Can phylogenetic distance of plant essential oils influence repellency to the specialist swede midge, Contarinia nasturtii (Diptera: Cecidomyiidae)?

Specialist insect herbivores have been widely shown to be attracted to particular volatile organic compounds emitted by their host plants. Although non-host plants have been shown to be repellent, much less is known about the nature of repellency and how it can be effectively utilized to mediate insect herbivore host acceptance. While host plants can be attractive and repellent to specialist insect herbivores, it is not known whether herbivorous species can actually detect and respond to both classes of compounds. Given that specialist insect herbivores are thought to be responsive to individual plant species, or genera, we hypothesized that increasing phylogenetic distance of plant odors would be more repellent to specialist insect herbivores. Swede midge, Contarinia nasturtii (Diptera: Cecidomyiidae) is an invasive specialist of Brassica crops. Using phylogenetic distance as a measure of chemical relatedness, we curated a list of plant essential oils to test in the lab and field that may influence oviposition by triggering aversive behavior. We tested the plant essential oils in laboratory assays and olfactometer tests, measuring if they repelled female midges from laying eggs on broccoli plants. We also measured the chemical profiles of each of the oils using gas chromatography. Mass spectrometry was performed on oils that successfully altered midge behavior in the lab. The results from these trials indicate oils derived from garlic and lemongrass as being the most successful in repelling the Swede midge from host plants.