Behavioral Responses of the Melon Fly, Zeugodacus curcubitae (Conquillett, 1849) (Diptera: Tephritidae) to Host Plant Volatile Organic Compounds

Abstract

The Melon fly, Zeugodacus cucurbitae (Coquillett, 1849) (Diptera: Tephritidae), is a major pest of Cucurbitaceae but relevant field observations suggest that Solanaceous plants such as tomato have also become a major host of the pest. Solanaceous plants are highly susceptible to Z. curcubitae damage which may range from 30-100% globally depending on the season. Management of this pest in the past has focused mainly on the application of synthetic chemical insecticides which have resulted in negative effects on the environment and non-target beneficial organisms. Non-chemical control options such as fruit bagging are also employed but are labor intensive and/or expensive to small scale farmers. The aim of this study therefore was to compare attraction levels of Z. cucurbitae toward tomato varieties and compare volatile organic compounds (VOCs) produced by most attractive tomato variety with those of the main host, cucumber in order to understand the chemical basis of host shift in the pest. Behavioral responses of sexually mature and immature male and female Z. curcubitae to VOCs from three tomato varieties viz. Moneymaker (MM), Anna F1and Cal-J were investigated using a dual choice olfactometer. Experimental insects (immature and mature male and female Z. curcubitae) and plants (vegetative and flowering Moneymaker, Anna F1, and Cal-J tomato varieties and Ashley Cucumber) were used. Volatiles were collected from the potted plants using super-Q, eluted using nitrogen gas under ice, and subsequently identified based on their mass spectral data and authentic standards using a Gas Chromatography- Mass spectrometer (GC-MS) with helium as a carrier gas. Antennal responses of immature and mature male and female flies to host plant VOCs were evaluated using Gas Chromatography-Electroantennographic Detection (GC-EAD). Results from olfactometer assays showed that both sexes of immature and mature Z. curcubitae were attracted to all varieties of tomato with Cal J being the most attractive in pairwise comparisons. The results further showed that there was no significant difference in attraction of Z. cucurbitae to tomato (Cal J) and cucumber. The results for GC-MS analysis showed similarities among VOCs released by the three tomato varieties and cucumber (variety Ashley). About 11 electro physiologically active compounds from the three tomato varieties were revealed in the GC-EAD results. A comparison of cucumber and Cal J tomato variety revealed seven active compounds which were among the shared VOCs. The results suggest that there exists host plant variety discrimination in attraction hence odor perception is the key for selection of most suitable host plant variety. Results further showed qualitative and quantitative differences among VOCs released by Anna F1, Cal-J and MM tomato varieties in vegetative and flowering stages of growth. This suggests that Cal J tomato variety can be highly susceptible to Z. cucurbitae infestation than the other two varieties in monoculture farming of the three tomato varieties. In conclusion, shared volatiles between tomato plant (Solanaceous) and Ashley cucumber plants (Cucurbitaceous) have made tomato plant become major hosts of Z. cucurbitae. A similarity of EAD active compounds among tomato and cucumber plant profiles may explain the preference of Z. cucurbitae to tomato plants.