Soybean Aphid Populations Research Articles

Effects of Grassland Habitat and Plant Nutrients on Soybean Aphid and Natural Enemy Populations

ABSTRACT The soybean aphid Aphis glycines Matsumura (Hemiptera: Aphididae) is an invasive economic pest of soybean (Glycine max [L.] Merrill) in the United States. Research has shown the endemic natural enemy community in the United States is capable of suppressing A. glycines below EILs, but this biological control is inconsistent, especially in simple agricultural landscapes. In the course of a 3-yr project (2006–2008) we sought to determine the affects of landscape composition, configuration, and plant nutrients (N, P, and K) on A. glycines and aphidophagous natural enemy abundance. Specifically, we tested whether nearby grasslands contribute to the abundance of natural enemies and the suppression of A. glycines. The study site was located around the Neal Smith National Wildlife Refuge, composed of >2,000 ha of reconstructed prairie, located in Jasper County, IA. We sampled A. glycines, natural enemies, and plant nutrients in 100 soybean fields while characterizing the landscape surrounding each field....

Lack of strong refuges allows top-down control of soybean aphid by generalist natural enemies

Refuges have been shown to be important mediators of predator–prey interactions, and in particular, have been proposed as a potential mechanism allowing herbivore populations to reach outbreak levels. However, very little research on the role of refuges has been conducted in systems dominated by generalist predators. We investigated the existence of refuges from predation for the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae) at multiple scales. This species invaded North America and in spite of previous studies demonstrating strong suppression by generalist natural enemies, its populations periodically cause significant economic losses. Using naturally occurring populations of soybean aphid and its natural enemies, we tested for the presence of A. glycines spatial and dynamic refuges at the within-field, single plant, and within-plant scale. At the within-field level, we found only weak and transient spatial patterns in aphid populations suggesting the lack of spatial refuges at this scale. Similarly, at the plant level we found no individual colonies that escaped predation and aphid suppression was 9- to 28-fold greater in comparison with caged controls regardless of initial aphid density. When high aphid populations were exposed to predation they were rapidly reduced to levels close to the average field density and showed reduced per capita growth rates, indicating an absence of dilution of predation risk at increased aphid density. Finally, we found a significant shift in the distribution of aphids to the lower portions of the plant in the presence of generalist predators, suggesting a partial refuge from predation at the within-plant scale. Overall, we found the naturally occurring community of generalist predators to exert strong top-down suppression of soybean aphid populations at multiple scales, and no evidence that the presence of refuges at the scales studied can lead to outbreak populations. The partial refuge from predation at the within-plant scale revealed in our study may have important consequences for the within-season population dynamics of A. glycines, since it may be associated with low plant quality tradeoffs, and therefore warrants further research.

Methyl Salicylate Attracts Natural Enemies and Reduces Populations of Soybean Aphids (Hemiptera: Aphididae) in Soybean Agroecosystems

Methyl salicylate, an herbivore-induced plant volatile, has been shown to attract natural enemies and affect herbivore behavior. In this study, methyl salicylate was examined for its attractiveness to natural enemies of the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), and for its direct effects on soybean aphid population growth rates. Methyl salicylate lures were deployed in plots within organic soybean [Glycine max (L.) Merr.] fields. Sticky card traps adjacent to and 1.5 m from the lure measured the relative abundance of natural enemies, and soybean aphid populations were monitored within treated and untreated plots. In addition, exclusion cage studies were conducted to determine methyl salicylate's effect on soybean aphid population growth rates in the absence of natural enemies. Significantly greater numbers of syrphid flies (Diptera: Syrphidae) and green lacewings (Neuroptera: Chrysopidae) were caught on traps adjacent to the methyl salicylate lure, but no differences in abundance were found at traps 1.5 m from the lure. Furthermore, abundance of soybean aphids was significantly lower in methyl salicylate-treated plots. In exclusion cage studies, soybean aphid numbers were significantly reduced on treated soybean plants when all plants were open to natural enemies. When plants were caged, however, soybean aphid numbers and population growth rates did not differ between treated and untreated plants suggesting no effect of methyl salicylate on soybean aphid reproduction and implicating the role of natural enemies in depressing aphid populations. Although aphid populations were reduced locally around methyl salicylate lures, larger scale studies are needed to assess the technology at the whole-field scale.

