyang 110161, China. Tel: +86 24 31021234; fax: +86 24 31025677; email: [email protected]
in China, which mainly attacks soybean plants (Glycine max) (Hill et al., 2004; Wang et al., 2005). In general, A. glycines not only decreases soybean yields through direct plant damage, such as stunting, leaf distortion and reduced pod sets, but also can transmit soybean mosaic virus and alfalfa mosaic virus to soybeans (Hill et al., 2001; Clark & Perry, 2002). This species is native to eastern Asia and is widely distributed in mainland and Taiwan of China, Japan, Korea, Indonesia, Malaysia, Philippines, and Thailand (Wang et al., 1994; van den Berg et al., 342
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1997). Nevertheless, A. glycines is an invasive species that has quickly spread over the northern United States and some provinces of southern Canada in recent years (Venette & Ragsdale, 2004). Thus, it seriously threatens the quality and quantity of soybeans in some of the main production areas worldwide. For example, in China, over 1.39 million ha of soybean in Heilongjiang Province were damaged by A. glycines in 2004 (Wang et al., 2005; Ragsdale et al., 2007; Hodgson et al., 2012). Moreover, its economic impact on soybean production annually has been estimated to range from US$3.6 to $4.9 billion in North America (Kim et al., 2008). Due to its short generation time, overlapping generations, high growth rate and lack of natural enemies, it is very difficult to effectively control the soybean aphid (Ragsdale et al., 2004; Tierney et al., 2007). A. glycines has a typical heteroecious holocyclic life cycle (Wang et al., 1962; Ragsdale et al., 2004). Rhamnus davurica and R. japonica are the main primary hosts in China and Japan (Takahashi et al., 1993), and R. cathartica and R. alnifolia in North America (Ragsdale et al., 2004). In general, overwintering eggs are deposited at the interface between the bud and twig of the primary host, and the fundatrices hatch in early spring and continue to breed for two or three generations before migrating to soybean. Many overlapping generations can occur on soybean, and alatae and apterae are produced throughout the growing season. In autumn, under the influence of reduced photoperiod and temperature, gynoparae and males migrate to Rhamnus, where the gynoparae produce nymphs that develop into oviparae (Ragsdale et al., 2004; Wu et al., 2004; Hodgson et al., 2005). Males emigrate from soybean plants in search of oviparae and mate with them. Finally, mated oviparae lay fertilized eggs at the base of the dormant Rhamnus shoot buds and in twig crevices (Ragsdale et al., 2004). Increasing our understanding of the population dynamics of A. glycines may improve predictions of its outbreaks and enhance management efforts. In general, population scouting is an important component of soybean aphid management and the standard sampling method for A. glycines is in situ field scouting. However, this method is labor-intensive and costly to growers. Fortunately, a network of more than 70 suction-traps, has been established in the western United States and Canada over the past few decades and has been effective in assessing aphid flights (Allison & Pike, 1988; Schmidt et al., 2012). Currently, in China, with the support of the Chinese National Special Fund for Agro-scientific Research in the Public Interest, a new aphid monitoring network system, including more than 20 suction-traps, has been constructed around the major soybean- and wheat-producing areas to monitor the C 2013
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movements of soybean aphids and cereal aphids (Qiao et al., 2011). Several studies have successfully monitored the migration of other small insects and determined the diversity of the arthropod community in agro-ecosystems (Harrington & Woiwod, 2007; Jiang et al., 2011; Zheng et al., 2011; Jiang et al., 2012). Temperature is a key abiotic factor that regulates insect population dynamics, developmental rates and seasonal occurrence (Campbell et al., 1974; McCornack et al., 2004). Several studies have demonstrated that knowledge of the intrinsic rate of increase, fecundity and survivorship schedules are essential for describing the effects of temperature on aphid population dynamics (Walgenbach et al., 1988; Aldyhim & Khalil, 1993; Asin & Pons, 2001). Although life tables for the soybean aphid under constant temperature conditions have been investigated (McCornack et al., 2004), the age-specific life table of A. glycines under fluctuating temperatures is still not well understood. To better understand the mechanisms of soybean resistance to aphids and develop aphid-resistant soybean varieties, it is necessary to investigate the physiological responses of soybeans to soybean aphid feeding. A limited number of studies has indicated that plants have different mechanisms of aphid resistance because different enzymes respond differently to aphid infestation (Chaman et al., 2001; Ni et al., 2001). For example, changes in the amount of malondialdehyde (MDA) and the activities of the defense-related enzymes superoxide dismutase (SOD), peroxidase (POD), phenylalanine ammonia-lyase (PAL) and polyphenol oxidase (PPO) in susceptible and resistant varieties are closely related to aphid resistance of alfalfa under the stress of A. medicaginis infestation, and these biochemical markers may be used as