Soybean Aphid Populations Research Articles

Effects of the termination of LC30 imidacloprid stress on the multigeneration adaptive strategies of Aphis glycines population.

Aphis glycines Matsumura (Hemiptera: Aphididae) is a major soybean pest that often poses a serious threat to soybean production. Imidacloprid is one of the commonly used insecticides to control the soybean aphid. To investigate the effect of termination of imidacloprid stress on the adaptive strategies of soybean aphid populations, we studied the growth, development, and related metabolism changes when the stress was terminated after 24 generations of imidacloprid stress on A. glycines. The results show that the A. glycines population accelerated its recovery and expanded its population size across generations. The longevity of the adults of the recovering population in the F12, F18, and F24 generations, respectively, was 1.11, 1.15, and 1.11 times longer than the control, while the fecundity was 10.38%, 11.74%, and 11.61% higher than that of the control. The net reproductive rate (R 0) of the recovering population was always significantly higher than that of the control in the F1 to F24 generations. In addition, metabolisms related to the regulation of cell proliferation and oocyte meiosis were significantly upregulated in the recovering population. Even when the imidacloprid pressure disappeared, intergenerational stimuli still affected the adaptive strategies of soybean aphid populations. This effect was manifested as inhibiting the growth and development of the soybean aphid in the early generations and improving the fecundity of the soybean aphid in the later generations. Adaptive soybean aphid populations would surge in the absence of imidacloprid pressure. This study provides an important reference for exploring the adaptability of the A. glycines population under termination of stress from low lethal concentrations of imidacloprid across generations. It also provides important data for monitoring the population dynamics of A. glycines in the field and analyzing the degree of pharmacodynamic stress.

Crop diversity and edge density benefit pest suppression through bottom-up and top-down processes, respectively

Agricultural intensification has resulted in the homogenization of agricultural landscapes, leading to the loss of crop and habitat diversity, and a higher rate of chemical inputs in the landscape, negatively impacting natural enemy communities and their ability to provide pest control services. Landscape heterogeneity is thought to benefit predator populations (increasing landscape complementation) and lower pest colonization (decreasing resource concentration); however, few studies have determined the relative importance of these two processes in pest suppression. Here, we tease apart direct and indirect impacts of landscape heterogeneity on experimental populations of soybean aphid excluded from and open to predation, in 23 soybean fields in Manitoba, in 2017 and 2018. Field plant counts and sweep net samples were conducted to determine aphid and predator abundances, respectively, and Malaise traps were used to quantify predator movement into soybean. Landscape effects differed between a soybean aphid outbreak year (2017) versus a non-outbreak year (2018). We found that crop diversity at a 1.5 km radius had negative impacts on aphid colonization during the outbreak year, resulting in greater aphid suppression but no impacts on predator movement. Edge density at a 1.5 km radius increased aphid suppression during the low aphid year. Predator movement and abundance were driven by aphid colonization in soybean, and syrphid and coccinellid abundance were 33-fold and 15-fold more abundant in the high compared to the low aphid year. Thus, our study revealed complementary effects of landscape heterogeneity on aphid suppression: a reduction of resource concentration associated with increased crop diversity and resulting in bottom-up effects during an outbreak, and an increase in landscape complementation associated with increased edge density and top-down control in a low aphid year. Increasing crop diversity seems a promising strategy to mitigate aphid outbreaks without compromising food production.

Model fitting of the seasonal population dynamics of the soybean aphid, Aphis glycines Matsumura, in the field

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is one of the greatest threats to soybean production, and both trend analysis and periodic analysis of its population dynamics are important for integrated pest management (IPM). Based on systematically investigating soybean aphid populations in the field from 2018 to 2020, this study adopted the inverse logistic model for the first time, and combined it with the classical logistic model to describe the changes in seasonal population abundance from colonization to extinction in the field. Then, the increasing and decreasing phases of the population fluctuation were divided by calculating the inflection points of the models, which exhibited distinct seasonal trends of the soybean aphid populations in each year. In addition, multifactor logistic models were then established for the first time, in which the abundance of soybean aphids in the field changed with time and relevant environmental conditions. This model enabled the prediction of instantaneous aphid abundance at a given time based on relevant meteorological data. Taken as a whole, the successful approaches implemented in this study could be used to build a theoretical framework for practical IPM strategies for controlling soybean aphids.

