Studies on the nymphal aggregation pheromone of Malagasy Migratory Locust, Locusta migratoria capito (Saussure, 1884) and its effects on adult maturation

Abstract

The Malagasy migratory locust, Locusta migratoria capito (Saussure, 1884), is the mostdestructive pest in the Malagasy agricultural production system. The recent plaguebetween 1996 and 2000 during which economic losses amounting to ca. US$ 50 millionmainly in rice fields, were recorded, is an indication that, the locust menaces is far frombeing resolved in Madagascar. Control operations against locusts have focused entirelyon large-scale application of synthetic chemical insecticides. Given that Madagascar hasa unique and rich biodiversity; such spraying has negative effects on the environmentand non-target beneficiary organisms.Recent research on the desert locust, Schistocerca gregaria (Forskäl) by the icipe teamfor over the last fifteen years has revealed the importance of pheromonal mediation inthe aggregation behaviour of adult and nymphal stages and in gregarizationsolitarizationdynamics. The aim of this study was to provide basic information on therole played by volatiles in the aggregation of nymphs of L. m. capito and to explorepossible applications of the chemical constituents for preventive control.The aggregation response of fifth instar nymphs to their own volatiles was investigatedusing a single-chamber olfactometer. When locust nymphs where reared together withtheir adult conspecifics, they aggregated weakly to their own volatiles. However, whenthe male and female nymphs were tested separately, nymphs of each sex respondednegatively to the volatiles. Nymphs reared separately from adult locusts showed strongavoidance to their own volatiles. The same pattern of responses was observed for thebody extract.To identify the constituents, volatiles were collected using Super-Q and analysed usingGas Chromatography (GC), coupled Gas Chromatography-Mass Spectrometry (GCMS),and coupled Gas Chromatography-ElectroAntennographic Detection (GC-EAD).The results revealed the presence of the following electrophysiologically activecompounds: 2,3-butanediol, hexanal, benzyl alcohol, nonanal, phenylacetonitrile andtwo unidentified compounds from live nymphs; 2,3-butanediol, anisole, guaiacol,phenylacetonitrile, beta ionone and one unidentified compound were identified fromvolatiles derived from the faeces. There were no sexual differences in the production ofvolatiles, both qualitative and quantitative.The identified compounds were further used in bioassays to assess the aggregationresponses of the nymphs. The results showed that, the synthetic full blend elicited thesame response as the crude volatiles from live nymphs and fresh faeces. However,exclusion of each compound at a time from the full blend modified the response of thenymphs from avoidance to attraction.In another experiment, the effect of the nymphs on the sexual maturation of young adultswas evaluated using a double-storey bioassay and aluminium standard cages. The resultsshowed significant delay in both mating and ovipositing time when young adults wereexposed to nymphs in the presence of visual, tactile and olfactory cues. In contrast, thenymphal volatiles alone did not cause retardation of sexual maturation of young adultswith regard to the above parameters. The results are discussed in terms of the relativeconcentration of the compounds in the volatiles