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1 April 2008 Influence of Seed Head–Attacking Biological Control Agents on Spotted Knapweed Reproductive Potential in Western Montana Over a 30-Year Period
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Abstract

Five insect biological control agents that attack flower heads of spotted knapweed, Centaurea stoebe L. subsp. micranthos (Gugler) Hayek, became established in western Montana between 1973 and 1992. In a controlled field experiment in 2006, seed-head insects reduced spotted knapweed seed production per seed head by 84.4%. The seed production at two sites in western Montana where these biological control agents were well established was 91.6–93.8% lower in 2004–2005 than 1974–1975, whereas the number of seed heads per square meter was 70.7% lower, and the reproductive potential (seeds/m2) was 95.9–99.0% lower. The average seed bank in 2005 at four sites containing robust spotted knapweed populations was 281 seeds/m2 compared with 19 seeds/m2 at four sites where knapweed density has declined. Seed bank densities were much higher at sites in central Montana (4,218 seeds/m2), where the insects have been established for a shorter period. Urophora affinis Frauenfeld was the most abundant species at eight study sites, infesting 66.7% of the seed heads, followed by a 47.3% infestation by Larinus minutus Gyllenhal and L. obtusus Gyllenhal. From 1974 to 1985, Urophora spp. apparently reduced the number of seeds per seed head by 34.5–46.9%; the addition of Larinus spp. further reduced seed numbers 84.2–90.5% by 2005. Path analysis indicated that both Larinus spp. and U. affinis contributed significantly to reduction of seed production over the 30-yr period. Spotted knapweed density may not decrease significantly until the seed bank falls below a critical threshold.

JIM M. STORY, LINCOLN SMITH, Janelle G. Corn, and Linda J. White "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, (1 April 2008). https://doi.org/10.1603/0046-225X(2008)37[510:IOSHBC]2.0.CO;2
Received: 3 March 2007; Accepted: 15 November 2007; Published: 1 April 2008
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