Feeding behavior by the soybean aphid (Hemiptera: Aphididae) on resistant and susceptible soybean genotypes

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is a major pest of soybean, Glycine max (L.) Merr. Since 2000, when A. glycines was detected in the United States, several studies on this insect have been done in different areas, but there is no report of any studies of stylet penetration behavior by A. glycines on resistant and susceptible soybean. Assessment of feeding behavior of this aphid species was compared on four resistant entries (K1639, Pioneer 95B97, Dowling, and Jackson) and a susceptible check (KS4202) by using the electrical penetration graph (EPG) technique. Feeding behavior of A. glycines adults was recorded during a 9-h period. The average time needed to reach the first sieve element phase by A. glycines was 3.5 h in KS4202, whereas it was 7.5 h in the resistant entries. The total duration in the sieve element phase was longer than an hour in KS4202, and only 2 to 7 min in the resistant entries. These results suggest that morphological or chemical factors in the phloem tissue of resistant plants affect stylet penetration activities of A. glycines. In the majority of the recordings, however, the aphid stylet reached the xylem phase before penetrating the sieve element, and the time that aphids spent ingesting xylem sap was not different among all entries. Therefore, it is possible that xylem sap in the resistant entries may contain toxic substances that change aphid behavior and that affect further activities in the sieve element phase.     

Resistance to Aphis glycines (Hemiptera: Aphididae) in various soybean lines under controlled laboratory conditions

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), a pest of soybean, Glycine max (L.) Merr., native to Asia, has recently become a principal pest of this crop in many areas of North America. Insecticides are currently used to manage A. glycines, but host plant resistance is a potential alternative management tool. Tests were conducted to determine resistance to A. glycines among soybean lines. 'Cobb,' 'Tie-feng 8,' and 'Jackson' were resistant to population growth of A. glycines compared with 'Cook' and '91B91,' a susceptible control. Antibiosis was evident in Cobb, Jackson, and Tie-feng 8 from lowered survival of first generation A. glycines, and in Cobb, Jackson, Tie-feng 8, and 'Braxton' from diminished reproduction by first generation aphids. Antixenosis was apparent in Cobb and Jackson during initial infestation of aphid population growth tests, because A. glycines were unsettled and dispersed readily from placement points on unifoliolate leaves. Decreased nymphiposition by A. glycines occurred on Cobb and Jackson, and it may have been caused by antibiotic chemicals in these lines, failure of aphids to settle, or both. Differences in distribution of A. glycines between unifoliolate leaves and other shoot structures suggest that unifoliolate leaves were acceptable feeding sites on 91B91 and Cook, whereas unifoliolate leaves and other shoot structures were roughly equally acceptable feeding sites on Braxton, Tie-feng 8, Jackson, and Cobb. However, Jackson and Cobb had relatively low counts of A. glycines on shoots that may have been due to abandonment of plants by aphids, decreased aphid survival, or both. Results confirm earlier findings that Jackson is a strong source of resistance to A. glycines, and they suggest that Tie-feng 8, Braxton, and especially Cobb are potentially useful sources of resistance.     

Chlorophyll loss caused by soybean aphid (Hemiptera: Aphididae) feeding on soybean

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is a worldwide pest of soybean, Glycine max (L.) Merr. Studies to find control methods were initiated in 2000 when it was first detected in North America. A. glycines can reduce yields by as much as 50%, and it is the vector of several viral diseases. A. glycines removes phloem sap, which can result in a reduction of chlorophyll content. Quantification of chlorophyll loss caused by A. glycines feeding on soybean is of vital importance. The SPAD-502 chlorophyll meter is a device that has been used to measure chlorophyll loss caused by nonchewing insects. Chlorophyll loss was studied in no-choice tests on the infested and uninfested leaves of a susceptible check (KS4202). The minimum combined number of days and aphids needed to detect significant chlorophyll loss was 30 aphids confined for 10 d. In a similar experiment, seven resistant entries and two susceptible checks were evaluated. There was no significant chlorophyll reduction between infested and uninfested leaves of five of the resistant entries (K1621, K1639, Pioneer 95B97, Dowling, and Jackson). Percentage of loss of chlorophyll in the susceptible checks was approximately 40%; Jackson and Dowling had a significantly lower percentage loss (13 and 16%, respectively) compared with the susceptible checks. The percentages of chlorophyll loss of K1621, K1639, and Pioneer 95B97 were not statistically different from the percentage of loss of Jackson.     

