DAMAGE POTENTIAL OF THE SWEETCLOVER WEEVIL, SITONA CYLINDRICOLLIS (COLEOPTERA: CURCULIONIDAE), IN THE CANADIAN PRAIRIES

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Abstract
Adults of the sweetclover weevil, Sitona cylindricollis Fähr., injure seedling, vegetative, and yearling sweet clover, Melilotus officinalis (L.) Lam. In field cage tests an infestation of 1 weevil: 7 seedlings at the cotyledon stage destroyed less than 5%, and an infestation of 1 weevil: 1 seedling destroyed 100% of the seedlings. Significant reduction of seedlings was caused by infestations of I weevil: 3 or 5 seedlings depending upon growing conditions. An infestation of 4.5 to 9 weevils per emergent second-year plant caused a significant reduction, and an infestation of 18 weevils per plant caused almost total destruction of second-year forage yield. In the Canadian mid-west, weevil populations seldom exceed these minimum threshold values, therefore the weevil should not be considered a deterrent to either stand establishment or to stand retention in the second year. Defoliation of vegetative clover by weevils at the beginning of and mid-way through the critical period, which extends from 15 August to 20 September, caused a 50% loss of second-year forage yield. Because of cultural practices associated with sweet clover production in western Canada this may be the most severe type of injury wrought by the weevil.