Response of the parasitoid Eupelmus vuilleti to the odours of the phytophagous host and its host plant in an olfactometer

Eupelmus vuilleti (CRW) is a solitary ectoparasitoid which, under our experimental conditions, developed at the expense of the larvae and pupae of Bruchidius atrolineatus (Pic) (Coleoptera, Bruchidae). The larvae of this beetle are found inside the seeds of cowpea, Vigna unguiculata (Walp). Using a tubular olfactometer, we determined the olfactory stimuli involved in the orientation of females of the hymenopterous species E. vuilleti towards one of its hosts; B. atrolineatus. Several odours were tested including those from host plants of B. atrolineatus, from non-host plants, from the host itself and from the waste products of the host. The odours of pods or dry seeds of V. unguiculata (host plant of the phytophagous insect) affected the locomotor behaviour of females of E. vuilleti. The odours from the plant in the vegetative stage had no effect. Kairomones from larvae, pupae or faeces of B. atrolineatus were attractive to the parasitoid females. Odours from adult male or female B. atrolineatus, as well as the sex pheromone emitted by females were not attractive to E. vuilleti females. This sex pheromone did however stimulate the locomotor activity of the parasitoid. The adaptive significance of these olfactory stimuli is discussed. Resume Reponse du parasitoide Eupelmus vuilleti aux odeurs de son hote phytophage et de la plante-hote dans un olfactometre Eupelmus vuilleti (CRW) est un ectoparasitoide solitaire qui se developpe dans nos conditions experimentales aux depens des larves et des nymphes de Bruchidius atrolineatus (Pic) (Coleoptere: Bruchidae). Les larves de ce Coleoptere se trouvent a l'interieur des graines d'une Legumineuse Vigna unguiculata (Walp). Grâce a un olfactometre tubulaire, nous avons determine les differents stimuli olfactifs intervenant dans l'orientation des femelles de cet hymenoptere, vers l'un de ses hotes; Bruchidius atrolineatus. Plusieurs odeurs emanant soit de plantes-hotes ou non-hotes de B. atrolineatus, soit de l'hote lui-meme ou resultant de son metabolisme ont ete testees. Les odeurs de gousses ou de graines seches de Vigna unguiculata Walp (plante-hote du phytophage) influencent le comportement locomoteur des femelles. Par contre, les odeurs de plantes au stade vegetatif n'ont pas d'effet. Les kairomones emanant de larves, nymphes ou feces de larves de B. atrolineatus exercent un effect attractif sur les femelles parasitoides. Les odeurs emanant d'adultes mâles ou femelles de B. atrolineatus ainsi que la pheromone sexuelle emise par les femelles ne sont pas attractives pour les femelles de E. vuilleti. Cette pheromone sexuelle, par contre, stimule l'activite locomotrice des parasitoides. La signification adaptative de ces differents stimuli olfactifs est discutee.