Performance and prospects of Rag genes for management of soybean aphid

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an invasive insect pest of soybean [Glycine max (L.) Merr. (Fabaceae)] in North America, and it has led to extensive insecticide use in northern soybean‐growing regions there. Host plant resistance is one potential alternative strategy for managing soybean aphid. Several Rag genes that show antibiosis and antixenosis to soybean aphid have been recently identified in soybean, and field‐testing and commercial release of resistant soybean lines have followed. In this article, we review results of field tests with soybean lines containing Rag genes in North America, then present results from a coordinated regional test across several field sites in the north‐central USA, and finally discuss prospects for use of Rag genes to manage soybean aphids. Field tests conducted independently at multiple sites showed that soybean aphid populations peaked in late summer on lines with Rag1 or Rag2 and reached economically injurious levels on susceptible lines, whereas lines with a pyramid of Rag1 + Rag2 held soybean aphid populations below economic levels. In the regional test, aphid populations were generally suppressed by lines containing one of the Rag genes. Aphids reached putative economic levels on Rag1 lines for some site years, but yield loss was moderated, indicating that Rag1 may confer tolerance to soybean aphid in addition to antibiosis and antixenosis. Moreover, no yield penalty has been found for lines with Rag1, Rag2, or pyramids. Results suggest that use of aphid‐resistant soybean lines with Rag genes may be viable for managing soybean aphids. However, virulent biotypes of soybean aphid were identified before release of aphid‐resistant soybean, and thus a strategy for optimal deployment of aphid‐resistant soybean is needed to ensure sustainability of this technology.     

Field and laboratory evaluations of soybean lines against soybean aphid (Hemiptera: Aphididae)

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is a major pest of soybean, Glycine max (L.). Merr., that significantly reduces yield in northern production areas of North America. Insecticides are widely used to control soybean aphid outbreaks, but efforts are underway to develop host plant resistance as an effective alternative management strategy. Here, previously identified resistant lines were evaluated in laboratory tests against field-collected populations of soybean aphid and in field-plot tests over 2 yr in South Dakota. Six lines previously identified with resistance to soybean aphid--Jackson, Dowling, K1639, Cobb, Palmetto and Sennari--were resistant in this study, but relatively high aphid counts on Tie-feng 8 in field plots contrasted with its previously reported resistance. Bhart-PI 165989 showed resistance in one of two laboratory tests, but it had relatively large aphid infestations in both years of field tests. Intermediate levels of soybean aphid occurred in field plots on lines previously shown to have strong (Sugao Zairai, PI 230977, and D75-10169) or moderate resistance to soybean aphid (G93-9223, Bragg, Braxton, and Tracy-M). Sugao Zairai also failed to have a significant proportion of resistant plants in two laboratory tests against aphids field-collected in 2008, but it was resistant in laboratory tests with aphids collected in 2002, 2005, and 2006. Overall, results showed that lines with Rag (i.e., Jackson) or Rag1 gene (i.e., Dowling) had low aphid numbers, whereas lines with Rag2 (i.e., Sugao Zairai, Sennari) had mixed results. Collectively, responses of soybean aphid populations in laboratory and field tests in 2008 resembled a virulence pattern reported previously for biotype 3 soybean aphids, but virulence in soybean aphid populations was variable and dynamic over years of the study. These results, coupled with previous reports of biotypes virulent to Rag1, suggest that deployment of lines with a single aphid-resistance gene is limited for soybean aphid management, and that deployment strategies relying on multiple resistance genes may be needed to effectively use plant resistance against soybean aphid.     

