Precipitation and the Interaction of Seedhead Biological Control Insects for Spotted Knapweed in the Rocky Mountain Front Range

Biological control of spotted knapweed (Centaurea stoebe) is an ongoing topic of research that holds ecological importance due to the extent to which knapweed has spread across the Western United States. Using herbivores (insects) as biological controls of spotted knapweed can be successful at reducing plant density in certain habitats. However, the exact relationship among the insects, knapweed, and abiotic factors affecting knapweed growth such as precipitation, remains uncertain. Through collection and dissection of knapweed seedheads, I estimated densities of two insects, Larinus minutus and Urophora affinis, as well as the average number of seeds produced by seedheads of spotted knapweed at a long-term study site in the Front Range of Colorado The data I collected were then added to a data set collected at this site extending back to 2002, which represents the longest data set of its kind presently available. After inclusion of my data, I found a positive trend between precipitation and seed production that differed from the findings of previous work. Insect presence decreased the number of seeds per seedhead, and this effect was independent of the amount of precipitation. I also found that the interaction of the two species of insects in the seedheads negatively affected the individual effectiveness of each insect, although not significantly, which supports previous research. My findings will benefit model predictions for knapweed spread and inform land management decisions about spotted knapweed infestations in the Rocky Mountain Front Range.