Parasitoid

Abstract: The extent to which an organism is selected to invest in defences against pathogens and parasites depends on the advantages that ensue should infection occur, but also on the costs of maintaining defences in the absence of infection. The presence of heritable variation in resistance suggests that costs exist, but we know very little about the nature or magnitude of these costs in natural populations of animals1. A powerful technique for identifying trade-offs between fitness components is the study of correlated responses to artificial selection2,3. We have selected Drosophila melanogaster for improved resistance against an endoparasitoid, Asobara tabida. Endoparasitoids are insects whose larvae develop internally within the body of other insects, eventually killing them, although their hosts can sometimes survive attack by mounting a cellular immune response4,5,6. We found that reduced larval competitive ability in unparasitized D. melanogaster is a correlated response to artificial selection for improved resistance against A. tabida. The strength of selection for competitive ability and parasitoid resistance is likely to vary temporally and spatially, which may explain the observed heritable variation in resistance.

Abstract: A parasitoid’s decision to reject or accept a potential host is fundamental to its fitness. Superparasitism, in which more than one egg of a given parasitoid species can deposit in a single host, is usually considered sub-optimal in systems where the host is able to support the development of only a single parasitoid. It follows that selection pressure may drive the capacity for parasitoids to recognize parasitized hosts, especially if there is a fitness cost of superparasitism. Here, we used microsatellite studies of two distinct populations of Cotesia vestalis to demonstrate that an egg laid into a diamondback moth (Plutella xylostella) larva that was parasitized by a conspecific parasitoid 10 min, 2 or 6 h previously was as likely to develop and emerge successfully as was the first-laid egg. Consistent with this, a naive parasitoid encountering its first host was equally likely to accept a healthy larva as one parasitized 10 min prior, though handling time of parasitized hosts was extended. For second and third host encounters, parasitized hosts were less readily accepted than healthy larvae. If 12 h elapsed between parasitism events, the second-laid egg was much less likely to develop. Discrimination between parasitized and healthy hosts was evident when females were allowed physical contact with hosts, and healthy hosts were rendered less acceptable by manual injection of parasitoid venom into their hemolymph. Collectively, these results show a limited capacity to discriminate parasitized from healthy larvae despite a viability cost associated with failing to avoid superparasitism.

Abstract: The parasitic mode of life is very productive and has evolved in numerous orders of arthropods. Many parasitic insects differ from other parasitic organisms in that the adults of parasitic insects are free living, a limited number of progeny develop per host, and the host does not survive the encounter. These parasitic insects have been referred to as parasitoids to distinguish them from other parasitic organisms in which the adults and their progeny are parasitic, many individuals of different generations may occur on the same host, and the host is usually not killed. Unlike the parasite-host relationship, the evolution of the parasitoid-host relationship is of little consequence for the host, which does not survive the association to pass on its genetic information (203). The coevolution of the parasitoid and host has resulted in a number of unique associations. There are a number of steps necessary for the success of the parasitoid­ host relationship. Doutt (44) divided the events into four steps: (a) host habitat location, (b ) host location, (c) host acceptance, and (d ) host suit­ ability. Vinson (203) added a fifth step, referred to as host regulation. Whether a parasitoid is able to develop within a host may depend on the ability or inability of the parasitoid to regulate the host's physiology. The