Soybean Aphid (Hemiptera: Aphididae) Feeding Behavior is Largely Unchanged by Soybean Mosaic Virus but Significantly Altered by the Beetle-Transmitted Bean Pod Mottle Virus.

The soybean aphid (Aphis glycines Matsumura) is an economically important invasive pest of soybean. In addition to damage caused by soybean aphid feeding on the phloem sap, this insect also transmits many plant viruses, including soybean mosaic virus (SMV). Previous work has shown that plant viruses can change plant host phenotypes to alter the behavior of their insect vectors to promote virus spread, known as the vector manipulation hypothesis. In this study, we used electropenetography (EPG) to examine the effects of two plant viruses on soybean aphid feeding behavior: SMV, which is transmitted by many aphid species including the soybean aphid, and bean pod mottle virus (BPMV), which is transmitted by chrysomelid and some coccinellid beetles but not aphids. These two viruses often co-occur in soybean production and can act synergistically. Surprisingly, our results showed little to no effect of SMV on soybean aphid feeding behaviors measured by EPG, but profound differences were observed in aphids feeding on BPMV-infected plants. Aphids took longer to find the vascular bundle of BPMV-infected plants, and once found, spent more time entering and conditioning the phloem than ingesting phloem sap. Interestingly, these observed alterations are similar to those of aphids feeding on insect-resistant soybean plants. The cause of these changes in feeding behavior is not known, and how they impact virus transmission and soybean aphid populations in the field will require further study.

Soybean aphid infestation and crop yield in relation to cultivar, foliar insecticide, and insecticidal seed treatment in South Dakota

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), remains an important pest of soybean, Glycine max (L.) Merr., in areas of the Midwestern United States and southern Canada. Options for managing soybean aphid are currently limited to insecticides and aphid-resistant varieties. Farmers commonly employ a prophylactic treatment of soybean seed with a neonicotinoid insecticide such as thiamethoxam and often a remedial, late-season application of foliar insecticides such as the pyrethroid, λ-cyhalothrin. Environmental concerns about detrimental side effects of neonicotinoids and the substantiation of pyrethroid resistance in field populations of soybean aphid have strengthened the case for planting aphid-resistant soybean cultivars. We conducted a three-year field study in eastern South Dakota, United States, that evaluated commercially-available active substances and enhanced plant materials for soybean aphid management that included a thiamethoxam seed treatment, spray application of λ-cyhalothrin, and an aphid-resistant soybean cultivar containing a pyramid of the Rag1 and Rag2 aphid-resistance genes. Soybean aphid infestations had sustained, economically injurious levels in 2015, but not the other two years. The three management tactics independently reduced soybean aphid population densities. Thiamethoxam and the resistant cultivar suppressed cumulative aphid-days (CAD) in all three years, whereas the λ-cyhalothrin spray decreased CAD in 2015 and 2017. By far, the resistant cultivar had the greatest impact on soybean aphid populations, reducing CAD by 28- to 150-fold year−1, with mean numbers of aphids much less than 100 soybean aphids plant−1. Thiamethoxam seed treatment reduced CAD by 1.7- to 3.5-fold year−1, and λ-cyhalothrin reduced it from 2.0- to 5.6-fold year−1. Soybean yield was not affected by treatments in 2015. Yield was greater for the susceptible than for the resistant cultivar in 2016, and yield was greater for plots without foliar insecticide in 2017. Implications for the use of these management tactics against soybean aphid are discussed.

Parasitoid-mediated indirect interactions between unsuitable and suitable hosts generate apparent predation in microcosm and modeling studies.

Parasitoids used as biological control agents often parasitize more than a single host species and these hosts tend to vary in suitability for offspring development. The population dynamics of parasitoids and hosts may be altered by these interactions, with outcomes dependent on the levels of suitability and acceptance of both host species. Parasitism of individuals of an unsuitable host species may indirectly increase populations of a suitable host species if eggs laid into unsuitable hosts do not develop into adult parasitoids. In this case, the unsuitable host is acting as an egg sink for parasitoids and this can reduce parasitism of suitable hosts under conditions of egg limitation. We studied parasitoid‐mediated indirect interactions between two aphid hosts, Aphis glycines (the soybean aphid) and A. nerii (the milkweed, or oleander aphid), sharing the parasitoid Aphelinus certus. While both of these aphid species are accepted by A. certus, soybean aphid is a much more suitable host than milkweed aphid is. We observed a drastic reduction of parasitoid offspring production (45%) on the suitable host in the presence of the unsuitable host in microcosm assays. Aphelinus certus females laid eggs into the unsuitable hosts (Aphis nerii) in the presence of the suitable host leading to egg and/or time limitation and reduced fitness. The impact of these interactions on the equilibrium population sizes of the three interacting species was analyzed using a consumer–resource modeling approach. Both the results from the laboratory experiment and the modeling approaches identified apparent predation between soybean aphid and milkweed aphid, in which milkweed aphid acts as a sink for parasitoid eggs leading to an increase in the soybean aphid population. The presence of soybean aphids had the opposite effect on milkweed aphid populations as it supported increases in parasitoid abundance and thus reduced the fitness and abundance of this aphid species.