The Influence of Lasius neoniger (Hymenoptera: Formicidae) on Population Growth and Biomass of Aphis glycines (Hemiptera: Aphididae) in Soybeans

In the United States, the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), are often tended by the aphid-tending ant, Lasius neoniger Emery (Hymenoptera: Formicidae). In this study, we examined the effects of tending by ants on the density and biomass of soybean aphids on soybeans in Kentucky. We performed cage studies that limited access by ants and/or natural enemies. We used a split-plot design with natural enemy access as the main plot and ant attendance as the sub plot. We found that natural enemy access negatively affected aphid population density in the presence of tending ants, seen as a three- to four-fold increase in aphid density when natural enemies were excluded. In addition, we found that ant tending positively affected aphid biomass, both when natural enemies were given access to aphids or when natural enemies were excluded, seen by a two-fold increase in aphid biomass when ants tended aphids, both in the presence or absence of natural enemies. Biomass accumulation is seen as an important measurement for assessing aphid performance, and we argue that aphid-tending by ants can have an influence on natural field populations of soybean aphids. Agronomic practices that affect ant abundance in soybeans may influence the performance and hence pest outbreaks for this economically important pest.

Development and Parasitism byAphelinus certus(Hymenoptera: Aphelinidae), a Parasitoid ofAphis glycines(Hemiptera: Aphididae)

Since its introduction in 2000, the soybean aphid (Aphis glycines Matsumura) has been a serious pest of soybean in North America. Currently, insecticide application is the only recommended control method. However, a number of natural enemies have the potential to regulate soybean aphid populations. In 2007, Aphelinus certus Yasnosh, a soybean aphid parasitoid native to Asia, was found in commercial soybean fields in Ontario. This is the first record of this species in North America. To evaluate the potential biological control services provided by A. certus for soybean aphid management, temperature-dependent developmental parameters and functional response to soybean aphid were determined. A. certus is capable of completing its development between temperatures of 15.3 and 30.2°C. The lower thresholds of development for the egg-mummy and mummy-adult life stages were determined to be 9.1 and 11.6°C, respectively. The lethal temperature of development for the egg-mummy and mummy-adult life stages were 29.5 and 31.0°C, respectively. In this temperature range, A. certus did not exhibit temperature-dependent mortality; however, parasitism rate increased with temperature. A. certus exhibited a type II functional response to the soybean aphid.

Population Growth of Soybean Aphid,Aphis glycines, Under Varying Levels of Predator Exclusion

Although soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), has caused economic damage in several Midwestern states, growers in Missouri have experienced relatively minor damage. To evaluate whether existing predatory insect populations are capable of suppressing or preventing soybean aphid population growth or establishment in Missouri, a predator exclusion study was conducted to gauge the efficacy of predator populations. Three levels of predator exclusion were used; one that excluded all insects (small mesh), one that excluded insects larger than thrips (medium mesh), and one that excluded insects larger than Orius insidiosus (Say) (Hemiptera: Anthocoridae), a principal predator (large mesh). Along with manipulating predator exposure, timing of aphid arrival (infestation) was manipulated. Three infestation times were studied; vegetative (V5), beginning bloom (R1), and beginning pod set (R3). Timing of aphid and predator arrival in a soybean field may affect the soybean aphid's ability to establish and begin reproducing. Cages infested at V5 and with complete predator exclusion reached economic threshold within two weeks, while cages with predators reached economic threshold in four and a half weeks. Cages infested at R1 with complete predator exclusion reached economic threshold within five weeks; cages with predators reached economic threshold within six weeks. Cages infested at R3 never reached threshold (with or without predators). The predator population in Missouri seems robust, capable of depressing the growth of soybean aphid populations once established, and even preventing establishment when the aphid arrived late in the field.

A New Soybean Aphid (Hemiptera: Aphididae) Biotype Identified

Shortly after its arrival, the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), became established as the most important insect pest of soybean, Glycine max L. (Merr.), in the northern part of the North American soybean production region. Soybean resistance is an environmentally sustainable method to manage the pest and new soybean aphid resistant cultivars are beginning to be deployed into production. However, an earlier study identifying a soybean aphid biotype that could colonize plants with the Rag1 resistance gene has raised concerns about the durability of soybean aphid resistance genes. Choice and nonchoice tests conducted in this study characterized the colonization of a soybean aphid isolate, collected from the overwintering host Frangula alnus P. Mill in Springfield Fen, IN, on different aphid resistant soybean genotypes. This isolate readily colonized plants with the Rag2 resistance gene, distinguishing it from the two biotypes previously characterized and indicating that it represented a new biotype named biotype 3. The identification of soybean aphid biotypes that can overcome Rag1 and Rag2 resistance, even before soybean cultivars with the resistance genes have been deployed in production, suggests that there is high variability in virulence within soybean aphid populations present in North America. Such variability in virulence gives the pest a high potential to adapt to and reduce the effective life of resistance genes deployed in production. The search for new soybean aphid resistance genes must, therefore, continue, along with the development of alternative sustainable strategies to manage the pest.

