Influence of host origin on host choice of the parasitoid Dinarmus basalis: does upbringing influence choices later in life?
The aim of this study was to investigate the influence of volatile compounds from four secondary host plants on the ability of Dinarmus basalis Rond. (Hymenoptera: Pteromalidae) to locate, recognize, and parasitize its host, 4th instar larvae or pupae of Callosobruchus maculatus F. (Coleoptera: Chrysomelidae). To examine this, strains of D. basalis were transferred from cowpea seeds (Vigna unguiculata (L.) Walp. (Fabales: Fabaceae)) to pigeon pea (Cajanus cajan (L.) Millsp.) and two varieties of Bambara groundnut (Vigna subterranea (L.) Verdc.) seeds. The ability of D. basalis females to recognize the volatile compounds emanating from their complex host plant was tested by using a Y-tube olfactometer and a three-dimensional device. The results suggest that when females have a choice between pure air and the air emanating from their complex host of origin, they are attracted to the air tainted by the volatile compounds they have become accustomed to. They spent significantly more time (p<0.0001) in the branch of the tube leading to the odorous air than in the tube leading to the pure air. When females from pigeon pea seed hosts were offered a choice between cowpea and pigeon pea seeds, all containing 4th instar larvae, the familiar odor of pigeon pea seeds were most attractive. When females from Bambara groundnut (white and striped) seed hosts were offered a choice between cowpea and pigeon pea seeds, all containing 4th instar larvae, they were significantly attracted to the odour of cowpea seeds. In the three-dimensional system, the females from the four strains did not appear to have any preference for a given type of seed containing 4th instar larvae or pupae. The parasitism rate remained high on all four types of seeds used. These results show that the use of D. basalis as a biological control agent is possible in host changing situations where C. maculatus starts to attack other legumes. The results of this study also provide information supporting the behavioral plasticity of D. basalis. Understanding the mechanisms involved in the adaptive phenomena of biological control agents is discussed in the context of the development of adequate methods of pest control.