1. Introduced common wasps (Vespula vulgaris) reach high densities in the beech forests (Nothofagus spp.) of the northern South Island, New Zealand, and may be having a severe impact on populations of native invertebrates. An experimental approach was used to test whether reducing the abundance of common wasps increases the probability of native invertebrates surviving. Garden orb‐web spiders (Eriophora pustulosa) were used because they were easy to collect and could be trained to build webs on a standard frame. Thirty orb‐web spiders were placed out on web‐frames in each of four study sites in beech forest during periods of low, medium and high wasp abundance, and their rate of removal was measured over a 4‐h period. Wasp numbers were reduced by poisoning in two study sites. Using wasp‐abundance data from weekly Malaise trap samples in the poisoned and non‐poisoned sites, the probability of spider survival over the entire wasp season was modelled and the level of wasp control necessary to protect natural populations of orb‐web spiders was estimated. 2. Wasp abundance and the probability of spider survival were negatively correlated, and smaller spiders were likely to survive longer than larger spiders. At the peak of the wasp season, significantly more spiders survived in the poisoned areas than in the non‐poisoned areas. 3. The probability of a spider surviving to the end of the wasp season was virtually nil in the non‐poisoned sites (5.08 × 10–18), but was also very unlikely in the poisoned areas (3.47 × 10–5). 4. The survival model predicts that wasp abundance would need to be reduced by 80–89.5% over the entire wasp season to protect populations of orb‐web spiders. 5. Extrapolation from the model predicts that the invertebrate taxa most vulnerable to wasp predation may have already been removed from the beech forest ecosystem during the 40 years of wasp occupation.

Sphecophaga vesparum burra (Cresson) (Hymenoptera: Ichneumonidae) has been released at seven sites in New Zealand as a biological control agent for wasps (Vespula spp.). Between 1996 and 1998, more than 35 000 overwintering cocoons were released nationwide, more than half of them at two South Island sites: Binser Track in Arthur's Pass National Park (13 200 cocoons) and Tennyson Inlet in the Marlborough Sounds (13 660 cocoons). Monitoring at these two climatically and altitudinally disparate sites revealed that over 5000 adult parasitoids emerged at each site over the 4 years. More than 300 wasp nests were dug and inspected for attack by S. v. burra, but there was no evidence of establishment.