Indirect interactions between an introduced and a native ladybird beetle species mediated by a shared parasitoid

We studied parasitism rates by Dinocampus coccinellae (Schrank) of the native species Coleomegilla maculata De Geer and the introduced Harmonia axyridis (Pallas) in the laboratory and in the field. The rate of successful parasitism for H. axyridis was lower than for C. maculata, but the proportion of the population that had immature stages of the parasitoid was similar for both species. We used a population dynamics model to predict interactions between C. maculata, H. axyridis, and D. coccinellae, incorporating the differences in suitability we found between the two hosts. In the model, parasitoid attacks on non-suitable hosts contribute to parasitoid egg depletion without causing host death or parasitoid recruitment. Thus, hosts with low suitability could act as sinks for parasitoid eggs. Simulations of this model suggest that there is a critical value of the growth rate (r) for C. maculata below which C. maculata goes extinct. The critical r value is an increasing function of the attack rate on C. maculata (as expected) and a decreasing function of the attack rate on H. axyridis because the strength of H. axyridis as an egg sink increases with the attack rate on this species. Simulations of the model using experimentally derived parameter values suggest that the presence of H. axyridis leads to an increase in equilibrium densities of C. maculata. This outcome is due to the wastage of parasitoid eggs in the relatively unsuitable H. axyridis hosts.