Host Diet Affects Extinctions and Colonizations in a Parasitoid Metapopulation

1. In the Aland islands of SW Finland, the parasitoid Cotesia melitaearum (Wilkonson) is a specialist on the Glanville fritillary butterfly, Melitaea cinxia (L.), which uses two host plants, Plantago lanceolata (L.) and Veronica spicata (L.). 2. Extensive survey data on the butterfly and parasitoid populations collected in 1994-97 were used to examine possible associations of parasitoid colonizations and extinctions, with host population isolation, host population size, and host diet (plant species). 3. The likelihoods of parasitoid colonization and host patch occupancy were found to increase with decreasing isolation from other parasitoid populations, increasing host population size and an increasing fraction of host larvae on V. spicata. The likelihood of parasitoid population extinction increased with decreasing host population size and decreasing fraction of hosts on V. spicata. The pattern of association was exceptional in one area of high-density parasitoid populations as well as during a year of many extinctions but few colonizations. 4. Among host populations occupied by C. melitaearum, the fraction of host larval groups parasitized was higher in host populations using the more rare host plant, V. spicata. If host larval groups were present on both host plant species within one host population, then the larval groups on V. spicata were more likely to be parasitized. 5. The large-scale population dynamics of the parasitoid wasp are influenced by the distribution of the two host plants of the butterfly, by host-plant-dependent parasitoid behaviour and by host butterfly oviposition preference. 6. Possible explanations for the apparent superiority of V. spicata for C. melitaearum as the host butterfly's host plant are discussed. These include host larval group size, larval web structure, plant spatial aggregation, plant chemistry, and host larval physiology and phenology.