Impact of a natural enemy overwintering refuge and its interaction with the surrounding landscape

1. Egg parasitoids in the genus Anagrus (Hymenoptera: Mymaridae) are important mortality factors for grape leafhoppers ( Erythroneura elegantula ; Homoptera: Cicadellidae) in California vineyards, yet must overwinter in habitats external to these vineyards. Existing evidence suggests that French prune trees, which harbour the overwintering host Edwardsiana prunicola , planted adjacent to vineyards may enhance early-season abundance of Anagrus.
2. Anagrus overwintering in French prune tree refuges were labelled with the trace element rubidium in four separate experiments. Rubidium-labelled Anagrus were captured in adjacent vineyards in two of the experiments, confirming that French prune trees contribute to early-season Anagrus populations. Anagrus from refuges were captured at the most distant sampling positions, 100 m from refuges.
3. Use of rare element labelling has, for the first time, enabled the relative contribution of different sources to early-season colonization by this parasitoid to be quantified. Refuges contributed 1% and 34% of Anagrus colonizing two of the experimental vineyards, respectively. The remainder originated from overwintering habitats external to the French prune/vineyard system.
4. The spatial patterns of Anagrus originating from external overwintering habitats suggest that the French prune trees are generating a 'windbreak effect'. Anagrus dispersing within the windstream colonized vineyards at a higher-than-average rate immediately downwind of refuges.
5. The amount of colonization by Anagrus from external overwintering habitats was apparently related to the distance to presumed overwintering habitats. These findings demonstrate that both the number of natural enemies emerging from a refuge and the composition of the surrounding landscape are important in determining the impact of local, small-scale habitat manipulations.