Honeydew as a contact kairomone for aphid parasitoids

A suspected chemical means of habitat selection byDiaeretiella rapae, a primary parasite of aphids on crucifer plants, was investigated in the laboratory with the aid of an olfactometer. Females (and, to a lesser extent, males) were found to be attracted by odor to leaves and leaf juices of a crucifer host plant and to dilute solutions of the mustard oil allyl isothiocyanate. Given a choice of aphid colonies (Myzus persicae) on leaves of beet (Chenopodiaceae — mustard oils not present) and collard (Cruciferae — mustard oils present), femaleD.rapaedisplayed a marked preference to oviposit in aphids feeding on collard. It is concluded that femaleD.rapaeparasites locate their hosts initially by a response to an odor emanating from the aphid host-plant, followed by visual searching. MaleD.rapaewere attracted by odor from femaleD.rapae, suggesting the existence of a sex pheromone in this species.Neither males nor females ofCharips brassicae, a hyperparasite ofD.rapae, were attracted to collard leaves, though females showed apparent attraction to femaleD.rapae. As withD.rapae, males ofC.brassicaeare attracted by odor to females of the same species.The host finding behavior ofD.rapaeis discussed in relation to that of other insect species belonging to the fauna associated with the plant family Cruciferae.