Aphid resistance is the future for soybean production, and has been since 2004: efforts towards a wider use of host plant resistance in soybean.

The soybean aphid (Aphis glycines) is an important pest of soybeans in the Midwestern US. The first aphid resistance genes were identified in the early 21st century and resistant varieties have been commercially available for 10 years, but have been very underutilized. Major seed companies have avoided commercializing aphid resistant soybean varieties for conventional farmers (i.e., not organic), in part because of the discovery of virulent biotypes in North America. The emergence of soybean aphid populations resistant to insecticides creates a greater incentive for the use of host plant resistance. New research on aphid genetics and markers, plant gene expression and in-plant refuges, suggest important avenues for insect resistance management (IRM) which may encourage more widescale commercialization of this valuable pest management tool.

A field-based assessment of the parasitoid Aphelinus certus as a biological control agent of soybean aphid in North America

Damaging outbreaks of soybean aphid continue to occur in North America despite the valuable biological control services provided by resident natural enemies. The adventive parasitoid Aphelinus certus Yasnosh (Hymenoptera: Aphelinidae) has recently established in North America and has been hypothesized to be a key component of the resident natural enemy community, but there have been few studies evaluating the efficacy of this parasitoid in suppressing soybean aphid populations. We used an exclusion cage study to quantify the effect of A. certus on soybean aphid population growth at four sites spanning western and east-central Minnesota from 2017 to 2019. There were minimal differences in soybean aphid population growth between experimental treatments that excluded natural enemies and control treatments, suggesting that parasitism of soybean aphid by A. certus did not have a strong impact on soybean aphid population growth during this study. Because, for example, A. certus larvae can reduce host reproduction prior to mortality (resulting in underestimates of effects in short-term studies), our results reflect the challenges of using exclusion cages to assess the effects of individual natural enemy species, especially those with complex life cycles.

Cultivar mixtures of soybeans have inconsistent effects on herbivore and natural-enemy populations

Mounting evidence suggests that increasing within-species (or genotypic) crop diversity by planting mixtures of cultivars is a promising tactic for managing herbivorous pests. The soybean aphid (Aphis glycines Matsumura) is an invasive species that has forced growers to increase use of insecticides across the Midwestern United States. There is a clear need for alternative and more sustainable management methods; therefore, the soybean-soybean aphid system provides a useful model for testing the utility of cultivar mixtures. We conducted a two-year field experiment with soybeans and soybean aphid to measure top-down and bottom-up ecological consequences of increasing plant genotypic diversity in crop fields. Soybean aphid populations can be suppressed by predators, but can also be influenced by aphid-tending ants. Specifically, we evaluated if cultivar mixtures influence soybean aphid populations, the associated arthropod community, predation, and crop yield. We compared monocultures of six soybean cultivars to all six possible five-cultivar mixtures using 82.8 m2 plots. We also assessed if cultivar mixtures influence populations of another herbivore, potato leafhopper. Separately for day and nighttime, we evaluated predator and ant populations, and measured predation. We also assessed soybean yield. Over the course of the study, aphid populations remained low, and cultivar mixtures did not alter season-long aphid pressure or yield, but did produce intermediate yield and aphid populations compared to monocultures. Crop genotypic diversity increased leafhopper populations, likely because of non-additive interactions involving one highly susceptible or attractive cultivar. Diversity did not influence aphid-tending ants. In both years, predators helped suppress aphid populations, and predator abundance and predation was influenced by genotypic diversity during specific parts of the season and for some sampling times, but results were mixed and inconsistent. Of the most abundant taxa, genotypic diversity increased abundance of Harmonia axyridis in 2010 during the day, but not the night. Overall, results from this study demonstrated that under low pest pressure, cultivar mixtures do not produce clear benefits. Mixtures did appear to produce intermediate aphid populations and yield, and also influenced other herbivore species, which could influence natural-enemy control of soybean aphids. Effects on herbivores and predators could become more important under high pest pressure, although this remains to be determined. Our study also provides some evidence that plant genotypic diversity in crop fields can influence higher trophic levels, paralleling work in natural systems that has demonstrated system-wide consequences of diversity.

