Insect taste, smell and repellents

Spotted-wing drosophila, Drosophila suzukii, is an invasive fruit fly. Most fruit flies lay eggs in overripe or rotting fruit, but D. suzukii females have a serrated ovipositor that cuts into ripe and ripening berries, so infested fruit can look ready to pick and be shipped unnoticed. The fly reached the Northeastern United States in 2011, and U.S. losses in small fruit are estimated at $718 million a year (Northeast SARE project LNE22-455R). Growers control it mainly with broad-spectrum insecticides.

Methyl benzoate is a volatile produced by many plants and an FDA-approved food additive, and it repels the fly. In a field trial in highbush blueberry with Rutgers University and the USDA, methyl benzoate dispensers reduced infestation when fly pressure was very high (Gale et al. 2024). The dispensers were the “push” of a push-pull system, which pairs a repellent on the crop with lure traps that pull the pest away; the traps alone did not give comparable protection, and adding them did not improve on the dispensers’ effect.

Methyl benzoate only works at high doses, though, and flies vary in how they respond to it. We are now screening structural analogs that differ from it in the ester group, a ring substituent, or both (Figure 1). Each compound is tested on female flies in a two-choice spatial repellency assay (Figure 2) and ranked by its RD50, the dose that repels half of the flies. Several analogs repelled flies more strongly and more consistently than methyl benzoate at lower doses (Heinbockel and Shields, NCUR 2026). A structure-activity paper is in preparation, and a high-throughput version of the assay that also scores knockdown has been presented at ABRCMS 2025.

We screened 27 essential oils against the house cricket, Acheta domesticus. The crickets moved freely in an open arena that held a bottle with the oil treatment inside, and repellency was scored from the share of crickets that entered the bottle, with fewer entries meaning a stronger repellent (Figure 3). Four strong repellents, peppermint, rosemary, cinnamon and lemongrass oils, were then tested at four doses and showed a clear dose-dependent effect, while the synthetic repellents DEET and IR3535 had no significant effect. Our 2026 review in Insects covers essential oils as alternatives to synthetic repellents.

The spongy moth, Lymantria dispar (called the gypsy moth in the lab’s earlier papers), feeds on the leaves of hundreds of tree species. Caterpillars have several kinds of taste sensilla on their mouthparts; the main ones for feeding are two on each galea, the medial and lateral styloconic sensilla, each containing four taste receptor cells (Figure 4). We record from them by tip recording, in which a glass electrode filled with the stimulus solution is placed over the sensillum tip; the same electrode delivers the stimulus and records the cells’ action potentials.

These recordings identified cells that respond to sugars, salt, inositol and plant alkaloids (Table 1). One cell in the medial sensillum, the deterrent cell, responds to caffeine, nicotine, strychnine and aristolochic acid, and also to salt (Martin and Shields 2012b); sucrose and inositol, which stimulate feeding, suppress its response (Martin and Shields 2012a). The same cell responds to the insect repellents DEET, IR3535 and picaridin, and each of them deterred feeding. This was the first report of insect repellents detected by a caterpillar’s taste receptor cells (Sanford et al. 2014).

In 2017 the lab received a $150,000 grant from the USDA for work on the brown marmorated stink bug, Halyomorpha halys, with USDA research chemist Aijun Zhang and research associate Nicholas Larson. The project set out to collect volatiles from the bugs and to test candidate attractants, feeding deterrents and repellents in electrophysiological and behavioral assays, to help design better traps (Towson University News, September 2017).

Dr. Shields’s work with John G. Hildebrand of the University of Arizona described eight types of sensilla on the antenna of the female tobacco hornworm moth, Manduca sexta. Recordings from single type-A trichoid sensilla, using a panel of 102 volatile compounds and three plant-derived odor mixtures, sorted these sensilla into three functional types.