Relationship of Soybean Aphid (Hemiptera: Aphididae) to Soybean Plant Nutrients, Landscape Structure, and Natural Enemies

In the north central United States, populations of the exotic soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), are highly variable across space, complicating effective aphid management. In this study we examined relationships of plant nutrients, landscape structure, and natural enemies with soybean aphid abundance across Iowa, Michigan, Minnesota, and Wisconsin, representing the range of conditions where soybean aphid outbreaks have occurred since its introduction. We sampled soybean aphid and its natural enemies, quantified vegetation land cover and measured soybean nutrients (potassium [K] and nitrogen [N]) in 26 soybean sites in 2005 and 2006. Multiple regression models found that aphid abundance was negatively associated with leaf K content in 2005, whereas it was negatively associated with habitat diversity (Simpson's index) and positively associated with leaf N content in 2006. These variables accounted for 25 and 27% of aphid variability in 2005 and 2006, respectively, suggesting that other sources of variability are also important. In addition, K content of soybean plants decreased with increasing prevalence of corn-soybean cropland in 2005, suggesting that landscapes that have a high intensification of agriculture (as indexed by increasing corn and soybean) are more likely to have higher aphid numbers. Soybean aphid natural enemies, 26 species of predators and parasitoids, was positively related to aphid abundance; however, enemy-to-aphid abundance ratios were inversely related to aphid density, suggesting that soybean aphids are able to escape control by resident natural enemies. Overall, soybean aphid abundance was most associated with soybean leaf chemistry and landscape heterogeneity. Agronomic options that can ameliorate K deficiency and maintaining heterogeneity in the landscape may reduce aphid risk.

Modeling Distribution and Abundance of Soybean Aphid in Soybean Fields Using Measurements From the Surrounding Landscape

Soybean aphid (Aphis glycines Matsumura) is a severe pest of soybean in central North America. Outbreaks of the aphid in Ontario are often spotty in distribution, with some geographical areas affected severely and others with few or no aphid populations occurring in soybean for the duration of the season. A. glycines spend summers on soybean and overwinter on buckthorn, a shrub that is widespread in southern Ontario and is commonly found in agricultural hedgerows and at the margins of woodlots. A. glycines likely use both short distance migratory flights from buckthorn and longer distance dispersal flights in the search for acceptable summer hosts. This study aims to model colonization of soybean fields by A. glycines engaged in early-season migration from overwintering hosts. Akaike's information criterion (AIC) was used to rank numerous competing linear and probit models using field parameters to predict aphid presence, colonization, and density. The variable that best modeled aphid density in soybean fields in the early season was the ratio of buckthorn density to field area, although dramatic differences in relationships between the parameters were observed between study years. This study has important applications in predicting areas that are at elevated risk of developing economically damaging populations of soybean aphid and which may act as sources for further infestation.

Effects of Foliar Fungicide and Insecticide Applications on Soybean in Ohio

Mid-season applications of Quadris with and without Warrior insecticide were evaluated in 2004 and 2005, and Headline, Folicur, Domark, or Headline plus Folicur were evaluated in 2006, 2007, and 2008 in 37 farm scale studies in Ohio. Producer cooperators selected fields, applied treatments at growth stage R3 and harvested fields. Each trial was rated at growth stage R5 or R6 for incidence and severity of brown spot and frogeye leaf spot. Aphids and foliar disease incidence was low in 2004. Soybean aphids were counted at stage R5 in 2005. Brown spot severity was reduced significantly by Headline in seven of the 13 locations. In the absence of soybean aphid, a single application of fungicide(s) increased yield significantly in only six of the 28 locations, of which only three had yield increases greater than 4.2 bu/acre. Populations of soybean aphids were high during 2005 at nine locations, and an insecticide application increased yield significantly at eight locations. These data indicated that foliar diseases and aphids contribute to yield loss in soybeans. However, more studies are required to determine action thresholds for brown spot and frogeye leaf spot. Accepted for publication 14 October 2010. Published 22 January 2010.

