Effects of the interaction of the biocontrol agent Agapeta zoegana L. (Lepidoptera: Cochylidae) and grass competition on spotted knapweed

Agapeta zoegana is a Eurasian root-mining moth introduced for biological control of spotted knapweed, Centaurea maculosa, in North America. A study was conducted during 1992 through 1994 to assess the establishment of the moth and to make a preliminary assessment of the combined effects of the moth and grass competition on spotted knapweed plant structure and density at two nearby sites in western Montana. The moth was well established at a release site in 1992 where it had been previously released, and it became comparably well established at a check site, 140 m away, by 1994. Knapweed plants at the A. zoegana release site had less above-ground biomass (43%), fewer stems per plant (29%), and fewer capitula per plant (43%) and were shorter (18%) than knapweed plants at the check site, which may represent impact of the moth on the plant. However, a comparison of infested versus uninfested knapweed plants throughout both sites showed that infested plants had more stems (15%), more capitula (40%), and more aboveground biomass (112%), were taller (7%), had thicker roots (92%), and were older (22%) than uninfested plants, suggesting that the moth preferentially attacked older, larger knapweed plants. Numbers of A. zoegana larvae per root were positively correlated with root diameter. Incidence of attack by A. zoegana was significantly greater in bolted knapweed plants than in rosettes, but larvae showed no preference for bolted plants over rosettes when root diameters were similar. A. zoegana had no effect on knapweed rosette density, but appeared to reduce the number of bolted knapweed plants in plots with low grass density (10% grass cover) by 39% and increase the density of knapweed seedlings in the spring by 65%. Plots with high grass density (50% grass cover) contained fewer bolted knapweed plants (35%), femer rosettes (38%), and fewer seedlings (50%) than plots with low grass density. The effects of A. zoegana and grass competition on bolted plant density were not additive. (C) 2000 Academic Press.