Mortality and Emergence Pattern of Overwintering Cocoons of the Wasp ParasitoidSphecophaga vesparum vesparum(Hymenoptera: Ichneumonidae) in New Zealand

Abstract

Sphecophaga vesparum vesparum (Curtis) has been released in New Zealand as a biological control agent for the introduced yellowjackets Vespula vulgaris (L.) and V. germanica (F.). The parasitoid has an overwintering cocoon stage that can remain in the subterranean nest cavity for 1–4 yr before emerging. A mathematical model of the potential effect of the parasitoid has suggested that the probability of survival of overwintering cocoons and the pattern of cocoon emergence are 2 key variables for predicting the ultimate suppression in yellowjacket abundance. Overwintering cocoons were buried in subterranean cavities to simulate natural conditions and rates of cocoon predation by rodents, plus mortality from other causes, and emergence patterns were recorded. Rodents killed 62% of cocoons over 3 yr, and flooding, insects, and unknown causes of mortality accounted for another 19.2%. The mean annual survival rate for overwintering cocoons was 0.56, considerably higher than the model predicted. Emergence of adults was delayed in comparison with above-ground parasitoid release boxes, with the majority of subterranean cocoons remaining dormant until their 3rd spring. Using the higher survival rate and delayed emergence pattern in the model for the effect of the parasitoid suggests that the maximum likely reduction of yellowjacket density may be greater than originally predicted but will take longer to achieve. However, data on the proportion of spring nests killed by the parasitoid is needed before the model can usefully predict the ultimate level of suppression of yellowjacket density.