Rapid evolution to host plant resistance by an invasive herbivore: soybean aphid (Aphis glycines) virulence in North America to aphid resistant cultivars.

Preventing rapid evolution of herbivores to plant traits that confer resistance is an area of active research for applied entomologists. The subfield of insect resistance management (IRM) uses elements of population genetics and ecology to prevent increases in the frequency of virulent (i.e. resistant) sub-populations of an insect pest. Efforts to delay such an increase include using highly lethal toxins (i.e., a high dose), combining multiple resistance traits in one cultivar (i.e., pyramids), and using susceptible plants (i.e. a refuge) within or near plantings of the resistant crop. Even if fully implemented, theoretical models suggest that IRM plans for asexually-reproducing insects (e.g. aphids) cannot limit the frequency of resistance to provide sustainable use of a pest-resistant cultivar. We discuss how feeding by conspecifics aphids induces susceptibility such that a "within plant" refuge is created, allowing both virulent and avirulent (i.e. susceptible) populations to persist. We use the soybean aphid (Aphis glycines Matsumura), and the rapid occurrence of virulence in the US to resistant cultivars of soybean (Glycine max). We describe how feeding by A. glycines on soybeans alters the quality of the plant as a host. These systemic changes to the plants' physiology allow avirulent A. glycines to thrive on resistant cultivars. We explore how the induction of susceptibility by a herbivore can slow an increase in the frequency of virulent populations to resistant host plants. We suggest that a within plant refuge, combined with standard IRM practices, can allow for sustainable use of plant resistance to asexually-reproducing insect pests.