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J. M. Story*, J. G. Corn, and L. J. White
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
should not be mowed at the bolting and flower bud stages if
large populations of seed head insects are present because
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
cause mortality of the insect larvae.
© 2012, Organic Agriculture Centre of Canada (OACC)