Implementation of a Diagnostic-Concentration Bioassay for Detection of Susceptibility to Pyrethroids in Soybean Aphid (Hemiptera: Aphididae).

Soybean aphid, Aphis glycines Matsumura, remains the most economically damaging arthropod pest of soybean in the midwestern United States and southern Canada. Foliar applications of a limited number of insecticide modes of action have been the primary management tactic, and pyrethroid resistance was documented recently with full concentration-response leaf-dip and glass-vial bioassays. Full concentration-response bioassays can be cumbersome, and a more efficient assessment tool was needed. In this study, we implemented a diagnostic-concentration glass-vial bioassay using bifenthrin and λ-cyhalothrin. Bioassays were conducted with field-collected soybean aphid populations to assess the geographic extent and severity of resistance to pyrethroids. In 2017, 10 of 18 and 11 of 21 field populations tested with bifenthrin and λ-cyhalothrin, respectively, had mean proportion mortalities less than the susceptible laboratory population. In 2018, 17 of 23 and 13 of 23 field populations tested with bifenthrin and λ-cyhalothrin, respectively, had mean proportion mortalities less than the susceptible laboratory population. Populations collected after reported field failures of a pyrethroid insecticide generally had mean proportion mortalities less than the susceptible laboratory population. In both years, there was a strong correlation between chemistries, which suggests cross-resistance between these insecticides. The diagnostic-concentration glass-vial bioassays reported here will provide the foundation for an insecticide resistance monitoring program with the ability to determine practical levels and geographic extent of insecticide resistance.

Detection of Stress Induced by Soybean Aphid (Hemiptera: Aphididae) Using Multispectral Imagery from Unmanned Aerial Vehicles.

Soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is a common pest of soybean, Glycine max (L.) Merrill (Fabales: Fabaceae), in North America requiring frequent scouting as part of an integrated pest management plan. Current scouting methods are time consuming and provide incomplete coverage of soybean. Unmanned aerial vehicles (UAVs) are capable of collecting high-resolution imagery that offer more detailed coverage in agricultural fields than traditional scouting methods. Recently, it was documented that changes to the spectral reflectance of soybean canopies caused by aphid-induced stress could be detected from ground-based sensors; however, it remained unknown whether these changes could also be detected from UAV-based sensors. Small-plot trials were conducted in 2017 and 2018 where cages were used to manipulate aphid populations. Additional open-field trials were conducted in 2018 where insecticides were used to create a gradient of aphid pressure. Whole-plant soybean aphid densities were recorded along with UAV-based multispectral imagery. Simple linear regressions were used to determine whether UAV-based multispectral reflectance was associated with aphid populations. Our findings indicate that near-infrared reflectance decreased with increasing soybean aphid populations in caged trials when cumulative aphid days surpassed the economic injury level, and in open-field trials when soybean aphid populations were above the economic threshold. These findings provide the first documentation of soybean aphid-induced stress being detected from UAV-based multispectral imagery and advance the use of UAVs for remote scouting of soybean aphid and other field crop pests.

Soybean aphid (Hemiptera: Aphididae) response to lambda-cyhalothrin varies with its virulence status to aphid-resistant soybean.

