Studies on the aggregation pheromone of adult Malagasy migratory locust, Locusta migratoria capito (Saussure, 1884) and its effects on conspecific nymphal stages

Abstract

Locusta migratoria capito is the major agricultural pest to cereal production inMadagascar. Unlike other members of this species (e.g. L. migratoria migratorioides and L. migratoria manilensis), hardly any study has been done on its chemical ecology. This project sought to characterize the aggregation pheromone of the adult L. m. capito and to assess the role of its components on the behaviour of conspecifics under laboratory conditions. To assess responses of adult locusts to their own volatiles, bioassays were conducted in a single chamber olfactometer that allowed for dual-choice between air, enriched with volatiles vs. clean air. To identify the composition of the volatiles, emanations from the locusts were collected using Super-Q adsorbent and then analysed using Gas chromatography (GC), coupled Gas chromatography-electroantennographic detection (GC-EAD) and coupled Gas chromatography-mass spectrometric (GC-MS) techniques. The identified electroantennographically-active components were assessed for their behavioural and solitarizing effects. Among the identified compounds, phenylacetonitrile (PAN) was further tested for its role in aggregation and anemotaxis on fifth instars nymphs and adults of the Madagascar locust, L. m. capito.Like in the other locust species that have been studied previously, olfactory cues from body and faecal volatiles might play a role in modulating the aggregation behaviour of L. m. capito. Body volatiles from mature female locusts elicited moderate (A. I = 29 ± 9%) aggregation responses in both sexes of the adults, whereas only mature males significantly responded to their own volatiles with the highest aggregation index (A. I = 48 ± 9%) recorded in the tests. With regard to faecal volatiles, only the immature adults of L. m. capito showed aggregation responses to the volatiles while the sexually mature adults were repelled.In chromatographic profiles, no qualitative compositional differences were observed in the body volatile that were produced by mature male and female locusts. However,there were quantitative differences between amounts of components in volatiles from locusts of the two sexes. This was more evident in the production of a high amount of phenylacetonitrile in mature males compared to their female counterparts (0.38 ±0.04 ng/male vs. 0.04 ± 0.003 ng/female).In GC-EAD and GC-MS analyses, a total of nine electroantennographically active compounds (3-penten-1-ol, anisole, benzyl alcohol, veratrole, n-decanoic acid, (Z) 6-pentadecen-1-ol, palmitic acid, (Z) 9-octadecanoic acid methyl ester and anunidentified C27 or C28) were identified in the body volatiles of the adult locusts while five compounds (anisole, benzyl alcohol, guaiacol and beta-ionone and an unidentified compound) were identified in their faecal volatiles. Those from the body comprised of nine compounds of which five were of relatively high volatility and four of high molecular weights. The high volatility compounds included anisole, benzyl alcohol, veratrole and other two compounds whose identity was not fully confirmed (3-penten-1-ol and n-decanoic acid), whereas among the heavy ones ((Z) 6-pentadecen-1-ol, palmitic acid, (Z) 9-octadecanoic acid methyl ester and an unidentified C27 or C28), only palmitic acid was fully confirmed. For the body volatiles, female locusts were found to release about twice the amount of highly volatile compounds as their male conspecifics while the heavy compounds were produced in higher amounts by the mature males. There were five components in faecal of which four were identified to be anisole, benzyl alcohol, guaiacol and betaionone. The remaining compound was not identified.Aggregation bioassays using synthetic standards of the identified single compounds showed that all the compounds tested except benzyl alcohol elicited varying levels of aggregation responses in male and female adult L. m. capito. Blends of these synthetic compounds elicited similar patterns of responses as obtained previously with the crude body and faecal odours eliciting slightly lower responses. These compounds were also tested for their primer effects on phase shifts in conspecific nymphal locust. Exposure of fifth instar nymph to each of the compounds, with the exception of benzyl alcohol and palmitic acid, and also to blends of the identified components, induced the appearance of solitarious locust characters with regard to colour and morphometric changes in the emerging fledglings. Phenylacetonitrile tested at various concentrations elicited no clear pattern in aggregation and anemotactic responses of fifth instars nymphs and adults, L. m. capito, as opposed to the desert locust in which it plays a key role as an aggregant for adults. Further investigations to determine the precise role played by this compound in the biology and behaviour of this locust are strongly recommended prior to its use as a behaviour modifying agent for its management.In locust control, the use of pheromone by controlling the aggregation of locustsappears to be an important strategy for the integrated locust management. This would target the reduction of expansiveness of the areas infested with hoppers and swarms of adult locusts. Consequently, this would lead to minimizing of the quantities of deleterious chemical insecticide that are sprayed an thus, reduction of yield losses of agricultural products. Results of this study provide an insight into the understanding of the chemical ecology of L. m. capito and are also a contribution to the search for an environmentally safe pheromone-based strategy for locust management.