Olfactory And Oviposition Responses Of Two Specialist Herbivores Towards Volatiles Emitted Form Non-host Plants

Plant volatiles are the most important semiochemiccals in mediating the interactions between plants and phytophagous insects. Some chemicals emitted from non-host plants have been demonstrated to be repellent to foraging adults of insect herbivores, and show potential value for behavioural manipulation in integrated pest management. However, a few insects have been shown to display habituation or/and experience-induced preference for repellents derived from non-host plants. To help understand the chemical and behavioural mechanisms of the experience-altered behaviour of phytophagous insects towards non-host chemicals and non-host plants, we used laboratory experiments to examine the effects of experience of chemicals or crude extracts derived from non-host plants, Solanum lycopersicum and Chrysanthemum morifolium, on the olfactory and oviposition behaviour of two specialist herbivores, the diamondback moth Plutella xylostella and the melon worm Diaphania indica. The results are summarized as follows:(1) Naive females of P. xylostella and D. indica showed a significant aversion to the volatiles emitted from tomato plant, and D. indica was significantly repelled by the odors emanating from C. morifolium. However, when the ovipositing females were given a brief experience of the two non-host plants, they were not repelled any more, and they exhibited a habituation or an induced preference towards the volatiles emitted from the two non-host plants.(2) Neither naive P. xylostell and D. indica females nor the females with prior experience of non-host plant accepted tomato for oviposition. The experience of C. morifolium could induce D. indica to lay few eggs on this plant.(3) Tomato extract showed a significant repellency against naive females of P. xylostella, however the females of P. xylostella displayed a habituation to the repellent when provided with a brief experience of tomato extract. When tomato extract was applied on cabbage plants, naive females of P. xylostella were attracted by the treated cabbage, and laid more eggs on the treated cabbage than on control plants. Diaphania indica showed a significant aversion response to the tomato extract and chrysanthemum extract. However, when D. indica females were given a brief experience of the tomato/chrysanthemum extract, they were no longer repelled, instead they were attracted by the non-host extract or the non-host extract-treated host plants.(4) The combined results of olfactory and oviposition bioassays demonstrated thatβ-caryophyllene, (-)-camphor and cabbage plants treated with this two chemicals respectively showed significant repellency against naive females of P. xylostella. However when P. xylostella females had an experience of the two chemicals or the chemical-treated cabbage respectively, they displayed an induced preference for the repellents. Diaphania indica showed a significant aversion towards a-humulene or (-)-camphor or (-)-camphor-treated cucumber plants, however, the experience of the repellent chemicals or chemical-treated host plants could induced D. indica to display a preference to the repellents. Naive D. indica females were attracted by the a-humulene-treated cucumber and laid more eggs on the treated host plants than on the untreated host plants.(5) The mixture of P-caryophyllene and (-)-camphor showed a significant repellency to naive P. xylostella females, and a brief experience of this mixture did not significantly affect the olfactory and oviposition behaviour of the females. The experienced females still exhibited an aversion response towards the mixture. Unexperienced D. indica females were significantly repelled by the mixture of a-humulene and (-)-camphor, however, when they were given a brief experience of the mixture, they displayed a habituation for the repellents, and were not repelled any more.(6) Naive P. xylostella females were significantly attracted by allyl isothiocyanate, and their olfactory responses towards this chemical were not significantly affected by the experience of allyl isothiocyanate. a-humulene showed neither attractance nor repellency to naiveP. xylostella females, and the experience of this chemical had no influence on the olfactory responses of the females.(7) A total of 25 compounds emitted from tomato plant were tentatively identified by three methods including simultaneous distillation extract (SDE), dynamic headspace trapping (DHT), and headspace solid phase micro-extraction (HS-SPME) coupled with GC-MS. Thirty-eight volatile compounds emanating from C. morifolium were identified using DHT coupled with GC-MS. These results provided important basic information for the study of the volatiles emitted from tomato and chrysanthemum plants naturally in the interactions between plants and phytophagous herbivores.For the first time, this study demonstrated that experience could induce habituation and preference in P. xylostella and D. indica for the repellents derived from non-host plant, S. lycopersicum or C. morifolium. We found that experience-induced preference for preciously repellent compounds was a major mechanism that leads to behavioural change of ovipositing females towards non-host plants or their extracts and that increased acceptance of non-host plants or their extracts following experience did not have to involve initially attractive compound. We also found that a mixture of different repellent chemicals could help reduce the occurrence of habituation or induced-preference by insect herbivores for the repellents. These results provided important basic information of insect learning for repellents and are helpful for effective deployments of repellents derived from non-host plants in behavioral manipulation of insect pests.