Soybean aphid, Aphis glycines, is an invasive insect in North America, considered one of the most important pests of soybean. Their management relies heavily on foliar insecticides, but there is growing effort to expand these tools to include aphid-resistant varieties. We explored if the LC50 and LC25 of lambda-cyhalothrin varied between virulent (resistant to Aphis glycines (Rag) soybeans) and avirulent (susceptible to Rag-genes soybeans) populations of soybean aphid with a leaf-dip bioassay. We also investigated the response to the LC25 of lambda-cyhalothrin on adults (F0) and their progeny (F1) for both avirulent and virulent soybean aphid. The LC50 of the virulent aphid population was significantly higher compared with the LC50 of the avirulent population. The LC25 significantly reduced fecundity of the F0 generation of avirulent soybean aphid, but no significant effect was observed for virulent aphids. In addition, the LC25 significantly shortened the adult pre-oviposition period (APOP) and lengthened the total pre-oviposition period (TPOP) of avirulent aphids, while the mean generation time (T) was significantly increased. For the virulent aphid, sublethal exposure significantly lengthened development time of first and third instars, TPOP, and adult longevity. In addition, all demographic parameters of virulent soybean aphid were significantly affected when they were exposed to the LC25 of lambda-cyhalothrin. Our results demonstrate lambda-cyhalothrin is less toxic to virulent aphids and exposure to the LC25 can trigger hormesis, which may have implications for the long-term management of this pest with this insecticide as well as with aphid-resistant varieties of soybean. © 2019 Society of Chemical Industry.

Baseline susceptibility to afidopyropen of soybean aphid (Hemiptera: Aphididae) from the north central United States

Afidopyropen, a novel Group 9D insecticide, has proven effective for management of soybean aphid, Aphis glycines (Hemiptera: Aphididae). However, to ensure continued efficacy of this insecticide, insecticide resistance management programs are needed. The goal of this study was to evaluate baseline susceptibility of several populations of soybean aphid to afidopyropen using leaf-dip bioassays in 2017 and 2018. Overall, mortality of adult soybean aphid varied among populations. Lethal concentrations necessary to kill 50% of a population (LC50) for adults ranged from 0.0013 to 0.40 mg a.i. per liter of afidopyropen. Lethal concentrations necessary to kill 70% of a population (LC70) of adults ranged from 0.136 to 13.1 mg a.i. per liter of afidopyropen. Afidopyropen was effective against populations resistant to pyrethroid insecticides, suggesting a lack of cross-resistance between afidopyropen and pyrethroids. Furthermore, afidopyropen was effective against nymphs of soybean aphid with concentrations ranging from 0.0285 to 54.45 mg a.i. per liter of afidopyropen causing 100% mortality by 6 days after exposure. The results reported here can be used for establishment of programs to monitor for development of resistance to this insecticide by soybean aphid.

Effects of aphid-resistant soybean on nontarget herbivores in Minnesota

The use of aphid-resistant soybean varieties containing Rag genes can provide an effective pest management strategy against soybean aphid (Aphis glycines Matsumura). However, the compatibility between soybean containing Rag genes and the management of other potential soybean pests (i.e., nontarget herbivores) is poorly understood. Therefore, our goal was to (1) assess the efficacy of pyramided (Rag1+Rag2) aphid-resistant soybean for reduction of soybean aphid populations, and (2) determine if aphid-resistant soybean affects populations of nontarget herbivores under field conditions. To accomplish these goals, field experiments were conducted in southeast Minnesota from 2015 to 2017 for a total of five site-years using whole-plant counts to assess soybean aphid densities and sweep-net sampling to estimate nontarget herbivore densities in plots of near-isoline soybean with Rag1+Rag2 (i.e., resistant) or no Rag genes (i.e., susceptible). Plant genotype (i.e., resistant or susceptible) affected seasonal abundance of soybean aphid, with fewer soybean aphids on aphid-resistant than susceptible soybean. Furthermore, plant genotype affected seasonal abundance of bean leaf beetle (Cerotoma trifurcata Forster), green cloverworm (Hypena scabra Fabricius), and potato leafhopper (Empoasca fabae Harris). Aphid-resistant soybean was associated with increased seasonal abundance of bean leaf beetle and reduced seasonal abundance of potato leafhopper and green cloverworm. The abundance of these herbivores was also associated with seasonal abundance of soybean aphid as a covariate. Results presented here confirm the efficacy of aphid-resistant soybean with Rag1+Rag2 against soybean aphid and show that aphid-resistant soybean can influence the seasonal abundance of nontarget herbivores in soybean. However, the magnitude of impacts of aphid-resistant soybean on nontarget herbivores was relatively small and likely of little-to-no agronomic significance, which suggests compatibility of aphid-resistant soybean with management of other potential pests.

Adjacent habitat type affects the movement of predators suppressing soybean aphids.

