The Effect of an Interspersed Refuge onAphis glycines(Hemiptera: Aphididae), Their Natural Enemies, and Biological Control

Abstract

Soybean production in the north central United States has relied heavily on the use of foliar and seed applied insecticides to manage Aphis glycines (Hemiptera: Aphididae). An additional management strategy is the use soybean cultivars containing A. glycines resistance genes (Rag). Previous research has demonstrated that Rag cultivars are capable of preventing yield loss equivalent to the use of foliar and seed-applied insecticides. However, the presence of virulent biotypes in North America has raised concern for the durability of Rag genes. A resistance management program that includes a refuge for avirulent biotypes could limit the frequency at which virulent biotypes increase within North America. To what extent such a refuge reduces the effectiveness of aphid-resistant soybean is not clear. We conducted an experiment to determine whether a susceptible refuge mixed into resistant soybean (i.e., interspersed refuge or refuge-in-a-bag) affects the seasonal exposure of aphids, their natural enemies, biological control, and yield protection provided by aphid resistance. We compared three ratios of interspersed refuges (resistant: susceptible; 95:5, 90:10, 75:25) to plots grown with 100% susceptible or resistant soybean. We determined that an interspersed refuge of at least 25% susceptible seed would be necessary to effectively produce avirulent individuals. Interspersed refuges had negligible effects on yield and the natural enemy community. However, there was evidence that they increased the amount of biological control that occurred within a plot. We discuss the compatibility of interspersed refuges for A. glycines management and whether resistance management can prolong the durability of Rag genes.

Evaluating the role of insecticidal seed treatment and refuge for managing soybean aphid virulence

“…Including an insecticidal seed treatment with a refuge does not improve virulence management compared with untreated refuge and Rag plants. Our findings suggest that a refuge alone could maintain avirulent aphids, similar to previous studies 15,16 . Additional research should address other assumptions of IRM models to evaluate Rag durability, and the durability of HPR traits in general.…”

“…In this case (S Rt ), while the refuge fulfills its role in providing a suitable host for biotype 1, it may also allow biotype 4 to survive and then move to Rag plants at later time points when the insecticidal seed treatment is no longer effective. Of course, the strength of the fitness costs to virulent aphids may also be dependent on other conditions, such as variety, soybean background and potentially the presence of natural enemies 15,16 . Estimating fitness costs is crucial to IRM durability models for insects that reproduce asexually, 14 but might prove difficult in the soybean aphid system due to variability in soybean varieties, agronomics, and environmental factors that may impact model robustness.…”

“…The high‐dose/refuge strategy has several assumptions including: (i) the high‐dose of the insecticide kills over 95% of individuals and makes resistance effectively recessive 9–11 ; (ii) any resistant individuals that survive will mate randomly with individuals from the refuge 12 ; (iii) resistance is recessive and rare (less than 1% frequency); and (iv) resistant individuals have fitness costs that make them less competitive in a refuge 13,14 . Although the HPR system may violate some assumptions of the high‐dose/refuge strategy applied in GM crops (such as a lack of high‐dose and non‐rare frequency of virulence), experimental data and population modeling (i.e., modified Hardy–Weinberg) suggest that a refuge could prevent adaptation to HPR, especially if virulent biotypes have lower fitness than avirulent biotypes on susceptible plants 14–16 …”

“…Previous research suggests a refuge can maintain avirulence in a soybean aphid population, 15,16 and an insecticidal seed treatment is effective against early season aphid infestation (prior to 42 days after planting, see Esquivel et al 18 . and McCarville and O'Neal 26 ).…”

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

“…Fitness costs are an important component to insect resistance management. Indeed, IRM models show that virulence can be managed, and durability of Rag resistance can be extended, in the presence of fitness costs (Varenhorst and O'Neal 2016). Given that both biotypes perform poorly on aphid susceptible soybean treated with thiamethoxam (<35 DAP), any refuge strategy to manage virulent biotypes cannot include treated susceptible soybean.…”