Genetic mapping revealed two loci for soybean aphid resistance in PI 567301B

The soybean aphid (Aphis glycines Matsumura) is the most damaging insect pest of soybean [Glycine max (L.) Merr.] in North America. New soybean aphid biotypes have been evolving quickly and at least three confirmed biotypes have been reported in USA. These biotypes are capable of defeating most known aphid resistant soybean genes indicating the need for identification of new genes. Plant Introduction (PI) 567301B was earlier identified to have antixenosis resistance against biotype 1 and 2 of the soybean aphid. Two hundred and three F_7:9 recombinant inbred lines (RILs) developed from a cross of soybean aphid susceptible cultivar Wyandot and resistant PI 567301B were used for mapping aphid resistance genes using the quantitative trait loci (QTL) mapping approach. A subset of 94 RILs and 516 polymorphic SNP makers were used to construct a genome-wide molecular linkage map. Two candidate QTL regions for aphid resistance were identified on this linkage map. Fine mapping of the QTL regions was conducted with SSR markers using all 203 RILs. A major gene on chromosome 13 was mapped near the previously identified Rag2 gene. However, an earlier study revealed that the detached leaves of PI 567301B had no resistance against the soybean aphids while the detached leaves of PI 243540 (source of Rag2) maintained aphid resistance. These results and the earlier finding that PI 243540 showed antibiosis resistance and PI 567301B showed antixenosis type resistance, indicating that the aphid resistances in the two PIs are not controlled by the same gene. Thus, we have mapped a new gene near the Rag2 locus for soybean aphid resistance that should be useful in breeding for new aphid-resistant soybean cultivars. Molecular markers closely linked to this gene are available for marker-assisted breeding. Also, the minor locus found on chromosome 8 represents the first reported soybean aphid-resistant locus on this chromosome.     

Parasitism of the soybean aphid, Aphis glycines by Binodoxys communis: the role of aphid defensive behaviour and parasitoid reproductive performance

The Asian parasitoid, Binodoxys communis (Gahan) (Hymenoptera: Braconidae), is a candidate for release against the exotic soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), in North America. In this study, we examined preferences by B. communis for the different developmental stages of A. glycines and investigated consequences of these preferences for parasitoid fitness. We also determined to what extent aphid defensive behaviours mediate such preferences. We found that B. communis readily attacks and successfully develops in the different A. glycines developmental stages. Binodoxys communis development time gradually increased with aphid developmental stage, and wasps took longest to develop in alates. An average (+/-SE) of 54.01+/-0.08% of parasitized A. glycines alatoid nymphs transformed into winged adult aphids prior to mummification. No-choice assays showed a higher proportion of successful attacks for immature apterous A. glycines nymphs compared to adults and alatoid nymphs. Also, choice trials indicated avoidance and lower attack and oviposition of adults and alatoid nymphs. The different aphid stages exhibited a range of defensive behaviours, including body raising, kicking and body rotation. These defenses were employed most effectively by larger aphids. We discuss implications for the potential establishment, spread and biological control efficacy of A. glycines by B. communis in the event that it is released in North America.     

Genetic linkage mapping of the soybean aphid resistance gene in PI 243540

The soybean aphid (Aphis glycines Matsumura) is a pest of soybean [Glycine max (L.) Merr.] in many soybean growing countries of the world, mainly in Asia and North America. A single dominant gene in PI 243540 confers resistance to the soybean aphid. The objectives of this study were to identify simple sequence repeat (SSR) markers closely linked to the gene in PI 243540 and to position the gene on the consensus soybean genetic map. One hundred eighty-four F(2) plants and their F(2:3) families from a cross between the susceptible cultivar Wyandot and PI 243540, and the two parental lines were screened with the Ohio biotype of soybean aphid using greenhouse choice tests. A SSR marker from each 10-cM section of the consensus soybean map was selected for bulked segregant analysis (BSA) to identify the tentative genomic location of the gene. The BSA technique was useful to localize the gene to a genomic region in soybean linkage group (LG) F. The entire F(2) population was then screened with polymorphic SSR markers from this genomic region and a linkage map with nine SSR markers flanking the gene was constructed. The aphid resistance gene was positioned in the interval between SSR markers Satt334 and Sct_033 on LG F. These SSR markers will be useful for marker assisted selection of this gene. The aphid resistance gene from PI 243540 mapped to a different linkage group than the only named soybean aphid resistance gene, Rag1, from 'Dowling'. Also, the responses of the two known biotypes of the soybean aphid to the gene from PI 243540 and Rag1 were different. Thus, the aphid resistance gene from PI 243540 was determined to be a new and independent gene that has been named Rag2.     

