In biological control programmes introduced natural enemies compete with indigenous enemies for hosts and may also engage in intraguild predation when two species competing for the same prey attack and consume one another. The large pine weevil, Hylobius abietis L. (Coleoptera: Curculionidae), is an important pest of coniferous reforestation in Europe. Among its natural enemies, the parasitoid Bracon hylobii Ratz. (Hymenoptera: Braconidae) and entomopathogenic nematodes have potential as biological control agents. Both parasitoid and nematodes target the weevil larvae and, hence, there is potential for competition or intraguild predation.
In this study, we examine the interaction of B. hylobii with the nematode Heterorhabditis downesi Stock, Griffin and Burnell (Nematode: Heterorhabditidae), testing the susceptibility of larvae, pupae and adults of B. hylobii to H. downesi and whether female parasitoids discriminate between nematode-infected and uninfected weevils for oviposition. In choice tests, when weevils were exposed to nematodes 1–7 days previously, no B. hylobii oviposited on nematode-infected weevil larvae. Up to 24 h, healthy weevils were twice as likely as nematode-infected ones to be used for oviposition. Bracon hylobii females did not adjust clutch size; nematode-infected hosts were either rejected or the parasitoid laid a full clutch of eggs on them.
When nematodes were applied to the parasitoid feeding on weevil larvae, the nematodes parasitized the parasitoid larvae, there was a reduction in cocoon formation and fewer cocoons eclosed. Eclosion rate was not reduced when nematodes were applied to fully formed cocoons, but nearly all of the emerging adults were killed by nematodes.