Nine alkaloids (acridine, aristolochic acid, atropine, berberine, caffeine, nicotine, scopolamine, sparteine, and strychnine) were evaluated as feeding deterrents for gypsy moth larvae (Lymantria dispar (L.); Lepidoptera: Lymantriidae). Our aim was to determine and compare the taste threshold concentrations, as well as the ED 50 values, of the nine alkaloids to determine their potency as feeding deterrents. The alkaloids were applied to disks cut from red oak leaves (Quercus rubra) (L.), a plant species highly favored by larvae of this polyphagous insect species. We used two-choice feeding bioassays to test a broad range of biologically relevant alkaloid concentrations spanning five logarithmetic steps. We observed increasing feeding deterrent responses for all the alkaloids tested and found that the alkaloids tested exhibited different deterrency threshold concentrations ranging from 0.1 mM to 10 mM. In conclusion, it appears that this generalist insect species bears a relatively high sensitivity to these alkaloids, which confirms behavioral observations that it avoids foliage containing alkaloids. Berberine and aristolochic acid were found to have the lowest ED 50 values and were the most potent antifeedants.

The gypsy moth, Lymantria dispar (L.) (Lepidoptera: Lymantriidae), is a major defoliator of forest habitats in North America. These larvae feed on the foliage of hundreds of plants, but prefer leaves from deciduous hardwood trees such as sweetgum, maple, and oak. Plants contain a variety of phytochemicals, some of which are insect feeding deterrents. Deterrent compounds influence the food selection of many phytophagous insects including gypsy moth larvae. Deterrent compounds, such as alkaloids, are generally unfavored and typically avoided by these larvae. We tested the effects of eight alkaloids using two‐choice feeding bioassays. Each alkaloid was applied to glass fiber disks and red oak leaf disks. All eight alkaloids tested on glass fiber disks were deterrent to varying degrees. When these alkaloids were applied to leaf disks, however, only seven were still deterrent. Of these seven, five were less deterrent on leaf disks compared with glass fiber disks, indicating that their potency was dramatically reduced when they were applied to leaf disks. The reduction in deterrency may be attributed to the phagostimulatory effect of red oak leaves in suppressing the negative deterrent effect of these alkaloids, suggesting that individual alkaloids may confer context‐dependent deterrent effects in plants in which they occur. We also tested the effects of these eight alkaloids at six concentrations spanning four logarithmetic steps to determine deterrent threshold effects. This study provides novel insights into the feeding behavioral responses of gypsy moth larvae to selected deterrent alkaloids. Our results may be potentially useful in designing crop protection strategies from insect pests and will help to improve our understanding of insect feeding behavior. Supported by NIH grant 1 R15 DC007609‐01 to V.S. and Bridges NIH grant 5R25GM058264‐03.