Comment on “Soybean Aphid Population Dynamics, Soybean Yield Loss, and Development of Stage‐Specific Economic Injury Levels” by M. A. Catangui, E. A. Beckendorf, and W. E. Riedell, Agron. J. 101:1080–1092 (2009)

Comment on “Soybean Aphid Population Dynamics, Soybean Yield Loss, and Development of Stage‐Specific Economic Injury Levels” by M. A. Catangui, E. A. Beckendorf, and W. E. Riedell, Agron. J. 101:1080–1092 (2009)

Non-target impacts of soybean rust fungicides on the fungal entomopathogens of soybean aphid

Soybean aphid, Aphis glycines, has caused serious economic damage to soybean across the North Central US since its introduction to North America in 2000. The management of another invasive soybean pest, Asian soybean rust, Phakopsora pachyrhizi, using foliar fungicide applications has the potential to impact soybean aphid populations by suppressing beneficial fungal entomopathogens. In 2005 and 2006, we applied recommended soybean rust fungicide treatments, consisting of strobilurin and triazole fungicides, to small soybean plots in two locations to assess if such applications might suppress aphid fungal epizootics. In Lamberton, MN, in 2005, during the epizootic, fungicide-treated plots averaged 2.0 ± 0.7% (mean ± SE) disease prevalence while untreated plots averaged 14.2 ± 5.6%. In 2007, we applied strobilurin and strobilurin–triazole mix fungicides to single-plant microplots either before or after release of Pandora neoaphidis, the most commonly observed aphid pathogen in 2005 and 2006. Treatments that contained a mixture of two active ingredients significantly lowered peak and cumulative aphid disease prevalence in both early and late reproductive stage soybeans indicating that fungicide mixtures used to manage soybean rust can negatively impact an aphid-specific fungal pathogen. However, no consistent soybean aphid population response was observed in these studies of low levels of aphid fungal infection.

Soybean Aphid Population Dynamics, Soybean Yield Loss, and Development of Stage‐Specific Economic Injury Levels

Stage‐specific economic injury levels (EILs) form the basis of integrated pest management for soybean aphid (Aphis glycines Matsumura) in soybean [Glycine max (L.) Merr.]. Experimental objectives were to develop a procedure for calculating EILs of the soybean aphid specific to the R2 (full bloom), R4 (full pod), and R5 (beginning seed) soybean development stages using the law of the diminishing increment regression model. Soybean aphid population growth over time appeared to follow the symmetrical bell‐shaped and logistic growth curve models. Peak soybean aphid population levels and rates of increase occurred at the R5 development stage and then declined sharply thereafter. Highest peak soybean aphid populations were 21,626 aphids plant−1 for infestations starting at V5, and 6446 aphids plant−1 for infestations starting at R2. Highest maximum aphid‐days plant−1 recorded were 537,217 for V5‐introduced soybean aphids and 148,609 aphid‐days plant−1 for R2‐introduced soybean aphids. On average, the calculated maximum possible yield loss was 75% for soybean aphid infestations starting at the V5 (five node) stage and 48% for soybean aphid infestations starting at the R2 stage. Interrelationships among the current or predicted market value of soybean, cost of soybean aphid control, and the yield potential of the soybean field were considered in the calculations of the stage‐specific EILs. Practical examples for calculating stage‐specific EILs are presented. Economic injury levels were calculated both as soybean aphids plant−1 and soybean aphid‐days plant−1. Use of these stage‐specific EILs may enable growers to manage soybean aphids more accurately.

Predation byCoccinella septempunctataandHarmonia axyridis(Coleoptera: Coccinellidae) onAphis glycines(Homoptera: Aphididae)

