Abstract
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 on Rag3, 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.
Highlights
In 2000, Aphis glycines Matsumura was first observed in the US
We confirmed our hypothesis that the population densities of A. glycines biotype-2 and biotype-3 would be lower on susceptible soybean when compared to biotype-1
Our results indicate that fitness costs exist for biotype-2, biotype-3 and biotype-4 A. glycines on the susceptible soybean cultivar (IA3027) that was used for these experiments when compared to cultivars on which they are virulent
Summary
In 2000, Aphis glycines Matsumura was first observed in the US. Prior to 2000, insecticide use in north central US soybean was infrequent [1]; after the establishment of A. glycines insecticide use on soybean in north central US dramatically increased [2]. The reason for the increase in insecticide use is due to soybean yield reductions of up to 40% caused by A. glycines. Insecticides are effective at reducing A. glycines populations and preventing associated yield loss while being cost effective [3, 4]. The insecticides commonly used to manage A. glycines populations are broad-spectrum and reduce populations of natural enemies present in soybean during application [5, 6, 7]. Concerns for the future development of insecticide-resistant A. glycines populations if insecticides are consistently used suggest a need for additional management tools. An alternative strategy that is potentially more cost effective with negligible effects on natural enemies is soybean that contain one or more resistant to A. glycines genes (or Rag genes) [3, 8, 9]. There is evidence that Rag genes are effective, a limited number of varieties containing Rag, Rag, or Rag1+Rag are commercially available and their adoption is limited [8, 9, 10]
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