Fumigant Toxicity of Essential Oils against Frankliniella occidentalis and F. insularis (Thysanoptera: Thripidae) as Affected by Polymer Release and Adjuvants

SIMPLE SUMMARY: Frankliniella occidentalis is among the most economically significant pests of greenhouse crops. In contrast, F. insularis is a relatively minor pest with a narrower distribution and host range. We conducted a series of fumigation assays to assess the vulnerability of both species to fumigation with essential oils released from hydrogels. These hydrogels contained either (R)-linalool, (S)-linalool, racemic linalool, or a binary mixture of (R)-linalool with one of twelve other essential oils. Solanum lycopersicum seedlings were screened for their sensitivity to the most potent fumigants, as determined from thrips bioassays. The least saturated hydrogels conditioned in essential oils were the most effective, and both species of thrips were more sensitive to (R)-linalool than to (S)-linalool. Frankliniella occidentalis was significantly more resistant to all treatments than F. insularis. Treatment of S. lycopersicum with the same concentrations of oils required to control thrips resulted in reduced root and hypocotyl lengths, most severely in seedlings exposed via foliar sprays than as fumigants. While our study demonstrates that essential oils are a promising alternative to conventional insecticides for thrips control, the resistance demonstrated by F. occidentalis underlines the need for judicious use of essential oils as part of broader pest control programs. ABSTRACT: Frankliniella occidentalis is among the most economically significant pests of greenhouse crops, whose resistance to conventional insecticides has created demand for biopesticides such as essential oils. We assessed the fumigant toxicity of linalool against F. occidentalis, F. insularis, and Solanum lycopersicum. Thrips were fumigated with polyacrylamide hydrogels containing either (R)-linalool, (S)-linalool, racemic linalool, or a binary mixture of (R)-linalool with one of twelve adjuvants (i.e., peppermint, cedarwood, neem, clove, coconut, jojoba, soybean, olive, α-terpineol, 1,8-cineole, trans-anethole, or (R)-pulegone). Solanum lycopersicum seedlings were exposed to (R)-linalool or a mixture of (R)-linalool and peppermint oil via conditioned hydrogels or foliar spray. For F. insularis, (R)-linalool was more toxic than (S)-linalool, with LC(50) values of 11.7 mg/L air and 16.7 mg/L air, respectively. Similarly for F. occidentalis, (R)-linalool was more toxic than (S)-linalool, with LC(50) values of 29.0 mg/L air and 34.9 mg/L air, respectively. Peppermint oil and α-terpineol were the only synergists, while the other adjuvants exhibited varying degrees of antagonism. All seedling treatments demonstrated phytotoxicity, but symptoms were most severe for foliar sprays and mixtures containing peppermint oil. While hydrogels conditioned in linalool may be a favorable substitute to conventional insecticides, the cross-resistance demonstrated herein indicates that expectations should be metered.