Investigation into the cumulative stress hypothesis: A case study with spotted knapweed (Centaurea stoebe ssp. micranthos) and Larinus minutus

The Cumulative Stress Hypothesis suggests that successful biological control of weeds projects is largely due to a combination of stresses when multiple natural enemy species are released.  Another hypothesis, The Lottery Model, states that the likelihood of releasing the one (or few) natural enemies that are effective in controlling the weed is increased by the number of species released.  The successful knapweed biological control program in the northwestern United States and southwestern Canada has been used to support both the Cumulative Stress Hypothesis and the Lottery Model in areas where multiple knapweed biological control agents are present.  We investigated if Larinus minutus is will cause a change in spotted knapweed infestations in an area where only one other biocontrol agent is present (Urophora quadrifasciata) and has been shown to be ineffective.  L. minutus reduced rosette density during 2011 through 2012 and seed production from 2010 through 2011, but did not show a significant change over the entire study (2010 through 2012).  We believe that the lack of response over the entire study could be a result of the extreme drought conditions over the study area in 2012 and a loss of replicates in the experimental group due to mowing activities.