The impacts of biological control on the performance of Lythrum salicaria 20 years post-release

Abstract

Biological control has the potential to be an effective tool in the control of invasive plants. However, because it involves the introduction of a novel species to an ecosystem, predicting the potential impacts and overall effectiveness can be difficult, especially at the outset. For this reason, long term follow up of programs is essential. Unfortunately, few programs include official follow up, and those that do are often not collecting all the information necessary to assess the benefits and risks. Here we describe a study where we performed plant population surveys to estimate the impacts of a large scale release of two species of chrysomelid beetles, Neogalerucella calmariensis and N. pusilla on the control of the invasive wetland plant, Lythrum salicaria in Ontario, Canada. We surveyed 18 populations that varied in their colonization history, including a set that are among the original sites of the Ontario government’s planned release program initiated in 1992. We discovered that L. salicaria plants at the original release sites exhibited more herbivore damage and fewer primary inflorescences relative to those at naïve (never colonized) or recent (those apparently colonized via migration) sites. We also discovered that, across sites, quadrats with a heavier load of Neogalerucella spp. tended to have lower overall plant species richness, which was likely due to the fact that beetles tend to congregate in areas with high target plant density. Surprisingly, many of the factors associated with a successful release program, such as reduced target plant density, reduced target plant fruit production, and reduced above- and/or belowground plant biomass at release sites were not detected in our study. We discuss the potential reasons for our findings and make recommendations for the application of biocontrol and follow up programs.