a physiological index for aphid resistance appraisal (Huang et al., 2007). The activities of SOD, POD and PPO in the leaves of three varieties of alfalfa seedlings increased significantly with increasing Therioaphis trifolii densities (Liu & Lan, 2009). The herbivory of cabbage leaves by Brevicoryne brassicae increased the activities of PPO and POD while reducing the activities of SOD and catalase (CAT) in the leaves. The enhanced PPO activity increased the capacity for scavenging free oxygen species (Khattab, 2007). However, to date, little is known about the changes in these enzymes under the stresses of different feeding times and infestation densities of A. glycines. The objectives of this study were: (i) to investigate the population dynamics of A. glycines in North China using field surveys and suction-trap monitoring; (ii) to construct age-specific life tables under fluctuating temperature conditions in the greenhouse; and (iii) to determine the responses of soybeans to this aphid
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feeding for different periods and at different infestation densities. Materials and methods Field surveys to investigate the population dynamics of A. glycines
fresh plants for rearing aphids. The experimental plants were grown individually in a pot and were watered every 3 days. Soybean aphids were collected from the soybean field in Xiuyan Manchu Autonomous County, Liaoning Province and reared in a growth chamber at 27 ± 1°C, with 75% ± 5% RH and with a photoperiod of 14 : 10 h L : D on soybean plants.
A 0.5-ha field of conventional soybeans (Dandou – 14), located at Xiuyan Manchu Autonomous County, Liaoning Province in northeastern China (40°19 N, 123°19 E) was used for this study. Soybeans were seeded on 8 May during 2009–2012, planted in 0.6-m rows to achieve final stands of approximately 15 000 soybean plants per ha. The field was hand-weeded periodically during the soybean growing season, and no chemical pesticides were applied. In this experimental field, 20 locations were selected as fixed sampling sites, each one containing five plants. Samples were collected every 5 days from 25 June to 31 August each year. Each plant was visually examined and the total number of A. glycines counted.
Experimental design The research was conducted in the greenhouse in which the temperature fluctuated between 18°C and 28°C. A clip-cage was used to permit uniform infestation, which provided good ventilation and protection against predators. When parthenogenetic adults were transferred into the cage, they settled rapidly in the confined area. Nymphs were observed the day after infestation, when adults were removed using a camel-hair brush. In each repetition, a total of 60 first-instar nymphs of A. glycines was transferred individually onto soybean plants when the plants reached the V3 stage (three unfolded trifoliolate leaves). Adult survival rate and fecundity were recorded daily and the experiment was continued until all females died. The experiments were replicated three times.
Monitoring the population dynamics of alate soybean aphids by suction-trapping
Age-specific life table parameters Age-specific life tables were constructed based on the age (x), age-specific survival rate (lx ) and average progeny produced in x age class (mx ). The most important parameter of an agespecific life table is the intrinsic rate of natural increase (r), which was estimated using the following equation (Birch, 1948):
An 8.8-m-tall suction traps (Keyun ST – 1B fan power: 0.12 kW; air volume: 2 000 m3 /h; fan velocity: 1450 r/m; inner diameter of the pipe: 244 mm) developed by the Institute of Zoology, Chinese Academy of Sciences (Miao et al., 2011), was built near the soybean field in Xiuyan County. A. glycines samples were collected weekly from 8 April to 19 November during 2009–2012. The main weather parameters, such as mean daily temperature, precipitation, wind direction and speed, and rainfall were noted. All of the aphid samples were stored in 95% ethanol at −20°C and deposited in the Institute of Zoology of the Chinese Academy of Sciences in Beijing, China. Greenhouse trial to determine age-specific life tables of A. glycines Plants and insects Soybeans (cultivar ‘Dandou – 14’) were grown in 10-cm diameter plastic pots in a growth chamber (MGC350HP, Shanghai Yiheng Technical Co., Ltd, Shanghai City, China) from early May to late September in 2012. The chamber was maintained at 27 ± 1°C, with 75% ± 5% RH and a photoperiod of 14 : 10 h L : D. Soybeans were planted every week to provide C 2013
e−r x l x m x = 1,
where r = intrinsic rate of natural increase, lx = age-specific survival, mx = age-specific number of female offspring, x = age in days. Other parameters of the age-specific life tables were calculated (Birch, 1948), including: net reproduction rate (Ro = ࢣlx mx ), mean generation time (T = lnRo /r) and doubling time (DT = ln2/r), as well as finite rate of increase (λ = er ). Physiological responses of soybean plants to A. glycines feeding Insect feeding treatments Two experiments were conducted to investigate the physiological responses of soybean plants to soybean aphid feeding. In the first Institute of Zoology, Chinese Academy of Sciences, 21, 342–351
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Fig. 1 Population dynamics of Aphis glycines in a soybean field in Xiuyan County during 2009–2012.
