Behavioral responses of tuta absoluta to a wild and cultivated tomato plants and characterization of the mediating semiochemical blends
Miano, Raphael Njurai
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Tomato is rated the second most important horticultural crop after potato in most parts of the world. However, its cultivation is threatened by infestations of Tuta absoluta (Lepidoptera: Gelechiidae). The pest originated from South America and is now invading fields and greenhouse production sites in the world. Tuta absoluta was first officially reported in Kenya in March 2014 at Isiolo and has spread to all parts where tomato is grown. The pest has been nicknamed tomato ‗Al–shabaab‘ as it leaves unimaginable damage of the crop after infestations. Chemical methods used to control the pest have led to high levels of residues, hence risking consumers, harming the ecology and the environment. The present study was based on field observations that a wild tomato, Lycopersicon esculentum var. cerasiforme, which grows in the tea zones of Mount Kenya region, Kenya, is not attacked by T. absoluta, unlike the cultivated commercial tomato varieties. It was hypothesized that the wild variety may be actively avoided by gravid females because of the presence of constituents that deter gravid T. absoluta females. The objective of the present study was to compare the behavioral responses of T. absoluta to wild and cultivated tomato plants and characterize their mediating semiochemical blends. The responses of gravid T. absoluta females to the wild tomato and cultivated tomato, Solanum lycopersicum L. (Rambo F1 variety) intact plants in a dual–choice olfactometer was conducted where the gravid females were attracted to the cultivated species but repelled by the wild species, PI = -45.45%, X2 = 10.47, df = 1, p < 0.05. The levels of infestation of the pest in mono–crop and intercrops of the two varieties were also compared. There was significant reduction in the levels of infestation in the intercrop arrangements (P<0.001, at α=0.05). Gas chromatography–linked mass spectrometry (GC–MS) of the headspace volatiles collected from the two tomato species revealed large differences in their chemical profiles. A total of 162 compounds were positively identified and quantified: 85 from cultivated tomato‘s day volatiles, 73 from wild tomato‘s day volatiles, 68 from cultivated tomato‘s night volatiles and 64 from wild tomato‘s night volatiles. Principle component analysis (PCA) resolved the compounds into 12 distinct principle component (PC) clusters. Of these clusters, PC1 and PC2 captured over 79.0% of the total variation. MANOVA and ANOVA tests on PC1 and PC2 revealed that there were significant differences in the volatile compositions, P < 0.00001, α = 0.05. Gas chromatography–linked electroantennography (GC–EAD) showed a large proportion of electroantennography (EAG)–active compounds from the two species of tomato plants. Of these, hexanal, trans-3-hexenol, verbenene, 4-keto-isophorone, camphor, citronellal, isopulegol, limonene oxide, linalool propanoate, germacrene A, β-elemene, germacrene B, germacrene D, and β-bisabolene were unique to the wild tomato. A blend of available compounds, at the time of study, (trans–3–hexenol, camphor, citronellal and limonene oxide) showed dose-dependent repellence to gravid T. absoluta females in the dual–choice olfactometer. The study lays down some groundwork for exploiting semiochemical traits of the tomato species in novel management of T. absoluta.