Intra- and interspecific host discrimination by host-seeking larvae of coleopteran parasitoids

Intraspecific host discrimination is widespread in solitary parasitoids who se adult females forage for and evaluate host suitability, whereas interspe cific discrimination is less common. In some parasitoid species, mostly Dip tera and Coleoptera, the larva performs the last step of host searching. It has been suggested that host discrimination will rarely occur in such host -seeking larvae because their low mobility results in a low host encounter rate. We determined the extent to which the larvae of Aleochara bilineata G yllenhal (Coleoptera: Staphylinidae), a solitary parasitoid of aggregated D iptera pupae: (1) discriminated between unparasitized hosts and hosts paras itized by conspecifics; (2) used semiochemical cues to discriminate; (3) we re influenced by life expectancy, presence of conspecifics and host availab ility in their host acceptance decision; and the extent to which (4) A. bil ineata and A. bipustulata L., a species exploiting the same hosts and occur ring sympatrically, showed interspecific host discrimination. A. bilineata larvae were able to discriminate between unparasitized hosts and hosts para sitized by conspecifics in a choice experiment. Such behavior has never pre viously been described for a coleopteran parasitoid or for a parasitoid spe cies whose larvae perform host searching. Host discrimination in this speci es was not based on the presence of visual or tactile cues (e.g., entrance holes) but rather on chemical cues. The life expectancy of A. bilineata lar vae was significantly shorter in the presence than in absence of hosts, and older larvae had lower parasitism success than young larvae in a 24-h expe riment. However, the host acceptance decision of A. bilineata larvae was no t influenced by larval age or the presence of conspecifics when the ratio o f hosts per larva was greater than or equal to 1. When hosts were scarce, t he degree of superparasitism increased significantly with the number of for aging conspecifics and the age of the larvae. Both species of Aleochara sho wed intra- and interspecific host discrimination in a choice experiment. In contrast to A. bipustulata, A. bilineata larvae more frequently parasitize d hosts parasitized by A. bipustulata than those parasitized by conspecific s. We suggest that host discrimination will be frequent in solitary parasit oids with host-seeking larvae when hosts are aggregated.