experiment, aphids were released on soybean leaves at three densities (0, 10 and 30 adult aphids per leaf) when the soybean plants reached the V2 stage (two sets of unfolded trifoliolate leaves). In every aphid density treatment, after introducing the aphids, the soybean leaf was covered with a clip-cage to prevent the aphids from escaping. After 7 days, the aphids were carefully removed from the damaged leaves, which were individually excised and flash frozen using liquid nitrogen, and then stored at −80°C for later processing. Each treatment was replicated three times. In the second experiment, 30 soybean aphids were released on a soybean leaf which was then covered with a clip-cage, and samples were taken at five harvest dates (0, 1, 3, 5 and 7 days) after the initiation of feeding damage by the aphids. Upon collection, the samples were immediately placed in liquid nitrogen and stored at −80°C for later processing. Each treatment was replicated three times.
The disappearance of H2 O2 was measured by the decrease in the absorbance at 240 nm for 1 min at 25°C. For the PPO activity assay, the protocol of Zhang et al. (2006) was followed. The change in absorbance at 398 nm was measured for 2 min at 25°C.
Soybean biochemical assays Biochemical assays were conducted to test whether there was a significant change in the amount of MDA and the activities of four defense-related enzymes (SOD, POD, CAT and PPO) in soybean leaves under different stresses of soybean aphid feeding time and density. For the MDA assay, the protocol of Wang et al. (2011) was followed. The SOD activity assay was based on the method of Dhindsa et al. (1980), which measured the inhibition of the photochemical reduction of nitroblue tetrazolium (NBT) spectrophotometrically at 560 nm. The POD activity assay was based on the method of Zhang et al. (2002), which measured the increase in absorbance at 470 nm over 3 min. For the CAT assay, the protocol of Beer & Sizer (1952) was followed.
Results
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Statistical analyses All of the data collected in this study were analyzed using SPSS for Windows, Version 12.0 (SPSS, Chicago, IL, USA) statistical software. The data were subjected to a one-way analysis of variance (ANOVA) to determine the differences between the mean values, which were compared using the least significant differences (LSD) test. The test data were transformed prior to conducting the LSD test where appropriate, to satisfy the assumptions of normality. The data are all expressed as the mean values ± SE, and the level of significance was set at 0.05.
Fields surveys to investigate the population dynamics of A. glycines On the whole, only one annual population peak of A. glycines occurred in the soybean field in Xiuyan County during 2009–2012 (Fig. 1). The aphid colonized the soybean plants on 11 June during 2010–2012. Soybean aphid density per leaf did not increase until 2 weeks later in each of the 3 years. In contrast, A. glycines colonized the soybean plants on 6 July in 2009. In general, as the temperature increased in early July, the population density of A. glycines increased rapidly, reaching a peak
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Table 1 Population parameters in the age-specific life table of Aphis glycines fed on soybeans in the greenhouse. Population parameter Ro r T DT λ
Mean values ± SE 7.36 0.15 13.57 4.73 1.16
± ± ± ± ±
0.98 0.01 0.30 0.21 0.01
than in the other 3 years. In contrast, only seven alate soybean aphids were collected in 2012 during the whole year.
Greenhouse trials to determine the age-specific life tables of A. glycines
Ro , net reproductive rate (female/female·generation); r, intrinsic rate of increase (female/female·days); T, mean generation time (days); DT, doubling time (days); λ, finite rate of increase (female/female/day).
of 64.0, 816.3, 46.1 and 112.3 aphids per plant on 16 July in 2009 and 2010, 6 July in 2011 and 1 August in 2012, and then the population declined rapidly so that there were fewer than eight aphids per plant in late August.
