PHYTOCHEMICAL ATTRACTANTS AND REPELLENTS FROM SELECTED KENYAN VARIETIES OF MANGO (Mangifera indica L.) AS CONTROL TOOLS FOR MANGO FRUIT FLY, Bactrocera invadens (DIPTERA: TEPHRITIDAE)

Abstract

Infestation of mangoes (Mangifera indica L.) by the mango fruit fly, Bactocera invadens (Drew, Tsuruta and White) is a big threat to mango production in Kenya. The pest infestation leads to poor quality fruits that cannot fetch good prices both locally and internationally. This study was carried conducted at Cheptebo in Kenya where observations in the mango orchard revealed variation in the infestation of the fruit fly among different mango varieties. The aim of this study was to identify the plant host chemical cues that possess the attractive or repellent effects to the fruit fly. The essential oils were extracted from six mango varieties: Ngowe, Apple, Keitt, Boribo, Tommy Atkins and Van Dyke by hydro distillation using a modified Clevenger-like apparatus. Volatiles from the six varieties together with Kent and Sabre were collected from the fruit juices on porapak Q adsorbent filters (mesh 40-80μm) using air entrainment kit. The oils and the volatiles were analyzed through gas chromatograph fitted with a flame ionization detector and coupled to a mass spectrometer. It was found that the chemical profile of all the mango varieties were qualitatively and quantitatively different. The oils were rich in monoterpenes and sesquiterpenes with minor quantities of their analogues. Trace amounts of nonterpenoid and oxygenated hydrocarbons were also identified. Among the identified compounds, δ-3-carene was most dominant in the leaf mango oils of Keitt, Tommy Atkins and Van Dyke. Five compounds namely; α-pinene, β-pinene, α-copaene, α-gurjunene and α-humulene were present in significant but varying amounts in all the oils of the six varieties. The aroma components of the eight mango volatiles were classified on the basis of their richness in -pinene, myrcene and δ-3-carene as the dominant compounds. The compounds that were selected for bioassay were those that were major both in the oil and the volatile profiles. The fruit flies were subjected to crude essential oils, fruit juices and blends of chemical standards comprising esters (ethylbutanoate, ethyl hexanoate ethyloctanoate and methyl salicylate), nonoxygenated monoterpenes (myrcene, δ-3-carene and α-pinene) as lures in both Laboratory Dual choice olfactometric and field bioassays. The bioassay of the individual compounds was also done in the field. Methyl eugenol was used as positive control. There was no significant difference (P>0.05) in the mean number of flies (4.75±0.25) lured by the blend of myrcene, α-pinene, and ethyl butanoate and the positive control (5.25±0.25) of methyl eugenol in the laboratory test. In the field bioassay there was no significant difference (P>0.05) in the attractiveness of Keitt juice volatiles (mean catch of 4.571±1.445) and the positive control (mean catch of 5.857±1.724) of fruit flies. In terms of individual components, Boribo oil, fruit juices and ethyl butanoate exhibited attractive effects while Ngowe oil and methyl salicylate showed repellent effects. The individual pure compounds were not attractive to the fruit fly in the field bioassay.This study shows that mango contains volatile compounds that can be exploited as natural, environmentally friendly lures for the fruit fly and the compounds can only be effective when used as a blend.