Submitted to: Biocontrol Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 24, 2004
Publication Date: August 1, 2005
Citation: Smith, L., Mayer, M.G. 2005. Field cage assessment of interference among insects attacking seed heads of spotted and diffuse knapweed. Biocontrol Science and Technology. 15(5):427-442. Interpretive Summary: Spotted knapweed is an alien weed that has invaded 7.5 million acres in the western U.S. and Canada. It causes an estimated annual loss of $14 million dollars in Montana alone. Eight species of insect biological control agents have been introduced that attack seed production of the plant. Whether or not these insects interfere with each other or complement each other to achieve better control of the weed is not known. Field studies were conducted to determine the interaction of two recently introduced weevils with a widespread, previously released gall fly. Results show that reproduction of neither weevil was reduced by the presence of the fly, but fly's reproduction was reduced by one of the weevils. One weevil, Bangasternus fausti, increased the total infestation of flower heads on both plants and the other weevil, Larinus minutus, increased it on diffuse knapweed, but reduced it on spotted knapweed. Both weevils should contribute to increased control of diffuse knapweed. Larinus minutus may actually interfere with the fly and reduce its effectiveness on spotted knapweed.
Technical Abstract: Successful biological control of alien weeds often depends on establishing several species of agents that complement each other to attain progressively higher levels of control. Field studies were conducted to determine the interaction of two recently introduced weevils, Larinus minutus and Bangasternus fausti, with a widespread, previously released gall fly, Urophora affinis. Larinus minutus produced almost three times as many progeny on diffuse knapweed as on spotted knapweed. Reproduction of both weevils was not reduced by competition with the fly. Fly reproduction was reduced by up to 77% by L. minutus but not by B. fausti. Mean production of mature seeds in infested flower heads was reduced by up to 79% and 86% by L. minutus, 97% and 85% by B. fausti and 83 and 98% by U. affinis on spotted and diffuse knapweed, respectively, compared to uninfested capitula. Production of mature seeds per cage was reduced by 61 to 95% by L. minutus, 87 to 99% by B. fausti and 0 to 98% by U. affinis. The additional release of B. fausti reduced spotted knapweed seed production per cage by 81% compared to that caused by U. affinis alone, but L. minutus did not cause a significant additional effect on either plant.