Resistance of Glycine species and various cultivated legumes to the soybean aphid (Homoptera: Aphididae)

The soybean aphid, Aphis glycines Matsumura, is a new pest of soybean, Glycine max (L.) Merr., in North America. It has become widespread on soybean in North America since it was first identified in the Midwest in 2000. Species of Rhamnus L. (buckthorn) are the primary hosts of A. glycines, and soybean is known as a secondary host. There is limited information about the secondary host range of A. glycines. Aphid colonization on various legume hosts was compared in choice experiments. Aphid colonization occurred on species in the genus Glycine Wild. No colonization occurred on Lablab purpureus (L.) Sweet, Lens culinaris Medik, Phaseolus vulgaris L., Pisum sativum L., or species of Vicia L. and Vigna Savi. Colonization was limited or aphids were transient on species of Medicago L., Phaseolus L., and Trifolium L. There were significant differences in aphid colonization among Medicago truncatula accessions with numbers ranging from 7 to 97 aphids per plant. Six Glycine soja Sieb. & Zucc. accessions were as resistant as G. max accessions to A. glycines; these may represent novel sources of A. glycines resistance not found in G. max. Antibiosis was found to play a large role in the expression of resistance in three of the G. soja accessions. Results of this study indicated that G. max and G. soja were the best secondary hosts of A. glycines; however, its secondary host range may include other leguminous species. Therefore, A. glycines did not seem to have a highly restricted monophagous secondary host range.     

Alfalfa living mulch advances biological control of soybean aphid

Despite evidence for biological control in North America, outbreaks of the invasive soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), continue to occur on soybean (Glycine max L. Merr.). Our objectives were to determine whether natural enemies delay aphid establishment and limit subsequent population growth and whether biological control can be improved by altering the within-field habitat. We hypothesized that a living mulch would increase the abundance of the aphidophagous community in soybean and suppress A. glycines establishment and population growth. We measured natural enemy and A. glycines abundance in soybean grown with and without an alfalfa (Medicago sativa L.) living mulch. Soybean grown with an alfalfa living mulch had 45% more natural enemies and experienced a delay in A. glycines establishment that resulted in lower peak populations. From our experiments, we concluded that the current natural enemy community in Iowa can delay A. glycines establishment, and an increase in aphidophagous predator abundance lowered the rate of A. glycines population growth preventing economic populations (i.e., below the current economic threshold) from occurring. Incorporation of a living mulch had an unexpected impact on A. glycines population growth, lowering the aphids' intrinsic rate of growth, thus providing a bottom-up suppression of A. glycines. We suggest future studies of living mulches or cover crops for A. glycines management should address both potential sources of suppression. Furthermore, our experience suggests that more consistent biological control of A. glycines may be possible with even partial resistance that slows but does not prevent reproduction.     

Impacts of thiamethoxam seed treatment and host plant resistance on the soybean aphid fungal pathogen, Pandora neoaphidis

Since the introduction of soybean aphid, Aphis glycines Matsumura, from Asia, insecticide use in soybean has increased substantially in the north central United States. Insecticide seed treatments and aphid resistant soybean varieties are management tactics that may reduce reliance on foliar applications of broad-spectrum insecticides. Exploring potential nontarget impacts of these technologies will be an important step in incorporating them into aphid management programs. We investigated impacts of thiamethoxam seed treatment and Rag1 aphid resistant soybean on a fungal pathogen of soybean aphid, Pandora neoaphidis (Remaudière & Hennebert) Humber, via open plot and cage studies. We found that although thiamethoxam seed treatment did significantly lower aphid pressure in open plots compared with an untreated control, this reduction in aphid density translated into nonsignificant decreases in fungal disease prevalence in aphids. Furthermore, when aphid densities were approximately equal in seed treated and untreated soybean, no impact on aphid fungal disease was observed. In open plots, Rag1 resistant soybean experienced lower aphid pressure and aphid disease prevalence compared with a nonresistant isoline. However, in cages when aphid densities were equivalent in both resistant and susceptible soybean, resistance had no impact on aphid disease prevalence. The addition of thiamethoxam seed treatment to resistant soybean yielded aphid densities and aphid disease prevalence similar to untreated, resistant soybean. These studies provide evidence that thiamethoxam seed treatments and Rag1 resistance can impact P. neoaphidis via decreased aphid densities; however, this impact is minimal, implying use of seed treatments and host plant resistance are compatible with P. neoaphidis.     

