The invasive wetland weed, purple loosestrife (Lythrum salicaria) has been the target of a biological control project throughout North America. At many sites, beetle populations have increased and brought about significant reductions in weed infestations. After these large increases, beetles have dispersed to other locations, at times more than 2 km distant. This dispersal offers the potential of the agents filling in gaps in release sites, thus giving more-complete coverage. However, the dispersal may also lead to a rebound in weed populations. We are investigating this dynamic and how it may affect both short-term and long-term control of the weed.