Evaluating the impact of a biological control parasitoid on invasive Vespula wasps in a natural forest ecosystem

The overall impact of the parasitoid Sphecophaga vesparum vesparum on invasive Vespula wasps in New Zealand native beech forest was evaluated by assessing the levels of parasitism achieved and the parasitoid’s effect at nest level and population level. The maximum proportion of nests parasitised was 17%, but there was no significant increase with time (r2 = 0.139; p = 0.115). However, there was an exponential reduction in the number of parasitoids produced per parasitised nest from a peak of 570 (SE = 143) parasitoids per nest in 1990, declining to only 15 (SE = 6) parasitoids per nest in 2004. Even when parasitoid density was high, the parasitoid had no detectable impact on the number of small cells or the total host nest size, but it halved the number of large (reproductive) cells produced. This may have resulted in fewer queens produced per parasitised nest. Wasp nest density was highly variable from year to year, but there was no evidence that the wasp population density at the parasitised site (Pelorus Bridge) had declined relative to the five sites where the parasitoid had not established. We conclude that the parasitoid is unlikely to have had any significant effect on wasp populations hitherto, nor is it likely to impact host populations in the future. We recommend other biological control programs adopt pre-release assessment of per capita impact as a way of identifying agents that are more likely to be successful and hence minimising economic and potential ecological costs of biocontrol.