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The wasp parasitoid Sphecophaga vesparum vesparum has been released in many parts of New Zealand since 1987 as a biological control agent for German (Vespula germanica) and common (V. vulgaris) wasps. We checked whether the parasitoid had established at 33 release sites. At 26 of these sites, we inspected enough nests (at least 20) to be more than 80% certain that we would have detected the parasitoid if at least 8.5% of the nests had been attacked. We suggest that the probability of 5. v. vesparum successfully invading a site is greatly increased when a large number of founders are released and there is a high density of wasp nests. The parasitoid has established in at least two sites, Pelorus Bridge (Marlborough) and Ashley Forest (Canterbury). S. v. vesparum has been established at Pelorus Bridge since 1988 and annually infests an average of 8.5% of nests within 2 km of the release site. The parasitism rate has increased since the parasitoid first established. Despite this increase, we did not record a reduction in wasp nest density that related to parasitoid abundance at Pelorus Bridge. We recommend no further releases of S. v. vesparum, but plan to release a subspecies, S. v. burra.