As shown in Figure 3, there are irregular reproductive peaks during the A. glycines adult lifetimes. As the aphids aged, their survival rate declined and their fecundity gradually increased. In addition, the results indicated that the population of A. glycines had a high intrinsic rate of natural increase (r = 0.15 ± 0.01 female/female/day), finite rate of increase (λ = 1.16 ± 0.01 female/female/day), and short doubling time (DT = 4.73 ± 0.21 day). These findings mean that 7.36 ± 0.98 nymphs were produced by a soybean aphid adult during its lifetime (13.57 ± 0.30 days), doubling every 4.73 days (Table 1). Physiological responses of soybean plants to soybean aphid feeding
Monitoring the occurrence of alate soybean aphids in the population by suction-traps Over the 4-year period, a total of 72 alate soybean aphids were collected by suction-traps in Xiuyan County (Fig. 2). The earliest alate soybean aphid capture occurred during the week beginning 28 May in three of the years (2009, 2011 and 2012) and during the week beginning 4 June in 2010. In 2011 (between 25 June and 6 August), a larger number of alate aphids was collected in the suction-trap
As shown in Figure 4, aphid infestation increased the amount of MDA and the activities of CAT and PPO in the leaves, whereas it reduced the activities of SOD and POD. The amount of MDA and the activities of four defenserelated enzymes in leaves infested with A. glycines significantly changed between 0 day and 7 days (MDA: F4,10 = 5.79, P = 0.021; CAT: F4,10 = 63.25, P = 0.000; PPO: F4,10 = 127.53, P = 0.020; SOD: F4,10 = 22.12,
Fig. 2 Alate Aphis glycines collected by suction-trap in Xiuyan County during 2009–2012.
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Fig. 3 Survival rate (lx ) and fecundity (mx ) of Aphis glycines under greenhouse conditions. Table 2 Changes in the amount of MDA and the activities of four defense-related enzymes in soybean leaves caused by Aphis glycines feeding at different densities. Aphid density per leaf
0 10 30
Contents of MDA and activities of enzyme activity MDA (μmol/g)
SOD (U/g)
POD (U/g·min)
CAT (U/g·min)
PPO (U/g·min)
50.27 ± 3.42 a 20.33 ± 0.57 c 29.80 ± 4.45 b
198.00 ± 24.07 b 176.57 ± 18.60 b 436.63 ± 58.81 a
4.90 ± 0.95 a 5.80 ± 0.78 a 5.40 ± 1.11 a
128.43 ± 11.05 b 388.13 ± 145.20 a 458.43 ± 133.87 a
10.6 ± 0.66 a 10.9 ± 1.77 a 12.53 ± 1.37 a
Mean values in a column indicated by different lowercase letters are significantly different (LSD test, P < 0.05). MDA, malondialdehyde; SOD, superoxide dismutase; POD, peroxidase; CAT, catalase; PPO, polyphenol oxidase.
P = 0.003; and POD: F4,10 = 48.80, P = 0.000). The PPO and CAT activities in the leaves increased more dramatically after 1 day of feeding by A. glycines. Furthermore, little change in the CAT activity occurred after 3 days. Significantly increased SOD and CAT activities in the leaves were found in the soybean leaves after feeding by A. glycines for 7 days (F2,6 = 42.646, P = 0.000; F2,6 = 6.950, P = 0.027), whereas there was no significant change in the activities of POD and PPO (F2,6 = 0.663, P = 0.549; F2,6 = 2.269, P = 0.185). In contrast, the amount of MDA in the soybean leaves was reduced by A. glycines infestation (Table 2).
Discussion In the present study, the population dynamics of A. glycines followed a unimodal curve distribution. However, the peak soybean aphid densities in Xiuyan County be C 2013
tween the 4 years were highly variable, ranging from early July to early August. In addition, A. glycines colonization of soybean plants was delayed until early July in 2009, approximately 1 month later than in the other 3 years. We suggest that this delay was possibly due to the low temperature in the early spring and the limited success of aphid overwintering. Due to the suitable climate conditions in this region (22.7–24.5°C daily mean temperature during June to August), the population densities of A. glycines increase quickly. In particular, the optimum climate for soybean aphid development was reported in 2010, with a relatively high daily mean temperature (20.6–23.9°C) and low daily mean rainfall and wind speed (1.6–15.2 mm, 1.5–1.8 m/s) during June to August; Thus, the number of soybean aphids was the largest among the 4 years – more than 800 aphids per plant in mid-July, which surpassed the suggested economic threshold (273 ± 38 aphids per plant) (Ragsdale et al., 2007). The mean number of aphids in the other 3 years did not reach more than 120 aphids per plant.
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Fig. 4 Changes in the amount of malondialdehyde (MDA) and the activities of four defense-related enzymes caused by the stress of Aphis glycines feeding for different times. A: amount of MDA (μmol/g); B: activity of superoxide dismutase (SOD) (U/g); C: activity of peroxidase (POD) (U/g·min); D: activity of catalase (CAT) (U/g·min); E: the activity of polyphenol oxidase (PPO) (U/g·min).
