Additive effects of aboveground and belowground herbivores on the dominance of spotted knapweed (Centaurea stoebe)

Abstract

Spotted knapweed (Centaurea stoebe) is found in over 3 million ha of rangeland and forests across North America, and evidence supporting the use of biological control as a regional method to reduce infestations and their associated impacts remains inconclusive. Several species of insects have been reported to reduce plant densities in some areas; however, rigorous studies that test combinations of these species and the influence of resource availability are lacking. We examined the singular and combined effects of herbivory by a root weevil (Cyphocleonus achates) and a flower head weevil (Larinus minutus) on the growth and flower production of C. stoebe. We also manipulated soil resource fertility as an additional factor that could explain the outcomes of contradictory biological control herbivore effects on C. stoebe. In a greenhouse study, herbivory by C. achates decreased flower production for plants across all resource environments. In a caged common garden study, the negative effects of herbivory also did not interact with soil nutrient status. However, the presence of plant competition further decreased knapweed growth, and the negative effects of concurrent herbivory by C. achates and L. minutus on plant biomass and flower production were additive. Derived within the context of variable levels of soil nutrient availability and competing vegetation, these results support the cumulative stress hypothesis and the contention that combined above- and belowground herbivory can reduce spotted knapweed densities and reduce the ecological and economic impacts of this species in rangelands of western North America.

Biological control and precipitation effects on spotted knapweed (Centaurea stoebe): empirical and modeling results

“…(Sheley et al 1998, Story et al 2008, Knochel and Seastedt 2010. One of the most effective combinations of biological control insects for this plant has been C. achates and L. minutus: collectively these insects increase spotted knapweed plant mortality and reduce seed production , Crowe and Bourchier 2006, Knochel et al 2010bA. P. Maines, D. G. Knochel, and T. R. Seasteadt, unpublished manuscript).…”

“…were absent from the flower for all reported years (Knochel and Seastedt 2010). Other parameter estimates came from a common garden experiment where plants in enclosures had biological controls excluded or added at densities observed in the field (Knochel et al 2010b) and from a field experiment where known densities of spotted knapweed seeds were sown and the resulting number of seedlings and rosettes were assessed (Knochel et al 2010a; A. P. Maines, D. G. Knochel, and T. R. Seastedt, unpublished manuscript). The yearly seed bank germination probability was the average of estimates reported by Knochel et al (2010a) and A. P. Maines, D. G. Knochel, and T. R. Seastedt (unpublished manuv www.esajournals.org script).…”

“…Further description of the study site is reported in sources ( Table 1). The common garden study was conducted at the University of Colorado at the Boulder greenhouse (Knochel et al 2010b).…”

“…Herbivory has been shown to reduce rosette survival, but has no effect on the rate of rosette growth to the adult stage (A. P. Maines, D. G. Knochel, and T. R. Seastedt, unpublished manuscript). The difference between adult mortality rates when biological controls were present or absent was subtracted from the 9-year lifespan (Knochel et al 2010b; Table 1). We obtained the estimate for seeds flower À1 when biological controls are present from the average the seed production for the two sites where biological controls were released (Knochel and Seastedt 2010).…”

“…In the effort to control spotted knapweed, thirteen different species of biological control insects have been introduced (Shea and Kelly 1998). Several of the introduced biological control agents for spotted knapweed have been shown to have additive negative effects (Knochel et al 2010b) and have been released in multiple locations across western North America for more than 10 years (Crowe and Bourchier 2006, Seastedt et al 2007. Thus an analysis of the population-level response of spotted knapweed to the released biological control agents is a valuable opportunity to evaluate long-term outcomes of biological control.…”