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The effects of feeding experience with antifeedants on larval feeding and adult oviposition behavior in generalist and specialist herbivores Akhtar, Yasmin

Abstract

The overall objective of this thesis was to assess the effects of larval experience with antifeedants upon feeding preference of larvae and oviposition preference of adults in generalist (Trichoplusia ni, Lepidoptera: Noctuidae) and specialist herbivores (Pseudaletia unipuncta, Lepidoptera: Noctuidae; Plutella xylostella, Lepidoptera: Plutellidae; and Epilachna varivestis, Coleoptera: Coccinellidae). Selection of plant extracts and pure allelochemicals used in the experiments was based on their growth inhibiting and antifeedant properties against the test species. Initial screening showed that an extract of Melia volkensii (Meliaceae) was the most effective growth inhibitor among all the antifeedants for both T. ni and P. unipuncta. It also acted as a strong antifeedant to T. ni, P. unipuncta, and E. varivestis (DC50 values {deterrency concentration causing 50% feeding deterrency compared to the control} = 8.3, 10.5, and 2.3ug/cm2 respectively). The effects of feeding experience with antifeedants on subsequent feeding preference showed that all instars of T. ni tested (second, third or fifth) exhibited a decreased feeding deterrent response to most of the antifeedants tested (M. volkensii, M. azedarach (Meliaceae), Origanum vulgare (Lamiaceae) and pure allelochemicals; xanthotoxin, toosendanin and thymol), following prolonged exposure. Cardenolides (digitoxin and cymarin) were the exceptions. Xanthotoxin, acting as a noxious stimulus, dishabituated (reversed) the decreased antifeedant response to M. volkensii. Feeding responses of specialist insects showed interspecific differences. Neither P. unipuncta nor P. xylostella showed a significant decrease in feeding deterrent response to M. volkensii in either choice or no-choice tests. However, there was a decrease in feeding deterrent response following prolonged exposure to M. volkensii by P. xylostella in no-choice test. In contrast, both species showed a significant decrease in feeding deterrent response to a pure allelochemical, thymol. Epilachna varivestis showed a decrease in feeding deterrent response to O. vulgare and thymol following prolonged exposure. Trichoplusia ni larvae also showed a generalization in feeding deterrent response to unrelated antifeedants following prolonged exposure in some instances. There was a significant decrease in feeding deterrent response to O. vulgare in larvae with previous exposure to M. volkensii extract and vice versa. Further investigation of feeding responses of T. ni larvae showed that there was a decrease in feeding deterrent response following prolonged exposure to plant extracts or pure allelochemicals when presented singly, but not to binary mixtures. Larval feeding experience influenced the oviposition behaviour of the adult moths. Comparison of ODIs (oviposition deterrence indices) of experienced and naive moths showed that there was a significant decrease in oviposition deterrent response by the experienced moths. The weight of F1 larvae from experienced female moths on the treated plants suggested that there was a positive correlation between larval growth performance and adult moth choice.

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