Isolation, identification and bioassay of plant chemicals mediating searching behavior by an insect parasitoid

Abstract

The parasitoid Campoletis sonorensis (Cameron) was used in studies to determine the influence of plants (the potential host habitat) on parasitoid searching behavior. Inferences regarding general host habitat location by parasitoids could be made from such studies. Laboratory studies showed the attraction of C. sonorensis (Hymenoptera: Ichneumonidae) to flowers and other parts of some common herbaceous plants. Some of the plants tested are food sources for the noctuid hosts of the parasitoid. Response varied significantly among plant species; some plants were attractive while others were not. Female parasitoids were also found to antennate and probe plant parts of certain species upon contact. Response was greater to flowers than to leaves or stems, and was also greater to damaged than undamaged leaves. Volatile and contact chemicals are involved in this attraction and these can be extracted with ethyl ether. Laboratory bioassays showed that C. sonorensis females were attracted to the following sesquiterpenes isolated from cotton essential oil: (alpha)-humulene, (gamma)-bisabolene, (beta)-caryophyllene oxide, spathulenol, (beta)-bisabolol and a naturally occurring bisabolene-like alcohol, isolated for the first time, which will be named gossonoral. This is also the first report of spathulenol in cotton. (beta)-caryophyllene, a major component of cotton, was not attractive to the parasitoids. The response of the parasitoids to these compounds is discussed. The possibility of augmenting parasitoid activity in the field by manipulation of plant secondary metabolites is considered. Gas chromatographic analysis revealed that synomones for C. sonorensis, previously identified in cotton, were not present in wheat germ diet-reared Heliothis virescens (F.) larvae. Diet-reared larvae fed cotton obtained the synomones from cotton with the consequence of enhanced kairomonal activity of the larvae and their frass to C. sonorensis. Parasitoids presented a choice between cotton, cotton plus hosts, hosts alone and control in an olfactometer, responded non-randomly, with the greatest number of responses to cotton plus hosts, and three times as many responses to cotton alone as to larvae alone. The role of the plant in the parasitoid-host relationship is discussed.