Contact toxicity of essential oils of Litsea pungens Hemsl. and L. cubeba (Lour.) Pers. (Lauraceae) and of blends of their major constituents was assessed against third-instar Trichoplusia ni larvae via topical application. Both oils showed moderate activity against T. ni larvae with LD(50) values of 87.1 and 112.5 microg/larva, respectively. 1,8-Cineole from the essential oil of L. pungens and gamma-terpinene from the oil of L. cubeba accounted for much of the toxicity of the oils to T. ni larvae. The toxicity of blends of selected constituents indicated a synergistic effect among putatively active and inactive constituents, with the presence of all constituents necessary for full toxicity of the natural oils. The results show that essential oils of L. pungens and L. cubeba and some of their constituents have potential for development as botanical insecticides.
Antifeedant and growth inhibitory effects of crude plant extracts (Melia volkensii and Origanum vulgare) and pure allelochemicals (digitoxin, cymarin, xanthotoxin, toosendanin, thymol and trans‐anethole) were investigated in the cabbage looper (Trichoplusia ni), and in the armyworm (Pseudaletia unipuncta) using different bioassays. Antifeedant effects of M. volkensii, O. vulgare and thymol were investigated in larvae of the diamondback moth (Plutella xylostella), and of O. vulgare and thymol in the Mexican bean beetle (Epilachna varivestis), using leaf disc choice bioassays. M. volkensii was the most potent growth inhibitor for T. ni and P. unipuncta (dietary EC50 = 7.6 and 12.5 p.p.m., respectively) of all the test substances. Cymarin was the second most potent growth inhibitor (EC50 = 132.0 p.p.m.) for T. ni. The most effective feeding deterrents for third instar T. ni larvae were xanthotoxin andM. volkensii (DC50 = 0.9 and 8.3 μg/cm2, respectively). M. volkensii was also the most potent feeding deterrent for third instar P. unipuncta, P. xylostella and adult E. varivestis (DC50 = 10.5, 20.7 and 2.3 μg/cm2, respectively). Because of interspecific differences in response to feeding deterrents and the lack of a strong relationship between EC50 and DC50 values, we recommend testing a battery of bioassay species with candidate compounds and the use of more than one bioassay. Based on their growth inhibitory and feeding deterrent properties, some of these plant extracts and pure allelochemicals have potential for use as alternative crop protectants against a number of pest species.
Leaf disc choice and oviposition bioassays were used to examine the effects of larval experience with a Hoodia gordonii latex on subsequent behaviors. The latex deterred feeding and oviposition by "naïve" cabbage looper (Trichoplusia ni, Noctuidae) larvae and moths with no previous exposure to the material. "Experienced" insects, reared on a diet with the H. gordonii latex (1000 ppm), exhibited lesser feeding deterrence relative to naïve insects. Experienced female moths actually preferred to lay eggs on treated rather than control leaves. There was no observed transfer of behavioral preferences from experienced parents to their offspring. Our results suggest that moths may be acquiring oviposition preferences from larval feeding experience as described by Hopkins' host selection principal (HHSP) or through chemical legacy.
The antifeedant, oviposition deterrent, and toxic effects of individual dialkoxybenzene compounds/sets and of hydroxy- or alkoxy-substituted allylbenzenes, obtained through Claisen rearrangement of substituted allyloxybenzenes, were assessed against the cabbage looper, Trichoplusia ni , in laboratory bioassays. Most of the compounds/sets strongly deterred larval feeding, with some exhibiting mild toxic and oviposition deterrent effects as well. Some of the compounds/sets were more active than the commercial insect repellent, DEET (N,N-diethyl-m-toluamide), as both feeding and oviposition deterrents against the cabbage looper. On the basis of the obtained oviposition data a general hypothesis was proposed regarding the oviposition sites: one binding mode with the alkyl and allyl groups on the same side of the benzene ring resulted in deterrence, the other with alkyl and allyl groups on opposite sides of the benzene ring resulted in stimulation. The results suggest some structure-activity relationships useful in improving the efficacy of the compounds and designing new, nontoxic insect control agents for agriculture.
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