Coccinella septempunctata L. and Harmonia axyridis Pallas are key natural enemies of soybean aphid, Aphis glycines Matsumura, in North America. Third instars, adult females, and adult males of both C. septempunctata and H. axyridis exhibited a type II functional response for predation toward adult soybean aphids at 26 +/- 1 degrees C. In C. septempunctata, the functional response curve of adult males differed from those of third instars and adult females, but there was no difference between third instars and adult females. In H. axyridis, the functional response curves of larvae, adult females, and adult males all differed significantly. Third instars and adult females consumed significantly more soybean aphids than did adult males at prey densities of 150 and 180 aphids per arena for C. septempunctata and at prey densities of 60, 90, 120, 150, and 180 aphids per arena for H. axyridis. The theoretical maximum daily predation rate of adult aphids by C. septempunctata was predicted to be 204 per third instar, 277 per adult female, and 166 per adult male, and 244, 156, and 73, respectively, for H. axyridis. Third instars and adult females of both species consumed significantly more aphids than did adult males on soybean plants with the recommended action threshold of 250 soybean aphids per plant. Both C. septempunctata and H. axyridis have high predation capacities and are important in suppressing soybean aphid populations.

Efficacy of Neonicotinoid Seed Treatments to Reduce Soybean Aphid Populations Under Field and Controlled Conditions in Nebraska

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is native to Asia and was recently (2000) detected in North America. Since then, it has become a significant threat to U.S. soybean production. Although neonicotinoid insecticide seed treatments, imidacloprid and thiamethoxam, have been suggested as a method of control, the season-long efficacy is still uncertain. Therefore, the use of imidacloprid and thiamethoxam seed treatments to control soybean aphid in Nebraska were examined. Soybean aphid populations were monitored weekly in an irrigated field study planted during the later half of the typical Nebraska planting window during 2005 and 2006. Imidacloprid and thiamethoxam were quantified by leaves through time, and leaf specific bioassays were conducted. In 2005, aphid populations were very low; however, in 2006 aphid numbers were significantly higher than 2005, reaching approximately 1,200 aphids per plant in the untreated plots. Aphid injury significantly reduced yield and individual seed size in 2006. Imidacloprid significantly reduced aphid densities in 2006 but not below the economic threshold. In 2006, thiamethoxam held aphid densities below the economic threshold. Leaf specific bioassays and leaf specific imidacloprid and thiamethoxam quantification indicated that thiamethoxam was present in the plant at higher concentrations and for a longer period.

Two Species of Symbiotic Bacteria Present in the Soybean Aphid (Hemiptera: Aphididae)

Aphids, which feed solely on plant phloem sap, have developed symbiotic associations with bacteria that provide them with the amino acids that are lacking in phloem. Three soybean aphid (Aphis glycines Mat samura) populations were screened for the presence of Buchnera aphidicola and three common species of secondary aphid symbionts (Serratia symbiotica, Hamiltonella defensa, and Regiella insecticola). Diagnostic polymerase chain reaction and subsequent DNA sequencing showed the presence of two species of symbiotic bacteria present in all three soybean aphid populations tested: B. aphidicola and Arsenophonus sp. Although Buchnera is commonly found in aphids, Arsenophonus is most commonly found in whiteflies (Hemiptera: Aleyrodidae), making the soybean aphid unique among aphids that have been tested for the presence of Arsenophonus.

Temporal relationships between the generalist predator, Orius insidiosus, and its two major prey in soybean

The generalist predator, Orius insidiosus (Say) is an important early-season predator of the soybean aphid, Aphis glycines Matsumura, a newly invasive pest of major concern in soybean crop management. We conducted a 3 year, multiple field study to characterize the dynamic relationships between the predator, the pest, and alternative prey in soybean. Using field sampling data, we showed that thrips were the only alternative prey to be well-established in fields prior to O. insidiosus arrival and were likely to promote predator colonization of soybean fields prior to the arrival of soybean aphid. The predator displayed a reproductive numerical response to thrips in one of the 3 years and a primarily aggregative response in another year. The predator did not respond numerically to soybean aphid in the majority of fields. Experimental manipulations of thrips populations in field plots temporarily reduced thrips densities but had a minimal effect on O. insidiosus densities, suggesting that the predator is resilient against temporary reductions in a major resource. In the 2 years O. insidiosus populations were well-established in fields prior to soybean aphid arrival, soybean aphid remained at low levels throughout the season. In the year soybean aphid arrived early with respect to the growing season and before O. insidiosus populations were established, soybean aphid reached outbreak levels in all fields. Future research efforts on the factors determining soybean aphid population dynamics need to address the relative importance of early-season soybean aphid colonization and generalist predator population dynamics on the potential for soybean aphid population outbreaks.