Landscape complexity influences soybean aphid suppression by generalist predators in North America, but the role of adjacent habitats as sources of these predators has not been studied directly. We quantified movement of aphidophagous predators between soybean and five adjacent habitats common in Manitoba using bi-directional Malaise traps. To test the contribution of predators from neighboring habitats to soybean aphid suppression, we performed experimental manipulations in adjacent soybean and alfalfa fields and monitored the movement of sevenspotted lady beetles, Coccinella septempunctata, using mark-release-recapture experiments. The identity of adjacent habitats affected the net movement of predators into soybean. The most abundant predators were hover flies (Diptera: Syrphidae), moving from woodlands to soybean. Similar (but non-significant) trends were found for lady beetles, minute pirate bugs, and green and brown lacewings. There was also a net movement of hover flies and green lacewings from soybean to canola. Lady beetles showed higher bidirectional movement in alfalfa and wheat borders than in woodland and canola borders in a high lady beetle abundance year. Soybean aphid populations in predator exclusion cages were 21- to 122- fold higher than populations exposed to predators, both in alfalfa and soybean fields. Aerial predators provide similar levels of aphid suppression as aerial and epigeal predators combined. Mark-release-recapture experiments showed high dispersal of C. septempunctata between soybean and alfalfa, with a net movement towards alfalfa, probably due to the lack of aphids in soybean. These results demonstrate that predator assemblages from both soybeans and alfalfa can suppress soybean aphids. Our findings indicate that the type of adjacent habitat and predator identity affect the directionality of predator movement into soybean. This study suggests that information on predator movement can be used to design the distribution of crops and natural habitats in agricultural landscapes that maximize pest control services.

A matrix model describing host-parasitoid population dynamics: The case of Aphelinus certus and soybean aphid.

Integrating elements from life tables into population models within a matrix framework has been an underutilized method of describing host–parasitoid population dynamics. This type of modeling is useful in describing demographically-structured populations and in identifying points in the host developmental timeline susceptible to parasitic attack. We apply this approach to investigate the effect of parasitism by the Asian parasitoid Aphelinus certus on its host, the soybean aphid (Aphis glycines). We present a matrix population model with coupled equations that are analogous to a Nicholson–Bailey model. To parameterize the model, we conducted several bioassays outlining host and parasitoid life history and supplemented these studies with data obtained from the literature. Analysis of the model suggests that, at a parasitism rate of 0.21 d−1, A. certus is capable of maintaining aphid densities below economically damaging levels in 31.0% of simulations. Several parameters—parasitoid lifespan, colonization timeline, host developmental stage, and mean daily temperature—were also shown to markedly influence the overall dynamics of the system. These results suggest that A. certus might provide a valuable service in agroecosystems by suppressing soybean aphid populations at relatively low levels of parasitism. Our results also support the use of A. certus within a dynamic action threshold framework in order to maximize the value of biological control in pest management programs.

Resistance of Soybean Plant Introductions to Three Colonies of Soybean Aphid (Hemiptera: Aphididae) Biotype 4.

Soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), infestations of soybean, Glycine max (L.) Merr. (Fabales: Fabaceae), and the associated yield loss have led to a large dependence on insecticidal management in soybean throughout the Midwestern United States. However, several populations of pyrethroid-resistant soybean aphids have recently been found in Iowa, Minnesota, North Dakota and South Dakota, which highlights the importance of alternative management approaches. One such alternative method is host-plant resistance, which uses naturally occurring plant defenses in crop cultivars to reduce the potential for yield loss from a pest population. Current soybean aphid-resistant cultivars do not protect against all soybean aphids due to the presence of virulent biotypes. In particular, soybean aphid biotype 4 is virulent to Rag1 and Rag2 resistance genes both individually and in combination. However, we hypothesized that resistance to biotype 4 may exist in previously identified, but uncharacterized resistant soybean plant introductions (PIs). To test this, we evaluated 51 previously identified but uncharacterized soybean aphid-resistant PIs for their resistance to colonies of soybean aphid biotype 4 collected in separate site-years (Lomira, WI 2013; Volga, SD 2015, 2016). Free-choice tests identified 14 PIs with putative resistance to 'Lomira13', two to 'Volga15', and eight to 'Volga16' soybean aphid colonies. Follow-up, no-choice tests corroborated two to three resistant PIs per colony, and PI 437696, which was resistant to each of the three colonies and could aid in breeding efforts and an integrated approach to soybean aphid management.

