Host plant resistance promotes a secondary pest population

Insecticides can cause secondary pest outbreaks that weaken the benefit of chemical pest control. These detrimental nontarget effects motivate the use of alternative pest management strategies such as host plant resistance and intercropping. However, when alternative pest management strategies effectively suppress primary pests, they also have the potential to promote secondary pest populations via competitive release. The potato leafhopper (Empoasca fabae) is a key pest of alfalfa, and leafhopper-resistant cultivars are being widely adopted by growers in the Midwest and Northeast United States. We conducted a field experiment comparing leafhopper-susceptible alfalfa, leafhopper-resistant alfalfa, and leafhopper-resistant alfalfa intercropped with orchardgrass. Leafhopper-resistant alfalfa reduced potato leafhopper abundance and protected the crop from protein loss, but there was no benefit of intercropping leafhopper-resistant alfalfa with orchardgrass. Importantly, the abundance of a secondary pest, the pea aphid (Acyrthosiphon pisum), was twice as high in the leafhopper-resistant plant treatments compared to the leafhopper-susceptible treatment. Field sampling and microcosm experiments confirmed that the increase in pea aphids was caused, at least in part, by release from competition with leafhoppers. These results suggest that, when host plant resistance against insects is employed, efforts to monitor and manage secondary pest populations are warranted.