Integrating soybean aphid and soybean cyst nematode management

Soybean aphid, Aphis glycines, and soybean cyst nematode (SCN), Heterodera glycines, can interact through the soybean plant resulting in increased SCN reproduction on both SCN-resistant and SCN-susceptible varieties. The management of SCN is heavily reliant on the planting of PI 88788-derived SCN-resistant varieties to limit yield loss to SCN in the current year and future years. Virulence to PI 88788 is increasing in SCN field populations due to its extensive use. Therefore, it is increasingly important to manage any factor that increases SCN reproduction on SCN-resistant varieties. Here I examined management tactics including host-plant resistance and insecticidal seed treatments to limit soybean aphid populations and disrupt the interaction between soybean aphids and SCN. Neither host-plant resistance incorporating a single resistance gene nor insecticidal seed treatments were able to prevent yield loss from soybean aphids. Furthermore, host-plant resistance incorporating a single resistance gene also failed to disrupt soybean aphid-SCN interactions. Host-plant resistance incorporating a pyramid of two resistance genes was, however capable of limiting yield loss to soybean aphids. The pyramid line also limited aphid population densities to below levels where we would expect to observe soybean aphid-SCN interactions. Future research will need to investigate the ability of a pyramid line to disrupt soybean aphid-SCN interactions in the field and the potential consequences for yield and long-term sustainable SCN population management

New Options for Soybean Aphid Host Plant Resistance

Host plant resistance for soybean aphid is the newest management tool for yield protection. In 2010, a single gene expression, called Rag1, was commercially released in the north central region. Aphids feeding on Rag1 plants do not live as long or produce as many offspring compared to when they feed on susceptible plants. In small plot evaluations of the Rag1 gene, there is a dramatic decrease in the seasonal accumulation of soybean aphid compared to aphids developing on susceptible varieties

Exploring soybean integrated pest management in a changing agricultural environment: the impacts of decreasing ecosystem services, invasive species and specialty cultivars

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae) is a recently invasive pest to North America. Integrated pest management strategies exist for minimizing the impact of the soybean aphid on yield. These strategies were developed from field-based research conducted under the environmental conditions of the time and using commodity soybean plants. The increased use of broad spectrum insecticides and the increased simplification of the landscape along with the release of altered fatty acid soybean cultivars have led to changes in the agricultural environment. How these changes affect soybean aphid population dynamics and interactions between the soybean aphid and other pests is investigated. The first objective was to determine how reduced natural enemy services may impact soybean aphid population growth between the economic threshold and economic injury level. The second objective was to determine how the soybean aphid could indirectly interact with the soybean cyst nematode, Heterodera glycines Ichinhoe and the brown stem rot fungus, Cadophora gregata Harrington and McNew. The third and fourth objectives were to determine the effect of altered fatty acid cultivars on soybean pests and pathogens. Objective four quantified the effect of these cultivars on the performance of the soybean aphid, soybean cyst nematode and brown stem rot. Objective five addressed how the alterations in fatty acid synthesis pathways present in these cultivars may affect plant volatile emissions and the downstream impact this has on host plant selection by the bean leaf beetle, Cerotoma trifurcata Forster (Coleoptera: Chrysomelidae)

A New Bean Leaf Beetle Threshold Calculator is Created

In April, an ICM News article predicted a slightly higher winter survivorship of bean leaf beetle in Iowa. Not surprisingly, this summer I have been hearing about a few soybean fields with bean leaf beetle. Most reports of overwintering and first generation beetles are at low densities with negligible defoliation. There are two generations of bean leaf beetle in Iowa (Fig. 1), with the second generation emerging in a few weeks. Scouting for bean leaf beetle and other defoliators should continue until seed set

Measuring the Benefit of Biological Control for Single Gene and Pyramided Host Plant Resistance for Aphis glycines (Hemiptera: Aphididae) Management

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an economically important pest in the north central United States. In the state of Iowa, economically damaging populations occurred in seven of 11 growing seasons from 2001 to 2011. The high frequency and economic impact of the soybean aphid makes it an ideal candidate for management by using host plant resistance. We compared an aphid-susceptible line to near-isolines that contain Rag1 and Rag2, both alone and pyramided together, to suppress aphid populations and protect yield. Each of four near-isolines, were artificially infested with aphids and grown in small plots in which the exposure to natural enemies was controlled by the use of cages, resulting in the following treatment groups: natural enemy free (only aphids), biocontrol (both aphids and natural enemies), and aphid free (no aphids or natural enemies). The seasonal accumulation of aphids and the population growth rates were measured for each line and an estimate of yield was measured at the end of the season. Soybean aphid population growth rate was reduced 20% by natural enemies alone, 44% by pyramided resistance, and 63% by the combination of natural enemies and pyramided resistance. This reduction in population growth rate resulted in a 99.3% reduction in the pyramid line\u27s seasonal exposure to aphids. In the presence of natural enemies, all three resistant lines maintained aphid populations below the economic injury level and prevented yield loss. This study demonstrates the compatibility of biological control with soybean aphid host plant resistance and its utility, especially for single resistance gene lines