Weekly Survivorship Curves of Soybean Aphid Biotypes 1 and 4 on Insecticidal Seed-Treated Soybean.

Thiamethoxam, an insecticide used in soybean seed treatments, effectively suppresses soybean aphids (Aphis glycines) Matsumura (Hemiptera: Aphididae) for a short time after planting. However, exactly when and how quickly soybean aphid populations could increase is unknown. Likewise, we lack data on virulent soybean aphid biotypes (that can overcome soybean resistance) when fed on seed-treated soybean. Determining the survival of soybean aphids over time on insecticidal seed-treated soybean is critical for improving soybean aphid management and may provide insights to manage aphid virulence to aphid resistant-soybean. In greenhouse and field experiments, aphid-susceptible soybean plants (with and without an insecticidal seed treatment) were infested at 7, 14, 21, 28, 35, and 42 days after planting (DAP). We compared aphid survival among biotypes 1 (avirulent) and 4 (virulent) and insecticide treatment 72 h after infestation. We also measured thiamethoxam concentrations in plant tissue using liquid chromatography-tandem mass spectrometry. As expected, soybean aphid survival was significantly lower on seed-treated soybean up to 35 DAP for both biotypes, which correlates with the decrease of thiamethoxam in the plant over time. Moreover, we found no significant difference between avirulent and virulent biotype survivorship on insecticidal seed-treated soybean plants, although we did find significantly greater survival for the virulent biotype compared with the avirulent biotype on untreated soybean in the field. In conclusion, our study further characterized the relative short duration of seed treatment effectiveness on soybean aphid and showed that survivorship of virulent aphids on seed-treated soybean is similar to avirulent aphids.

Impact of the parasitoid Aphelinus certus on soybean aphid populations

Aphelinus certus Yasnosh (Hymenoptera: Aphelinidae) is an accidentally introduced parasitoid of the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae) in North America and it has become one of the most common natural enemies of soybean aphids in its adventive range. It is unclear, however, if increased prevalence of A. certus has resulted in increased biological control. We conducted an exclusion-cage experiment designed to isolate the impact of parasitoids from that of other resident natural enemies (mainly predators) of the soybean aphid. We found that A. certus greatly outnumbered all other soybean aphid parasitoids, and that it significantly reduced soybean aphid populations over a time span of less than two weeks compared to controls. Moreover, parasitoids alone resulted in aphid densities that were statistically equivalent to the combined effect of predators and parasitoids. Across all treatment cages, there was a significant negative association between parasitism rate and aphid population growth, with predicted zero aphid growth occurring at a parasitism rate of 42.2%.

Profitability and efficacy of soybean seed treatment in Michigan

Soybean (Glycine max Merr.) is a globally important oilseed crop. Use of seed treatments to avert yield loss by managing seedling pathogens, early-season insects, and nematodes has become more common. Seed treatments may effectively protect plant stand and plant health in the presence of pathogens and pests, but the profitability of prophylactic seed treatment use across diverse environments remains in question, particularly when pests and pathogen populations are low or absent. Seed treatments, including a non-treated control (NTC), fungicide (F), fungicide-insecticide (FI), and fungicide-insecticide-biological nematode protectant (FIN), were evaluated on four soybean varieties at seven field sites in 2013, 2014, and 2015. In 2013, yield data was collected, and in subsequent years additional parameters were measured. FIN significantly improved plant stand at two sites in 2014 and three sites in 2015. Scant soybean aphid (Aphis glycines Matsumura) numbers were found at four sites across 2014 and 2015. Soybean aphid populations in FI plots were lower relative to the NTC at two of the four sites. Soybean cyst nematode (Heterodera glycines Ichinohe) (SCN) was present in one field in 2015, but FIN treatment did not significantly reduce SCN reproduction or population relative to the NTC. Yield across soybean varieties was significantly improved by FIN at the Allegan county sites in 2013 and 2015. Across sites in 2013, no seed treatment significantly improved net returns relative to the NTC. Across sites in 2014 and 2015, FIN significantly reduced net returns relative to the NTC. The probability that a seed treatment would result in economically neutral or positive outcomes was estimated by using Maximum Likelihood Estimation. Results from these planting dates and seeding rates indicate that seed treatments may not benefit all soybean growers. Seed treatment benefits may be affected by soybean variety, soil, environmental conditions, planting population, and planting date. Early planting dates and reduced seeding rates may see an increase in seed treatment profitability.