title: Influence of seed head-attacking biological control agents on spotted knapweed reproductive potential in western Montana over a 30-year period.
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/m(2)) was 95.9-99.0% lower. The average seed bank in 2005 at four sites containing robust spotted knapweed populations was 281 seeds/m(2) compared with 19 seeds/m(2) at four sites where knapweed density has declined. Seed bank densities were much higher at sites in central Montana (4,218 seeds/m(2)), 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.
authors.affiliation: Montana Agricultural Experiment Station, Western Agricultural Research Center, 580 Quast Ln., Corvallis, MT 59828, USA. jstory@montana.edu
authors.forename: Jim M
authors.initials: JM
authors.lastname: Story
authors.forename: Lincoln
authors.initials: L
authors.lastname: Smith
authors.forename: Janelle G
authors.initials: JG
authors.lastname: Corn
authors.forename: Linda J
authors.initials: LJ
authors.lastname: White
mesh_terms: D000818:Animals; D031179:Centaurea / Q000469:parasitology*; D003247:Conservation of Natural Resources; D005247:Feeding Behavior / Q000502:physiology; D007313:Insecta / Q000502:physiology*; D009009:Montana; D010572:Pest Control, Biological / Q000379:methods*; D011156:Population Density; D012098:Reproduction / Q000502:physiology; D012639:Seeds / Q000469:parasitology*; D013997:Time Factors