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

The dispersal of Microctonus hyperodae Loan, an introduced parasitoid of th e South American grassland pest Listronotus bonariensis Kuschel, was measur ed in Canterbury, New Zealand. Considering all directions, the mean annual dispersive increment was 1.9 +/- 0.9 km year(-1) as measured in the winters of 1993, 1994 and 1995. The parasitoid's ground distribution suggested tha t its movement was biased towards the south-west indicating probable wind-b orne dispersal. The overall relatively low rate of dispersal was thought to be related to the inhibitory effects of M. hyperodae parasitism on L. bona riensis flight. A generalized linear model fitted to percentage of L. bonar iensis parasitized 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 s ites measured in this study (P < 0.001) with a ratio of release site: dispe rsal site rates of 1.33: 1. The simplicity of the fitted generalized linear model indicated remarkable uniformity in both parasitism build-up and disp ersal from one year to the next; this finding indicated that all dispersal was 'natural' and unassisted by human activity. Despite such modest dispers al rates, by the winter of 1996, five years after its release, the parasito id had spread geometrically over an area of c. 140 km(2).