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A suction-trap is an effective tool for monitoring the aerial population dynamics of small insects, particularly aphids. There are many advantages to monitoring using a suction-trap. For example, sampling is not restricted by time, season or weather. In this study, although a small number of alate soybean aphids were collected from 2009 to 2012, our results strongly indicated that this method allows prediction of the occurrence of soybean aphids at least 2 weeks earlier than field surveying in early spring, when alates began to migrate to soybean fields. Thus, these results were consistent with those of other studies (Qiao et al., 2011). In contrast to the large number of A. glycines collected by suction-traps in North America (Rhainds et al., 2010; Schmidt et al., 2012), the number caught in Xiuyan County was exceedingly low. Over 4 years, a total of 72 alates were collected, and 15, 17, 33 and seven alates were collected in 2009, 2010, 2011 and 2012, respectively. Moreover, it is worth mentioning that soybean aphid catches form a very small proportion (< 0.4%) of the total aphid catch from the Xiuyan suction-trap. The reason for this is unclear, but may be related to the site of the suction-trap (Xiuyan is located in a mountainous region of Liaoning Province), the speed and direction of airflow, or the amount of seasonally migrating soybean aphids in Liaoning Province compared to elsewhere. Understanding the demography of A. glycines could advance the development of a successful integrated pest management program. In general, temperature is the key abiotic factor that regulates insect population growth (Campbell et al., 1974, Logan et al., 1976). Our data indicate that A. glycines tend to break out and cause serious damage to soybeans in suitable temperatures and when predators are excluded. Natural enemies (such as predators, parasitoids and pathogens) are thought to be the most significant biotic factor in regulating A. glycines populations (Claire et al., 2004; Rutledge et al., 2004; Brosius et al., 2007; Li et al., 2011) and populations often considered to be primarily controlled via top-down influences of generalist predators under a wide range of agricultural management systems (Costamagna & Landis, 2006). Future research efforts should focus on the fecundity parameters of A. glycines on resistant and susceptible soybean cultivars to detect their resistance modalities and the sources of resistance. Numerous studies indicate that the host plant’s resistance to herbivores can suppress herbivore population densities, which offers a promising approach to managing pests in a sustainable, economical and environmentally safe manner (Heng-Moss et al., 2004). Defense-related enzymes, including SOD, POD, PPO and CAT, are considered the most important enzymes involved in defensive Institute of Zoology, Chinese Academy of Sciences, 21, 342–351
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responses of plants to insects and pathogens (Weckx & Clisters, 1997; Nandwal et al., 2000). In general, SOD and POD exhibit simultaneous induction and decline, which may be due to their co-regulation (Shigeoka et al., 2002). Our results show that herbivory of soybean leaves by soybean aphids increased the activities of MDA, PPO and CAT, while reducing the activities of SOD and POD. Similar results have been reported for different plant species (Stout et al., 1999; Chaman et al., 2001; Ni et al., 2001; Khattab, 2007). Our study also demonstrated that the activities of these four defense-related enzymes increased under high densities of aphid-infestation stress, particularly those of SOD and CAT, which work to eliminate lipid peroxide and protect against cell membrane damage. Consistent with our results, previous studies reported that the activities of SOD, POD and PPO in the seedling leaves of three alfalfa varieties increased significantly along with increasing aphid densities (Liu & Lan, 2009). The enhanced activities of defense-related enzymes may increase the capacity for scavenging free oxygen species, allowing the infested plants to maintain their photosynthetic activity. These results can be used as practical biochemical parameters for the selection of aphid-tolerant soybean varieties. To date, by using field surveying and suction-trapping, we have investigated the population dynamics of A. glycines in Shenyang, Xiuyan, Xinmin and Changtu counties in Liaoning Province in northern China during 2009–2012, and herein, this work has been continued. Therefore, future research will focus on the relationship between the weather and soybean aphid abundance, forecasts of the timing and size of the spring migration, population sources, migration routes and landing areas. Furthermore, future research efforts should also focus on the constitutive and induced activities of the defense-related enzymes in aphid-resistant and aphidsusceptible cultivars of soybean. All of these efforts will help to determine appropriate management strategies and reduce the aphid’s negative impact on soybean production.
Acknowledgments We are grateful to Dr. X. L. Huang of the Key Laboratory of Zoological Systematics and Evolution, Institute of Zoology, Chinese Academy of Sciences, Beijing, China for providing valuable comments and suggestions. This study was financially supported by the Chinese National Special Fund for Agro-scientific Research in the Public Interest (No. 200803002, 201103002) and the Natural Science Foundation of China (No. 31101626). C 2013
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Disclosure The authors of this manuscript are not involved in any potential conflicts of interest, including financial interests, relationships and affiliations.
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Accepted December 16, 2013
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