Biochemical signalling in the tri-trophic interaction between anacardium occidentale, pseudotkeraptus wayi and oecophyua longinoda
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Date
2014-08-19
Authors
Kung'u, Caroline Wanjiku
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Abstract
The relationship between Pseudotheraptus wayi (cashew pest), Anacardium occidentale (Cashew plant), and Oecophylla longinoda (natural enemy) is a tritrophic interaction that is very important in cashew nut production. However, the chemical ecology of the interaction, specifically between the cashew plant and natural enemy as well as the pest and natural enemy was little investigated. Based on the hypothesis that both interactions were mediated by chemical signals, the objective of this study was to identify these signals and establish their relevance to the species in question. Head space volatiles of the cashew plant, the pest and natural enemy were collected, analyzed in Gas chromatography-mass spectrometry (GC-MS), confirmed and quantified using authentic standards. Behavioral assays were then conducted to establish the relevance of the bio- chemicals. In the pestnatural enemy interaction, the volatiles identified were mainly C6 compounds (aldehyde, alcohol and the corresponding esters). Hexanal, hexanol, hexyl acetate and hexyl hexanoate were identified in the head space volatiles of the natural enemy as well as the pest. Moreover they were also present in the metathoracic glands of the pest which are primarily devoted to producing chemicals for defense purposes. Previously, these same compounds were shown to constitute the alarm pheromone of the natural enemy as well as defense chemicals of various insect species in the Coreidae family. In another study these four components were also suggested to constitute a candidate sex pheromone of the pest, corroborated in this study by the finding that, the onset and pattern of both mating and production of these chemicals coincide. In the host plant-natural enemy interaction, volatiles from plant parts most vulnerable to herbivory (leaves, fruit and inflorescence) were predominantly monoterpenes and sesquiterpenes. The attractiveness of the crude volatile extracts of the individual plant parts was measured as, total time spent by the natural enemy in either the test or control arm of the Y-tube olfactometer expressed as a percentage of the total time allocated .Crude volatile extracts from leaves and inflorescence were significantly attractive compared to the control (ttest, 0.=0.05) .. However when the extracts were presented together with a food reward (sugar syrup) the responses of the natural enemy were significantly enhanced (ANOVA, a = 0.05). These fmdings demonstrate that while cashew volatiles are attractive to the natural enemy, they are most relevant when paired with a food reward; a response that is indicative of associative learning. Mirroring, these fmdings to the natural context suggests that, the natural enemy relies on cashew plant volatiles as predictors for food rewards. The rewards are usually nectar from extra floral nectaries located mainly on plant parts vulnerable to herbivory. Therefore, the rewards not only benefit the natural enemy but also confer an advantage to the plant, by motivating the natural enemy to spend more time around the vulnerable plant parts, effectively deterring herbivory. The wider implication is that the chemical signals identified in the interactions of the natural enemy with both the pest and plant can potentially be exploited for improved crop
protection,
Description
Department of Biochemistry and Biotechnology, 85p. 2014, QP 517 .C45K8