The difficulty of reducing introduced wasp (Vespula vulgaris) populations for conservation gains

Introduced common wasps (Vespula vulgaris) are widespread, abundant pests in New Zealand. They compete for food with native birds and feed on native invertebrates. We poisoned wasps annually over 4 years to see if it was possible to reduce their abundance in two 30-ha beech forest sites. Two different poisons (sodium monofluoroacetate and sulfluramid) were used, mixed with sardine catfood. There was no evidence that one poison was more effective than the other. Between 82 and 100% of the colonies were killed in the poisoned sites, but reinvasion by foraging workers meant that cumulative wasp biomass (measured using Malaise traps) was reduced by only 55- 70%. Individual wasps were about 16% heavier in the poisoned sites at the Peak of the wasp season (March) than in the non- poisoned sites, although this had a minimal effect on cumulative biomass over the entire season. Conservation gains need to be quantified in order to assess whether the expense of such poisoning operations is warranted.