Does host-plant resistance inadvertently increase secondary pest populations by reducing biocontrol?

Host-plant resistance in alfalfa (Medicago sativa) has warded off primary pests and natural enemies, while allowing secondary pests, such as the pea aphid (Acyrthosiphon pisum), to thrive. This study aims to investigate if glandular trichomes, the basis for host-plant resistance for potato leafhoppers (Empoasca fabae), on M. sativa, decreases the efficiency of conservation biocontrol of A. pisum by diminishing their primary natural enemy, parasitoid Aphidius ervi, and identifying the cultivar preferences of A. pisum with growth chamber and field experiments. The first objective is to quantify and compare A. pisum abundance on resistant and susceptible M. sativa with no-choice growth chamber tests. Second, the cultivar preferences of A. pisum will be identified with growth chamber choice tests. The third objective is to quantify differences in A. ervi and A. pisum abundance between the two cultivars with field experiments. Finally, parasitoid activity will be filmed to compare their behavior on stems with and without glandular trichomes. Preliminary growth chamber data reveals that A. pisum does not exhibit differences in survival, abundance, or preferences for either cultivar. Field experiment samples show that A. pisum were collected more frequently on the resistant cultivar hosts than the susceptible cultivar, both live and parasitized. It is hypothesized that resistant M. sativa cultivar hosts more A. pisum because of reduced biological control by A. ervi due to the presence of glandular trichomes. The results should expand current knowledge of how host-plant resistance and conservation biological control can negate each other’s benefits.