药剂包衣对苗期大豆蚜防治效果与安全性评价

【目的】大豆蚜Aphis glycines(Matsumura)是大豆上最重要的害虫之一。传统控制大豆蚜虫仍然以达到防治指标时大量喷洒化学药剂为主,危害人畜和环境安全。只有在大豆蚜发生初期进行有效防控,使其田间种群不能及时顺利的建立,从而实现无公害绿色防控。【方法】对筛选出的3种内吸式杀虫剂按不同浓度拌种包衣进行大田小区试验,调查分析包衣处理对大豆蚜、天敌以及大豆田其他害虫的影响和控制作用,同时对包衣处理后的大豆安全性、产量和品质进行评估。【结果】药剂拌种包衣处理能够显著压低苗期大豆蚜虫口基数,2014年对照区与处理区蚜量最高峰值比值最大达到448.15;同时对苗期大豆田间的双斑萤叶甲Monolepta hieroglyphica(Motschulsky)有很好的控制作用,处理区与对照区的受害株率差异极显著;并且保护了自然天敌种群;药剂拌种包衣处理在显著增产的同时还有效提升了大豆品质;经权威检测,收获后的籽粒在检出限内无药剂残留。【结论】药剂拌种包衣处理能有效控制苗期大豆蚜,不杀伤天敌,安全、无毒、无残留,而且增产显著,是比较理想的轻简无公害防控手段。     

Soybean cyst nematode effects on soybean aphid preference and performance in the laboratory

Herbivores on plants frequently interact via shared resources. Studies that have examined performance of herbivores in the presence of other herbivores, however, have often focused on above-ground feeding guilds and relatively less research has examined interactions between below- and above-ground consumers. We examine how soybean aphid, Aphis glycines (Matsumura) an above-ground phloem-feeding herbivore, interacts with a below-ground plant parasite, soybean cyst nematode, Heterodera glycines (Ichinohe) through their shared host plant, soybean (Glycine max L). Laboratory experiments evaluated the preference of alate (flight-capable) soybean aphids toward plants either infected with soybean cyst nematode or uninfected controls in a simple choice arena. Alate soybean aphids preferred uninfected soybean over soybean cyst nematode-infected plants: 48 h after the releases of alate aphids in the center of the arena, 67% more aphids were found on control soybean compared with nematode infected plants. No-choice feeding assays were also conducted using clip cages and apterous (flight-incapable) aphids to investigate effect of soybean cyst nematode infection of soybean on aphid performance. These studies had mixed results: in one set of experiments overall aphid population growth at 7 d was not statistically different between control and soybean cyst nematode-infected plants. A different experiment using a life-table analysis found that apterous aphids feeding on soybean cyst nematode-infected plants had significantly greater finite rate of increase (λ), intrinsic rate of increase (r(m)), and net reproductive rate (R(o)) compared with aphids reared on uninfected (control) soybean plants. We conclude that the below-ground herbivore, soybean cyst nematode, primarily influences soybean aphid behavior rather than performance.     

Soybean aphid and soybean cyst nematode interactions in the field and effects on soybean yield

