Can Host-Plant Resistance Lead to a Secondary Pest Outbreak Due to Competitive Release?

As the human population continues to increase, agricultural production must grow to meet food demands. Increasing crop yields can be accomplished by allocating more land to agriculture, engineering higher yielding and hardier crops, and decreasing insect damage to crops. The most common way to control insect pests is with insecticides. An environmentally friendly alternative to these chemicals is host plant resistance, the inherent ability of crop plants to restrict, retard or overcome pest infestations and thereby to improve the yield and/or quality of the harvestable crop. While host plant resistance against a single pest is effective, it has the potential to release secondary pests from competition. If large enough, these secondary pest outbreaks could reduce crop yield. In alfalfa, Medicago sativa L., host plant resistance for pea aphids, Acyrthosiphon pisum, and potato leafhoppers, Empoasca fabae, is available. Three varieties of alfalfa were studied: a susceptible cultivar(control), potato leafhopper resistant cultivar, and pea aphid and potato leafhopper resistant cultivar. A field study was conducted to examine the population levels of potato leafhoppers and pea aphids on the varying strains of alfalfa. It was found that potato leafhopper resistant alfalfa can induce a secondary pea aphid outbreak. A microcosm experiment indicated that the cause of this secondary pea aphid outbreak is release from competition with the potato leafhopper. Compared with no-insect control plants, the nitrogen content of plants with both pests was lower, suggesting that competition between these insect pests is likely over nitrogen. These results suggest that alfalfa cultivars with resistance to both potato leafhoppers and pea aphids may be warranted to avoid crop damage to potato leafhoppers and avoid a secondary pea aphid outbreak