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Use of cultivated sunflower (Helianthus annum L.) as a host by two species of seed weevils, Smicronyx fulvus LeConte, the red sunflower seed weevil, and S. sordidus LeConte, the gray sunflower seed weevil, was compared using dissection of achenes from naturally and artificially infested plants. The gray seed weevil oviposits on the tips of unopened sunflower florets before anthesis. Larvae migrate through the corolla tube, penetrate the developing achene, and begin extensive feeding at the achene base. Achenes infested by the gray sunflower seed weevil are enlarged, lack an embryo, and appear to be a type of gall. The red sunflower seed weevil oviposits internally in more mature achenes after anthesis has begun, and these achenes have well-developed seed embryos.