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

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.