How above- and belowground plant pests interact with each other and how these interactions affect productivity is a relatively understudied aspect of crop production. Soybean cyst nematode, Heterodera glycines Ichinohe, a root parasite of soybean, Glycine max (L.) Merr., is the most threatening pathogen in soybean production and soybean aphid, Aphis glycines Matsumura, an aboveground phloem-feeding insect that appeared in North America in 2000, is the key aboveground herbivore of soybean in the midwestern United States. Now, both soybean aphid and soybean cyst nematode co-occur in soybean-growing areas in the Upper Midwest. The objectives of this study were to examine aphid colonization patterns and population growth on soybean across a natural gradient of nematode density (range, approximately 900 and 27,000 eggs per 100 cm3 soil), and to investigate the effect of this pest complex on soybean productivity. Alate (winged) soybean aphid colonization of soybean was negatively correlated to soybean cyst nematode egg density (r = -0.363, P = 0.0095) at the end of July, at the onset of peak alate colonization. However, both a manipulative cage study and openly colonized plants showed that soybean cyst nematode density below ground was unrelated to variation in aphid population growth (r approximately -0.01). Based on regression analyses, soybean aphids and cyst nematodes had independent effects on soybean yield through effects on different yield components. High soybean cyst nematode density was associated with a decline in soybean yield (kg ha(-1)), whereas increasing soybean aphid density (both alate and apterous) significantly decreased seed weight (g 100 seeds(-1)).     

Antagonistic effects of soybean viruses on soybean aphid performance

Although there is long-standing recognition that pest complexes require different management approaches than individual pests, relatively little research has explored how pests interact. In particular, little is known of how herbivorous insects and plant pathogens interact when sharing the same host plant. The soybean aphid, Aphis glycines Mastumura, a recently introduced pest of soybean in the upper midwestern United States, and a complex of plant viruses vectored to soybean by insects have become a major concern for growers in the region. Given the abundance of soybean aphid and the increase in virus incidence in recent years, soybean aphids often use soybean infected by plant viral pathogens. We tested the hypothesis that soybean aphid performance is affected by virus infection of soybean plants. We conducted a series of field and laboratory experiments that examined how infection of soybeans with the common plant viruses, alfalfa mosaic, soybean mosaic, and bean pod mottle viruses, influenced soybean aphid performance. Soybean plants (in the field and laboratory) were hand inoculated with individual viruses, and aphids were allowed to colonize plants naturally in field experiments or added to the plants in clip-cages or within mesh bags in laboratory assays. In the field, aphid density on uninfected control soybean plants was nearly double that on infected plants. In laboratory assays, aphid population growth rates were on average 20% lower for aphids on virus infected compared with uninfected plants. Life table analyses showed that increased mortality on virus-infected plants likely explain differences in aphid population growth. Although there was some heterogeneity in the significance of treatment effects among different experiments, when independent experiments are taken together, there is on average an overall negative effect of these viruses on soybean aphids.     

Electrical penetration graph analysis of the feeding behavior of soybean aphids on soybean cultivars with antibiosis

The soybean aphid, Aphis glycine Matsumura (Hemiptera: Aphididae), is a major pest of soybean. In the current study, we used the Electrical Penetration Graph technique to study feeding behavior of soybean aphids on antibiotic-resistant soybean lines KS1621, KS1613, and KS1642, and a susceptible soybean line, KS4202. We observed that soybean aphids spent significantly shorter periods of time in the sieve element phase but slightly more times in nonprobing phases in all three resistant lines than in the susceptible control. Our study suggests that resistance factors exist in the phloem of the resistant soybean lines, and that these lines may contain antixenosis in addition to antibiosis.     

Soybean plant stage and population growth of soybean aphid

The soybean aphid, Aphis glycines Matsumura, is a newly invasive species of aphid in North America. Previous studies disagree as to whether soybean, Glycine max (L.) Merr., plant stage has an impact on aphid intrinsic rate of increase. Therefore, the growth rate of soybean aphids on soybean plants of different stages was examined at two different scales in the field. A planting date experiment was used to measure the population growth of soybean aphids on plants of different stages. Clip-cages were used to measure life history characteristics of individual aphids on plants of different stages. No differences were found in the population growth or dynamics of soybean aphids in the planting date experiment. The life history characteristics of individual aphids also showed no significant difference when feeding on different growth stages of soybean plants. The impact of these findings on soybean aphid management is discussed and the possible reasons why the results differ from previous estimates of the aphid growth-plant stage relationship are considered.     

