Linear patterns of dispersal and build up of the introduced parasitoidMicroctonus hyperodae(Hymenoptera: Braconidae) in Canterbury, New Zealand

Abstract

AbstractThe dispersal ofMicroctonus hyperodaeLoan, an introduced parasitoid of the South American grassland pestListronotus bonariensisKuschel, was measured in Canterbury, New Zealand. Considering all directions, the mean annual dispersive increment was 1.9 ± 0.9 km year–1as measured in the winters of 1993, 1994 and 1995. The parasitoid’s ground distribution suggested that its movement was biased towards the south-west indicating probable wind-borne dispersal. The overall relatively low rate of dispersal was thought to be related to the inhibitory effects ofM. hyperodaeparasitism onL. bonariensisflight. A generalized linear model fitted to percentage ofL. bonariensisparasitized took a simple form, with a quadratic increase in weevil infection, that with time, gradually decreased. The build-up of parasitism at the release site was significantly greater than the rates at the other sites measured in this study (P< 0.001) with a ratio of release site: dispersal site rates of 1.33: 1. The simplicity of the fitted generalized linear model indicated remarkable uniformity in both parasitism build-up and dispersal from one year to the next; this finding indicated that all dispersal was ‘natural’ and unassisted by human activity. Despite such modest dispersal rates, by the winter of 1996, five years after its release, the parasitoid had spread geometrically over an area ofc. 140 km2.