Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2007
Publication Date: 4/1/2008
Citation: Story, J.M., Smith, L., Corn, J.G., White, L.J. 2008. Influence of Seed Head-attacking Biological Control Agents on Spotted Knapweed Reproductive Potential in Western Montana over a 30-year Period. Environmental Entomology. 37(2):510-519
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. At least four of these are now widespread and abundant, but their impact on populations of knapweed in western Montana are unknown. This research documents decreases in the density of spotted knapweed at two locations and analyzes the role that the seedhead insects have played. We conclude that these biological control agents have reduced seed production by about 93%, which contributed significantly to the average 71% decrease in knapweed density. At two of eight sites monitored, the weed no longer occurs. The combined impact of the seedhead and root-feeding insects appears to be successfully controlling this invasive alien weed in a region where it was most dense. This will provide long-term self-perpetuating control of the weed, thus reducing application of herbicides and increasing biodiversity and the utility of rangeland for grazing and recreation.
Technical Abstract: Field experiments were conducted in 2006 to measure the direct impact of seed-feeding insects that were previously introduced as classical biological control agents. The results indicate that Larinus minutus, L. obtusus, Urophora affinis and U. quadrifasciata reduced seed production by 84%. Additional observations were made at field sites in western and central Montana to determine whether spotted knapweed had decreased since the introduction of the biological control agents and to determine if this was caused by the insects. From 1974, before the insects were abundant, to 2005, spotted knapweed seed production decreased by about 93% and density by 71% at two sites in western Montana. Path analysis showed that the Larinus species reduced seed production by about as much as the two Urophora species. There was a delay in the decrease of knapweed populations relative to the abundance of the seedhead insects because the plant is perennial and seeds persist in the soil for about eight years. Although the impact of root-feeding insects were not measured in this study, two of these have also been shown to affect the weed. The combined effect of all these insects appears to be achieving effective control of the weed in western Montana, where infestations were worst.