Host interaction and regulation in parasitoid wasps  (Page 4/7)

Examination of phylogenetic data and ecological constraints on parasitoid wasps reveals three main points. The first is that ectoparasitic idiobionts provided the original framework for parasitoid lifestyle from which other developmental strategies evolved. The second is that endoparasitoids developed physiological responses to overcome host immune and developmental factors, which allowed for greater specialization in endoparasitoid wasps. Finally, ecological factors such as host mortality risks have affected developmental traits such as offspring development times and parasitoid fecundities (Pennacchio and Strand, 2006).

Host location and oviposition

Once an adult parasitoid wasp emerges from its host, it must have an efficient way of finding its next host. Parasitoid wasps respond to a series cues to locate their next potential host. The adult parasitoid is able to determine the general area of their host using various environmental cues, namely temperature, shade or humidity, and semiochemicals associated with plants or other organisms in the place where the host might occur. Thus, adult parasitoids respond to either physical or chemical stimuli associated with the host’s habitat (Gauld and Bolton 1988). This response has been observed in numerous studies of braconids. The braconid Cardiochiles nigriceps , which parasitizes budworms, have been shown to respond to lighting cues in order to determine their host’s location. In an experiment conducted by Vinson, C. nigriceps eagerly searched for tobacco plants in sunlit places but rarely visited the same plants if they were located in heavily shaded areas (Vinson 1975).

The majority of parasitoid wasps locate hosts by responding to particular chemical stimuli, such as chemicals emitted from the host’s food source. For example, Read and colleagues found that the aphidiine braconid Diaretiella rapae , which parasitizes the cabbage aphid, is attracted to the odor of brassicas, the aphid’s food source (Read et al 1970). As a result of parasitoid reliance on physical or chemical cues, hosts that feed on relatively unusual foodplants can escape parasitism. Salt examined the ability of the parasitoid Cotesia glomerata to locate and attack the larvae of their host, the cabbage butterfly Pieris brassicae , based on the host’s source of food . C. glomerata easily located and parasitized pierid larvae when they fed on their normal foodplant, cabbage. However, the pierid larvae were able to escape parasitism by this parasitoid wasp if the larvae developed on sea rocket or on capers (Salt 1958).

Once a female parasitoid has located a potential host, she chooses whether or not to lay eggs on or inside the host using host-associated semiochemical cues. She initially examines the host by doing a simple physical exam using her antennae. As a result, the size, shape and surface texture of the prospective host are important characteristics involved in eliciting an oviposition response. Many females then probe the prospective host with their ovipositor and use semiochemical cues to determine that the correct host has been located prior to oviposition (Gauld and Bolton 1988).