The impact of an exotic social wasp (Vespula germanica) on the native arthropod community of north-west Patagonia, Argentina:: an experimental study

1. Biological invasions are usually thought to have a negative impact on native communities. However, data supporting this idea are often based on comparative studies between invaded and non-invaded areas, and are spatially and temporally limited. 2. The present study experimentally assessed the impact of an exotic wasp, Vespula germanica, on the native arthropod community of north-west Patagonia during 3 years in an area of 80 ha. Vespula germanica is an exotic social vespid that invaded north-west Patagonia 20 years ago. It has been suggested that its populations affect native arthropods because of its broad diet and also because Patagonia lacks natural enemies and potential competitors for these wasps. 3. Using wasp-specific toxic baits, V. germanica abundance was reduced in five sites of native woodlands during 3 consecutive years. The abundance, species richness, and composition of arthropods between non-poisoned (control) and poisoned sites was then compared, both before and after the wasps were poisoned. 4. Wasp abundance represented 6% of the total arthropod catches in non-poisoned sites and was reduced, on average, by 50% in the treated areas. The abundance, species richness, and composition of the arthropod community (305 species, 24 600 individuals) did not differ between control areas and areas where the abundance of V. germanica was reduced. Significant differences in response variables were found only before wasp poisoning had begun and were related to variations among sites. 5. These results suggest that V. germanica is not affecting the local arthropod assemblages, contradicting past work in other regions. The low relative abundance of wasps in Patagonia, when compared with other invaded regions, might explain the findings. 6. The present study provides further evidence for the importance of large-scale experimental work with before/after comparisons to fully understand the impact of invaders on natural communities.