After the introduction of an exotic species, it may compete with native species that were already present in the habitat. This may affect population densities of native species, and the rate in which the population of a new species can increase. Interactions can be direct (resource competition, predation) or indirect (for example competition mediated by a shared natural enemy).
Coleomegilla maculata (De Geer) and Harmonia axyridis (Pallas) are both generalist predators. They both feed mainly on aphids, but can use other types of prey such as insect eggs or pollen. They are both known to cannibalize, and are also able to feed on larvae and eggs of other coccinellid species.We studied direct interactions between both species in a controlled field experiment. We used field cages with high and low prey densities to investigate competition levels between larvae of both species. We also compared the levels of predation by H. axyridis larvae on C. maculata larvae (and vice versa) at high and low prey densities, and the level of cannibalism in both species.
The indirect interaction we investigated is parasitoid-mediated competition (or competition for enemy-free space). The Braconid parasitoid Dinocampus coccinellae (Schrank) attacks both species, but its level of success is not the same for both species. From preliminary studies, its appeared that D. coccinellae is not as successful (a lower fraction of parasitized hosts actually results in adult parasitoids) in H. axyridis than in C. maculata. We introduced parasitoids into cages with a single host species or both host species to investigate whether levels of parasitism levels differ between species, and also if parasitoids prefer one host over the other. We investigated the possible outcomes of parasitoid-mediated competition by using a model that is currently being constructed (Heimpel and Neuhauser, in preparation).
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA