An Induced Susceptibility Response in Soybean Promotes Avirulent Aphis glycines (Hemiptera: Aphididae) Populations on Resistant Soybean

Abstract

Observations of virulent Aphis glycines Matsumura populations on resistant soybean in North America occurred prior to the commercial release of Rag genes. Laboratory assays confirmed the presence of four A. glycines biotypes in North America defined by their virulence to the Rag1 and Rag2 genes. Avirulent and virulent biotypes can co-occur and potentially interact on soybean, which may result in induced susceptibility. We conducted a series of experiments to determine if the survival of avirulent biotypes on susceptible and resistant soybean containing the Rag1 or Rag1 + Rag2 genes was affected by the presence of either avirulent or virulent conspecifics. Regardless of virulence to Rag genes, initial feeding by conspecifics increased the survival of subsequent A. glycines populations on both susceptible and resistant soybean. Avirulent populations increased at the same rate as virulent populations if the resistant plants were initially colonized with virulent aphids. These results are the first to demonstrate that virulent A. glycines increase the suitability of resistant soybean for avirulent conspecifics, thus explaining the lack of genetic differentiation observed in North America between A. glycines populations on resistant and susceptible soybean. These results suggest the occurrence of virulence toward Rag genes in North America may be overestimated. In addition this may alter the selection pressure for virulence genes to increase in a population. Therefore, insect resistance management models for A. glycines may need to incorporate induced susceptibility factors to determine the relative durability of resistance genes.

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

“…There was a 2078 % increase in population density between the avirulent response population with no inducer treatment and the avirulent response population with a virulent inducer treatment. Obviation of Rag resistance by virulent A. glycines resulted in response populations of avirulent populations that were equivalent to that of a virulent response population with a virulent inducer (Varenhorst et al 2015). The consequences of both of these mechanisms are relevant in light of recent findings by Wenger et al (2014), which suggest an improvement in fitness for avirulent A. glycines on aphid-resistant soybean decreases the relative frequency of virulent biotypes.…”

“…Feeding by A. glycines induces susceptibility for subsequent A. glycines on both susceptible and resistant soybean varieties (Varenhorst et al 2015). This induced susceptibility was observed with both avirulent and virulent biotypes and could be divided into two different mechanisms: feeding facilitation (Denno and Benrey 1997) and obviation of resistance (Baluch et al 2012).…”

“…This induced susceptibility was observed with both avirulent and virulent biotypes and could be divided into two different mechanisms: feeding facilitation (Denno and Benrey 1997) and obviation of resistance (Baluch et al 2012). For A. glycines and soybean, both mechanisms were observed (Varenhorst et al 2015). For example, feeding facilitation was observed when A. glycines populations on aphid-resistant soybean had larger populations after initial herbivory by an inducer population of A. glycines when compared to a control lacking an inducer population, regardless of its biotype that resulted in a 714 % increase in population density.…”

“…These results align with the goal of an insect resistance management (IRM) plan to reduce/slow the spread of virulence. The extent that induced susceptibility can support a IRM plan is unknown, though initial modeling suggests that this phenomenon will reduce the frequency of virulent biotypes when a refuge of aphidsusceptible soybean is planted with the aphid-resistant cultivar (Varenhorst et al 2015).…”

“…In the previous experiments (Varenhorst et al 2015) with A. glycines, plants were co-infested with both an inducer population and a subsequent response population. In natural and agricultural settings, aphids form colonies on plants for brief periods before leaving due to emigration, predation, or an insecticide application.…”