Soybean Aphid (Aphididae: Hemiptera) Population Growth as Affected by Host Plant Resistance and an Insecticidal Seed Treatment

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae) is a significant soybean pest in the north central United States. Insecticidal seed treatments and host plant resistance are two commercially available management tools. Here we investigate the efficacy of both management tools throughout the season. Soybean lines containing the soybean aphid resistance genes Rag1, Rag2, or both Rag1 + Rag2 were compared with a near-isogenic aphid-susceptible line. Each line was grown in field plots both with and without thiamethoxam applied to the seed. Individual plants from each plot were caged and infested with soybean aphids to measure the efficacy and potential interaction of aphid resistance and thiamethoxam. Aphid population growth rate was measured for each caged plant for 9‐12 d after infestation. New cages were established each week from 34 d after planting (dap) to 92 dap to track seasonal variations in efficacy. Thiamethoxam reduced population growth only at the 42 dap time point and only for the susceptible, Rag1, and Rag2 lines. The lack of an effect of thiamethoxam on theRag1+ Rag2 line was likely because of already high mortality from two resistance genes. Aphid resistance alone reduced population growth compared with the susceptible line at least till 55 dap for single-gene resistance and 63 dap for the two genes combined. Aphid resistance provided suppression of soybean aphid population growth throughout the season unlike the insecticidal seed treatment

Reduced Fitness of Virulent Aphis glycines (Hemiptera: Aphididae) Biotypes May Influence the Longevity of Resistance Genes in Soybean

Sustainable use of insect resistance in crops require insect resistance management plans that may include a refuge to limit the spread of virulence to this resistance. However, without a loss of fitness associated with virulence, a refuge may not prevent virulence from becoming fixed within a population of parthenogenetically reproducing insects like aphids. Aphid-resistance in soybeans (i.e., Rag genes) prevent outbreaks of soybean aphid (Aphis glycines), yet four biotypes defined by their capacity to survive on aphid-resistant soybeans (e.g., biotype-2 survives on Rag1 soybean) are found in North America. Although fitness costs are reported for biotype-3 on aphid susceptible and Rag1 soybean, it is not clear if virulence to aphid resistance in general is associated with a decrease in fitness on aphid susceptible soybeans. In laboratory assays, we measured fitness costs for biotype 2, 3 and 4 on an aphid-susceptible soybean cultivar. In addition, we also observed negative cross-resistance for biotype-2 onRag3, and biotype-3 on Rag1 soybean. We utilized a simple deterministic, single-locus, four compartment genetic model to account for the impact of these findings on the frequency of virulence alleles. When a refuge of aphid susceptible was included within this model, fitness costs and negative cross-resistance delayed the increase of virulence alleles when virulence was inherited recessively or additively. If virulence were inherited additively, fitness costs decreased the frequency of virulence. Combined, these results suggest that a refuge may prevent virulent A. glycines biotypes from overcoming Rag genes if this aphid-resistance were used commercially in North America

Evaluation of Soybean Aphid-resistant Soybean Lines

The soybean aphid, Aphis glycines (Hemiptera: Aphididae), is a consistent pest of soybean in Iowa. Current management is heavily reliant on the use of insecticides, which can be expensive and time consuming to apply. Soybean varieties resistant to the aphid are available. These varieties primarily include one resistance gene (Rag1) for soybean aphid control. Varieties incorporating two genes (Rag1 + Rag2) have recently become available. We sought to compare soybean lines susceptible to the soybean aphid with lines carrying a single resistance gene (Rag1 or Rag2) and a line carrying two resistance genes (Rag1 + Rag2). We evaluated the lines based on aphid control and yield protection

Determining the duration of Aphis glycines (Hemiptera: Aphididae) induced susceptibility effect in soybean

Insect herbivores can increase the suitability of host plants for conspecifics by inducing susceptibility. Induced susceptibility can be separated into feeding facilitation, whereby herbivore feeding increases performance of conspecifics regardless of the genotype of the herbivore or plant, and obviation of resistance, whereby feeding by a virulent herbivore increases performance of avirulent conspecifics on resistant plants. Both forms occur between Aphis glycines (Hemiptera: Aphididae) and soybean. In natural and agricultural settings, A. glycines populations can colonize plants for brief periods before emigrating or being removed due to predation or insecticides. It is unclear if induced susceptibility lasts beyond the period when A. glycines are present on the plant. We measured the duration of induced susceptibility in the A. glycines-soybean system within a growth chamber by removing inducer populations after 24 h. We used an A. glycines-resistant soybean infested with an inducer population of either virulent, avirulent, or no aphids. Response populations of either virulent or avirulent aphids were added at three post-infestation times (24, 120, 216 h) and their densities measured 11 days after infestation. Feeding facilitation was lost within 24 h of the removal of avirulent inducer populations, and obviation of resistance diminished over time and was completely lost within 216 h of the removal of the virulent inducer populations. We discuss how these results support a hypothesis that virulence in A. glycines is due to effector proteins secreted by feeding aphids. We suggest that the duration of induced susceptibility may impact the durability of A. glycines resistance in soybean