Effect of Ant Attendance by <I>Monomorium minimum</I> (Buckley) (Hymenoptera: Formicidae) on Predation and Parasitism of the Soybean Aphid <I>Aphis glycines</I> Matsumura (Hemiptera: Aphididae)

Ant attendance is known to affect the population dynamics of aphids and may increase or decrease aphid populations through stimulation, predation, or protection. In this study, we performed a series of laboratory experiments to examine the effects of ant attendance on populations of the soybean aphid Aphis glycines. Aphid colonies were exposed to the predators Harmonia axyridis (Coccinellidae) and Orius insidiosus (Anthocoridae) and a parasitoid Aphidius colemani (Aphidiidae) in the presence and absence of attending Monomorium minimum (Formicidae). We also tested for direct effects of ant attendance in the absence of natural enemies. Ants attending soybean aphid populations were observed harassing or killing O. insidiosus and H. axyridis. Attendance interfered with both predator species, resulting in reduced predation and an increase in aphid numbers up to 10-fold in the presence of ants. Ants were not observed directly interfering with the parasitoid A. colemani, but the number of parasitized aphids was higher in aphid colonies that were left unattended by ants.

Potential for biotic interference of a classical biological control agent of the soybean aphid

The degree to which resident biota can inhibit the ability of an introduced biological control agent to establish and be effective is termed biotic interference. Studying biotic interference prior to a release using the actual agent is logistically difficult, however, due to quarantine restrictions. An alternative solution is to study biotic interference against a surrogate species in the intended range of introduction, with the expectation that biotic interference against the actual agent will be similar. This study assessed how biotic interference, mostly by generalist predators, may affect establishment of classical biological control agents of the soybean aphid, Aphis glycines Matsumura, in North America. The parasitoid Aphidius colemani Viereck was used as a surrogate for Asian aphidiine braconids such as Binodoxys communis (Gahan). We conducted a factorial field experiment that measured the effect of releasing A. colemani and of excluding resident natural enemies using field cages on soybean aphid populations. We also conducted molecular gut-contents analyses on predators collected in release plots to determine which species fed upon A. colemani. Releasing A. colemani in open field plots increased soybean aphid control beyond that observed in open field plots alone, despite indications that intraguild predation of A. colemani occurred. Thus, biotic interference was not sufficient to eliminate the contribution of A. colemani on soybean aphid suppression during the course of our experiment. Molecular gut-contents analysis revealed that at least two predators, Harmonia axyridis (Pallas) and Chrysoperla carnea Stephens, engaged in intraguild predation against A. colemani. The prolonged effect of intraguild predation on parasitoid establishment remains to be determined.

Seasonal Abundance of Resident Parasitoids and Predatory Flies and Corresponding Soybean Aphid Densities, with Comments on Classical Biological Control of Soybean Aphid in the Midwest

Seasonal abundance of resident parasitoids and predatory flies, and corresponding soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), densities were assessed in soybean fields from 2003 to 2006 at two locations in lower Michigan. Six parasitoid and nine predatory fly species were detected in 4 yr by using potted plants infested with soybean aphid placed in soybean fields. The parasitoid Lysiphlebus testaceipes Cresson (Hymenoptera: Braconidae) and the predatory flies Aphidoletes aphidimyza Rondani (Diptera: Cecidomyiidae), and Allograpta obliqua Say (Diptera: Syrphidae) were most numerous. Generally, L. testaceipes was more abundant late in the soybean growing season, but it also occurred during soybean vegetative growth; A. obliqua was more abundant during vegetative growth; and A. aphidimyza was common throughout the season. Soybean plants were visually inspected to estimate densities of soybean aphid, mummified aphids, and immature predatory flies. From 2003 to 2006, parasitism rates were inversely correlated with aphid density: percentage of parasitism was always very low (< or = 0.1%) at high aphid densities (> 100 aphids per plant), and higher parasitism, up to 17%, was observed at very low aphid densities (< 1 aphid per plant). Populations of immature predatory flies, particularly A. aphidimyza, generally increased in soybean fields with increasing soybean aphid populations, but aphids always outnumbered immature flies by 100-21,000-fold when flies were detected. Rearing field-collected aphid in 2006 substantiated that parasitism varied widely, with parasitism in most cases < 10%. Based on findings of low parasitism and predation, positive response to changing aphid densities by predatory flies but not parasitoids, early season abundance primarily of predatory flies, and past findings on these taxa's diversity and abundance, we discuss the potential use of exotic parasitoids and predatory flies to enhance soybean aphid biological control.