Field studies were conducted at two sites in western Montana during 2006 and 2008 to assess the compatibility of mowing with five seed head insect species introduced for biological control of spotted knapweed, Centaurea stoebe Lamarck subsp. micranthos. In 2006, mowing of spotted knapweed plants at the bolting and flower bud stages resulted in the development of new seed heads that contained significantly more seeds and significantly fewer larvae of each insect species than in seed heads in unmowed controls. No seed heads were produced in the plots mowed at the flowering stage. Seed numbers per seed head in 2008 were also significantly higher in plots mowed at the bolting stage than in unmowed controls, but between-treatment differences in insect numbers were more variable. The seed head insects Larinus spp. and Urophora affinis Frauenfeld were the primary cause of the reduced knapweed seed numbers per seed head in 2006. Spotted knapweed should not be mowed at the bolting and flower bud stages if large populations of seed head insects are present because mowing can result in the formation of new seed heads that are free from the insects' attack, thus allowing greater seed production. Mowing of spotted knapweed at the flowering stage and later can be conducted without a subsequent increase in seed production, but the mowing may cause mortality of the insect larvae.
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