Do onion thrips prefer to feed on healthy or anthracnose inoculated onion plants?

Onion thrips (Thrips tabaci Lindeman) is a major insect pest of onions (Allium cepa) causing direct feeding damage and vectoring several plant pathogens. A significant fungal pathogen of onions in Michigan is anthracnose (Colletotrichum coccodes) which causes tan lesions on the foliage, and infects bulbs, making them unmarketable. While thrips feeding can promote pathogen colonization and spread in onion fields, the preference and performance of thrips based on host infection status has not been investigated. We tested the preference of thrips for healthy versus inoculated plants in choice-tests to evaluate the mechanisms of the interactions between these two pests. Control plants were non-inoculated and inoculated plants were either asymptomatic or symptomatic at the start of the experiment. Adult thrips were released in choice-test arenas with access to a combination of two plants that were either healthy, asymptomatic, or symptomatic. Total number of thrips per plant and estimated % damage on each plant was recorded after ~2 weeks. Thrips numbers were higher on symptomatic plants than healthy plants, but there were no other differences in choice test. We tested thrips performance on each plant type in a no-choice test by releasing five adult thrips and recording the total number of larval and adult thrips on each plant after 13 days. Overall, thrips performed best on symptomatic plants. This indicates that there could be a beneficial relationship between pathogenic fungi and onion thrips. Our results on insect and pathogen interactions will help growers make informed management decisions to reduce multiple pests simultaneously.