Development of soybean aphid genomic SSR markers using next generation sequencing

Simple sequence repeats (SSRs) or microsatellites are very useful molecular markers, owing to their locus-specific codominant and multiallelic nature, high abundance in the genome, and high rates of transferability across species. The soybean aphid (Aphis glycines Matsumura) has become the most damaging insect pest of soybean (Glycine max (L.) Merr.) in North America, since it was first found in the Midwest of the United States in 2000. Biotypes of the soybean aphid capable of colonizing newly developed aphid-resistant soybean cultivars have been recently discovered. Genetic resources, including molecular markers, to study soybean aphids are severely lacking. Recently developed next generation sequencing platforms offer opportunities for high-throughput and inexpensive genome sequencing and rapid marker development. The objectives of this study were (i) to develop and characterize genomic SSR markers from soybean aphid genomic sequences generated by next generation sequencing technology and (ii) to evaluate the utility of the SSRs for genetic diversity or relationship analyses. In total 128 SSR primer pairs were designed from sequences generated by Illumina GAII from a reduced representation library of A. glycines. Nearly 94% (120) of the primer pairs amplified SSR alleles of expected size and 24 SSR loci were polymorphic among three aphid samples from three populations. The polymorphic SSRs were successfully used to differentiate among 24 soybean aphids from Ohio and South Dakota. Sequencing of PCR products of two SSR markers from four aphid samples revealed that the allelic polymorphism was due to variation in the SSR repeats among the aphids. These markers should be particularly useful for genetic differentiation among aphids collected from soybean fields at different localities and regions. These SSR markers provide the soybean aphid research community with the first set of PCR-based codominant markers developed from the genomic sequences of A. glycines.     

Population dynamics of soybean aphid and biotic mortality at the edge of its range

The soybean aphid, Aphis glycines Matsumura, was introduced to north central North America from Asia in 2000, and it has become a major pest of soybean, Glycine max (L.) Merr. Understanding how natural enemies impact aphid populations in the field is an important component in developing a comprehensive management plan. We examined the impact of naturally occurring predators in the field by using exclusion cages during July-August 2004 and 2005. Field cages of different mesh diameters were used to exclude different sizes of natural enemies from aphid-infested plots. Plots were surveyed twice weekly for A. glycines and natural enemies. Densities were recorded. Cage effects on mean temperature and soybean growth were found to be insignificant. Significant differences in aphid density were found between treatments in both years of the study (2004 and 2005); however, aphid densities between years were highly variable. Orius insidiosus (Say) was the most commonly occurring predator in the field. Other natural enemies were present in both years but not in high numbers. Parasitoids were present in both years, but their numbers did not suppress aphid densities. Treatment differences within years were related to the abundance of natural enemies. The large differences in aphid abundance between years were associated with the higher number of O. insidiosus found in the field in 2005 (416 total O. insidiosus) than in 2004 (149 total O. insidiosus). This study suggests that naturally occurring predators, primarily O. insidiosus, can have a large impact on A. glycines populations when predator populations are established before initial A. glycines colonization.     

Effect of three resistant soybean genotypes on the fecundity, mortality, and maturation of soybean aphid (Homoptera: Aphididae)

The fecundity, longevity, mortality, and maturation of the soybean aphid, Aphis glycines Matsumura (Homoptera: Aphididae), were characterized using three resistant soybean, Glycine max (L.) Merrill, genotypes ('Dowling', 'Jackson', and PI200538 'Sugao Zarai') and two susceptible genotypes ('Pana' and 'Loda'). Antibiosis in the resistant genotypes was demonstrated by a significant decrease in fecundity and longevity and increased mortality of A. glycines. Aphid fecundity, measured as number of offspring produced in the first 10 d by each viviparous aptera, was higher on Pana than on the resistant genotypes. Aphid longevity, the mean number of days a 1-d-old adult lived, was 7 d longer on Pana than on Dowling and Jackson. The mortality of both viviparous apterae and nymphs on resistant genotypes was significantly higher than on susceptible genotypes. A greater number of first instars survived to maturation stage (date of first reproduction) on susceptible plants than on resistant plants. None of the first instars placed on Dowling and PI200538 leaves survived to maturation. Observations of aphid behavior on leaves indicated that aphids departed from the leaves of resistant plants 8-24 h after being placed on them, whereas they remained indefinitely on leaves of susceptible cultivars and developed colonies. Reduced feeding due to ingestion of potentially toxic compounds in soybean may explain the possible mechanism of resistance to the soybean aphid.