Cause and Extent of Predation on Urophora spp. Larvae (Diptera: Tephritidae) in Spotted Knapweed Capitula

Field studies were conducted at 19 sites in western Montana during 1987 through 1990 to determine the cause and extent of predation on Urophora spp. larvae in spotted knapweed, Centaurea maculosa Lamarck, capitula. The extent of knapweed capitula removal was monitored at all 19 sites, and causes of capitulum removal were assessed at 1 site. Capitula removal and subsequent mortality of Urophora affinis Frauenfeld larvaeoccurred at all sites, with mean percentage removal ranging from 29 to 64%. In general, the percentage capitula removal increased with each sequential sampling date over all winters. Sampling date, winter, U. affinis gall density, and percentage U. affinis–infested capitula explained most of the variationin the percentage missing capitula.There was no relationship between percentage missing capitula and U . quadrifasciata (Meigen) gall density or percentage U. quadrifasciata infested capitula. Based on an assessment of bite marks and stem fracture patterns, =74% of the capitulum removal at the Teller Wildlife Refuge was caused by 3 predator/herbivores: the deer mouse, Peromyscus maniculatus (Wagner);the blackcapped chickadee, Parus atricapillus L.; and the white–tailed deer, Odocoileus virginianus (Zimmermann). Approximately 64% of the observed chickadee foraging activityin knapweed–infested areas during winter months was associated with knapweed capitula. Chickadee predation was selective for heavily infested capitula. Vegetation type had limited effect on predation.