DIFFERENCES IN SEX-PHEROMONE COMMUNICATION-SYSTEMS OF CLOSELY-RELATED SPECIES - SPODOPTERA LATIFASCIA (WALKER) AND S-DESCOINSI LALANNECASSOU AND SILVAIN (LEPIDOPTERA, NOCTUIDAE)

S. latifascia and S. descoinsi are closely related species that occur sympatrically over limited areas in French Guiana. We examined allopatric populations, S. latifascia originating from Barbados and S. descoinsi from French Guiana. Studies on nocturnal activity cycles showed temporal partitioning of female calling behavior, male sexual activity, and mating behavior. S. descoinsi were sexually active in the first half of the scotophase whereas S. latifascia were sexually active in the second half. Seven compounds (Z9-14: Ac, Z9,E12-14: Ac, Z11-16: Ac, E9,E12-14: Ac, Z9-14: Aid, Z9,E11-14:Ac and Z11-14:Ac) were identified in females of both S. latifascia and S. descoinsi extracts. Z9-14:Ac was a main pheromone component for the two species. The major difference between the pheromones of S. latifascia and S. descoinsi was the proportion of Z9,E12-14:Ac in the extracts: 7% for S. latifascia and 42% for S. descoinsi. The proportion of Z9,E12-14:Ac relative to the sum of Z9-14:Ac and Z9,E12-14:Ac in individual gland extracts was 4 +/- 1% (mean +/- standard deviation) for S. latifascia and 44.8 +/- 6% for S. descoinsi. Electrophysiological studies showed no major differences between species in the morphology and physiology of the pheromone receptors of males. Receptors were identified for Z9-14:Ac and Z9,E12-14:Ac, but no receptor was found for the other compounds. In the wind tunnel, synthetic blends with Z9-14:Ac and Z9,E12-14:Ac gave the same behavioral responses as conspecific female extracts for the males of the two species. Some cross-attraction was observed with synthetic blends and female extracts. Nethertheless, previous field trapping experiments in French Guiana were species-specific and suggested differences in the attractivity of males. In the laboratory, S. latifascia and S. descoinsi could hybridize in both reciprocal crosses. Female S. descoinsi x male S. latifascia mating rate was significantly lower than for the reciprocal cross, and 26.7% of female S. descoinsi could not separate from male S. latifascia after mating. These copulatory problems may involve genital incompatibilities between males and females. Several barriers against interbreeding between S. latifascia and S. descoinsi seem to combine including differences in nocturnal activity cycles, pheromone differences; and genital barriers. The study of sympatric populations will be necessary to define the role of sex pheromones in the reproductive isolation of S. latifascia and S. descoinsi.