The ecological consequences of social wasps (Vespula spp.) invading an ecosystem that has an abundant carbohydrate resource

Introduced Vespula wasps have successfully invaded beech (Nothofagus) fores ts in New Zealand. By collecting honeydew, an abundant carbohydrate resourc e, wasps can reach high numbers. Honeydew is produced by an endemic scale i nsect which infests about 1 million hectares of land, 15% of New Zealand's remaining native forest. At the peak of the wasp season, honeydew beech for ests had an average biomass of about 3.8 kg of introduced wasps ha(-1) (10, 000 workers ha(-1)). These abundant invaders reduce the standing crop of ho neydew by more than 90% for 5 months of the year and so compete with native species (such as birds and invertebrates) that also consume honeydew. The behaviour of three species of native bird is affected by this reduction in honeydew, but it is unknown if this affects the survival or reproductive su ccess of these birds. Nevertheless, to avoid altering the birds' behaviour, wasp density should not increase above a level where wasps revisit honeyde w threads more than once every 180-400 min. Additionally, the predation rat e of wasps on some invertebrate prey species is so high that the probabilit y of an individual surviving through the wasp season is virtually nil. Henc e wasps probably reduce or eradicate populations of some invertebrates, Was p abundance needs to be reduced by 80-90% to conserve some native invertebr ate species. Wasps could also affect nutrient cycling in the honeydew beech forest community by reducing the flow of carbon to micro-organisms in the phyllosphere and the soil, which ultimately could affect soil solution chem istry. Current control tools are unable to reduce wasp populations over lar ge tracts of forest. The challenge is to identify and develop new control t echniques to achieve widespread control For conservation gains. The impact of introduced social wasps provides a warning of the damage exotic ants cou ld cause if they were to invade honeydew beech forest. New Zealand needs to be vigilant to reduce the risk of an invasion by ants or other social wasp s. (C) 2001 Elsevier Science Ltd. All rights reserved.