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Stem Weevil Control in Sunflowers
Adult feeding and egg laying for black and spotted stem weevils commences around mid-June (2 to 4 leaf stage). Damage by the adults is insignificant, however, the organism that causes Phoma black stem in sunflowers has been isolated from both species. The black stem weevil has successfully transmitted Phoma under greenhouse conditions.
To date ONLY larvae of the SPOTTED STEM WEEVIL LARVAE have been found to cause serious stalk breakage. This occurs when 25 to 30 larvae are present in a stalk, weakening it when larvae make their overwintering cells in the stalk's base. Breakage is most likely to occur during drought stress or high winds.
The sunflower stem weevil is 3/16 inches in length, and grayish-brown with varying shaped white spots on the wing covers. The weevils emerge in mid to late June. Eggs are deposited in epidermal tissue of the stem. If controls are directed at the adults in order to minimize egg laying, treatments should be initiated during the first few days in July. About 50% of the eggs will be deposited by this weevil by mid July.
Scouting for these insects is difficult due to their size, coloration, and habit of "playing dead". Examine 5 plants each at 5 locations and keep record of the number of weevils found. Approach plants carefully to avoid alarming the weevils, causing them to drop to the ground. Scout from late June to mid July.
Threshold:
Treatment for sunflower stem weevils is recommended when scouting determines that an average of 1 adult per three plants is found.
Furadan 4F applied at planting has been shown to reduce larval populations in stalks, but a well-timed foliar application can also provide good control. Products registered for adult stem weevil control include Asana XL, Scout X-tra, Warrior, carbaryl, Furadan 4F and Lorsban 4E.
For more information consult the Insect Guide at http://www.ext.nodak.edu/extpubs/plantsci/pests/e1143w1.htm
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