Ecological Effects of the Biocontrol Insects, Larinus Planus and Rhinocyllus Conicus, on Native Thistles

Abstract

Biological control of invasive weeds is, by nature, a delicate balance between introducing effective biological control agents and not introducing another invasive species. A disconcertingly similar suite of traits is used to describe invasive insect species and to identify appropriate biological control agents (or candidates): good control agents and invasive exotic species are good dispersers, they are good colonizers, they have high reproduction rates, and they are suited for broad distribution. It shouldn't come as a surprise that two previously released weed biological control agents can now be characterized as invasive species. The purpose of this dissertation is twofold: first, it is to explore the ecological relationship between predispersal seed predation and plant population dynamics, and second, it is to elucidate the risks to native plants involved with introduction and redistribution of exotic species.

Rhinocyllus conicus and Larinus planus are Eurasian seed-head weevils, introduced and redistributed broadly across the western U.S. to control exotic thistles. Exclusion experiments on native thistles, including one that is rare and imperiled, at sites in Colorado present strong evidence that a decrease in seed production due to herbivory by both R. conicus and L. planus has lead to a reduction in recruitment of the thistles. The density of seedlings in both cases, even in the excluded units, was far below where density dependent effects may play a role in the dynamics of the thistle. Further, a survey of eight western states demonstrates established populations of L. planus and effects on seed production in multiple native species in four states. It also documents the near ubiquity and broad diet breadth of R. conicus.

The process for approval of phytophagous biocontrol agents has become more cautious and more efforts are made to prevent nontarget herbivory. Nevertheless, land managers still routinely redistribute previously approved, non-regulated agent insects that appear to pose a higher risk to the native flora. The results of this research will benefit resource managers who wish to consider use of phytophagous insects as biological control agents as well as help ecologists and environmental managers understand the risk probabilities of biological control applications.