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.
Bibliographical noteFunding Information:
We thank Dave Andow and Roger Moon for their input in this study, and Randy Hedlund, Natasha Leong, and Sam Lockner for help with the dissections. We also thank Joop van Lenteren and his staff for organizing the Anthonie van Leeuwenhoek symposium, Jaques Brodeur and Christine Müller for organizing the session in which this study was presented, and Ian Hardy for coordinating submission of the manuscripts from this session, and for comments on a earlier version of this manuscript. We thank two anonymous reviewers for their comments on an earlier version of the manuscript. This research was funded in part by the United States Department of Agriculture Grant No. 9802906 and N.S.F. Biocomplexity award NSF-DMS-0083468 to G.E.H. and has been supported in whole or in part by the University of Minnesota Agricultural Experiment Station.
Copyright 2008 Elsevier B.V., All rights reserved.
- Apparent competition
- Coleomegilla maculata
- Dinocampus coccinellae
- Harmonia axyridis
- Indirect interactions