Induced plant resistance among potato pests

Plant-mediated competition among insect herbivores occurs when one species induces changes in plant chemistry, nutrition, or morphology that render plants resistant to attack by others. In collaboration with Bob Denno and Galen Dively I investigated leafhopper-induced resistance to beetle herbivory. In mid-Atlantic U.S. potato fields the two primary insect pests are potato leafhoppers (Empoasca fabae) and Colorado potato beetle (Leptinotarsa decemlineata). Because of current management practices leafhoppers precede beetles and our research indicates that early-season leafhopper herbivory induces resistance to later-feeding beetles. Prior leafhopper damage adversely affects the preference/performance of beetles and increases their vulnerability to natural enemies. Thus, plants mediate interactions between these temporally-separated insect herbivores.

Economic injury levels for pest management have been developed primarily for single pest species, and interactive effects among pests are usually not considered. However, potato growers in the mid-Atlantic area of the United States are faced with intra-seasonal decisions concerning the control of both potato leafhopper and Colorado potato beetle. The current management approach assumes that the impacts of both pests are independent. Thus, control decisions regarding potato leafhoppers are based solely on current control costs and not on projected savings associated with the adverse induced resistance effects of leafhoppers on potato beetles. Current management practices for potato leafhoppers include the use of insecticides when densities reach a certain economic threshold, which has historically been rather conservative. Based on our findings, a potato grower should be able to accept a higher level of leafhopper injury, if losses due to Colorado potato beetle later in the summer can be prevented at a level sufficient to offset the direct costs of leafhopper damage. Thus, the action threshold for leafhopper control should be higher when the delayed benefits of beetle reduction via induced plant resistance are considered. Ultimately, to establish an effective management strategy for the multiple pests on potato, it will be essential to combine the positive indirect effects of induced resistance along with the negative direct effects on crop yield, a strategy which is not currently employed in potato, or in other crops at large.