Evaluating Broad-Scale Correlates of Biocontrol Success with Purple Loosestrife: Management Implications and a General Protocol for Post-Release Evaluation.

Weed biocontrol programs need to be monitored for years post-release to evaluate the reasons for eventual success or failure.  In 2008 we surveyed 46 wetlands in which the biocontrol agent Galerucella calmariensis (Coleoptera: Chrysomelidae) had previously been released to control the invasive purple loosestrife (Lythrum salicaria).  We found few correlates of biocontrol success and loosestrife remained abundant at most of the sites.  Variation in loosestrife cover among study wetlands was not explained by soil nutrients nor by a priori biocontrol release data.  Sites with high loosestrife cover also had higher G. calmariensis abundance, a pattern which is likely due to G. calmariensis requiring sufficient plant tissue to support their populations.  Overall, we were surprised to find that loosestrife was not more abundant in nutrient-enriched sites and that neither the number of releases nor the number of beetles released per site predicted loosestrife cover.  We propose a rigorous protocol for quantifying whether biocontrol agents are successful and in addition propose some revised ways to define biocontrol success.