The degree to which biocontrol agents impact invasive species varies widely among sites, often for unknown reasons. This is true even for generally-successful biocontrol programs that are implemented broadly. We formulated three hypotheses that could explain this variation: 1) that success is more likely where biocontrol efforts have been more pronounced, 2) that nutrient enrichment will lower biocontrol success regardless of biocontrol effort, and 3) that increasing biocontrol efforts will lead to increased success only in sites where nutrient levels permit. We tested our hypotheses by surveying 46 sites that had prior releases of Galerucella calmariensis (Coleoptera: Chrysomelidae), a biocontrol agent commonly released against purple loosestrife (Lythrum salicaria). At each site we recorded G. calmariensis abundance and damage, plant size for loosestrife and percent cover for loosestrife and three other community dominants. We also measured soil nutrient concentrations (nitrogen, phosphorous, potassium and calcium).

Results/Conclusions

Percent leaf damage was the only measure of biocontrol success that increased with increasing biocontrol effort regardless of nutrient availability. Every other measure suggested that increasing biocontrol efforts will improve success rates, but only in low-nitrogen sites. This pattern held for proximate estimates of success (increased biocontrol agent abundance) as well as ultimate outcomes for loosestrife (decreased plant size and cover) and the desired replacement community (increased cover of Typha spp.). This study has important implications for present and future biocontrol programs, both in the implementation and monitoring phases. Further, since nutrient enrichment is often tied to human activities our findings emphasize that local biocontrol success may be contingent on the degree to which humans impact the surrounding landscape.