Submitted to: Society for Ecological Restoration Abstracts
Publication Type: Abstract only
Publication Acceptance Date: 9/1/2003
Publication Date: 11/19/2003
Citation: Brockington, M.R., Sheley, R.L., Jacobsen, J.J., Svejcar, A.J., Jacobs, J.S. 2003. Influence of cyphocleonus achates (coloeptera: circulionidae), nitrogen, and sucrose on centaurea maculosa lam. (spotted knapweed) [abstract]. 15th Annual International Conference of the Society for Ecological Restoration, November 19-23, 2003, Austin, Texas. WWW.SER.org/pdf/2003_abstracts-A-L.pdf. Interpretive Summary: Since weeds thrive in high nutrient environments because they exploit their rapid growth rates, we tried to use a biological control agent to shift the system from growth rate driven to competition driven. We expected that insects would decrease spotted knapweed's ability to acquire soil nutrients, and that sucrose would decrease soil nutrient availability to plants because they would be tied up by soil microbes using the sucrose as an energy source. We found that sucrose effects were ephemeral and a more complex carbon source would probably be needed. The most compelling result was that the weevils effectively reduced spotted knapweed only where their density was high enough to infect a majority of the pants in the population, and in that case soil nutrients increased as predicted. In areas where the weevil density was low, spotted knapweed increased by self-thinning.
Technical Abstract: Competition is an important mechanism driving plant population dynamics. Our purpose was to use the influence of a natural enemy on plant available soil N as a mechanism for predicting its impact on an invasive weed population. We hypothesized that infection of spotted knapweed (Centaurea maculosa Lam.) by Cyphocleonus achates would increase plant-available soil nitrogen by reducing plant growth and N acquisition. Treatments consisted of N 'amendment and weevil presence or absence. N'amendments of 1) N addition (112 kg/ha), 2) sucrose (1000 kg/ha), and 3) an untreated control were arranged in a 2 x 3 factorial and replicated 5 times at each site. Two sites dominated by spotted knapweed in western Montana were established in 2001 and sampled in 2002. Additionally, a 7-year study comparing the effects of weevil presence or absence was sampled for extractable ammonium and nitrate. In the short-term study, ammonium increased with insect infection, and spotted knapweed biomass increased with weevils in the high N environment at site 1, but not site 2. In the long-term study, spotted knapweed biomass nearly doubled in the presence of the weevil, and soil ammonium decreased with weevils. Uninfected individuals within the population may be favored by increased availability of resources otherwise used by infected individuals. In the long-term study, a decrease in plant available soil N would have predicted an increase in plant biomass. Our study provides preliminary evidence that a natural enemies' influence on plant available soil N can be used to predict plant population dynamics.