IMPACT OF A NATURAL ENEMY OVERWINTERING REFUGE AND ITS INTERACTION WITH THE SURROUNDING LANDSCAPE

1. Egg parasitoids in the genus Anagrus (Hymenoptera: Mymaridae) are i mportant mortality factors for grape leafhoppers (Erythroneura elegant ula; Homoptera: Cicadellidae) in California vineyards, yet must overwi nter in habitats external to these vineyards. Existing evidence sugges ts that French prune trees, which harbour the overwintering host Edwar dsiana prunicola, planted adjacent to vineyards may enhance early-seas on abundance of Anagrus. 2. Anagrus overwintering in French prune tree refuges were labelled with the trace element rubidium in four separat e experiments. Rubidium-labelled Anagrus were captured in adjacent vin eyards in two of the experiments, confirming that French prune trees c ontribute to early-season Anagrus populations. Anagrus from refuges we re captured at the most distant sampling positions, 100 m from refuges . 3. Use of rare element labelling has, for the first time, enabled th e relative contribution of different sources to early-season colonizat ion by this parasitoid to be quantified. Refuges contributed 1% and 34 % of Anagrus colonizing two of the experimental vineyards, respectivel y. The remainder originated from overwintering habitats external to th e French prune/vineyard system. 4. The spatial patterns of Anagrus ori ginating from external overwintering habitats suggest that the French prune trees are generating a 'windbreak effect'. Anagrus dispersing wi thin the windstream colonized vineyards at a higher-than-average rate immediately downwind of refuges. 5. The amount of colonization by Anag rus from external overwintering habitats was apparently related to the distance to presumed overwintering habitats. These findings demonstra te that both the number of natural enemies emerging from a refuge and the composition of the surrounding landscape are important in determin ing the impact of local, small-scale habitat manipulations.