Sarah Sommerfeld, PAg, Regional Forage Specialist, Outlook
Saskatchewan Ministry of Agriculture
Alfalfa weevils are becoming a pest of increasing concern for forage producers. Through recent surveys of alfalfa fields conducted by Ministry of Agriculture Regional Forage Specialists, data shows that the alfalfa weevil can be found throughout Saskatchewan, with the exception of the North West area of the province.
Adult alfalfa weevils over-winter in plant debris and alfalfa stubble. Adult weevils emerge in the spring and begin feeding on alfalfa leaves. Through mid to late May, female adult weevils lay eggs in the alfalfa stems. Within one to three weeks the eggs hatch and the larvae begin feeding on new plant growth.
Mature alfalfa weevil larva are up to eight millimeters in length, green in color with a white stripe on the back and have a black head capsule. Mature larva chew on developing buds and leaves, leaving leaves with a skeletonized appearance. Feeding damage stunts plant growth and the field takes on a silvery, sheen appearance. Extensive damage may result in some fields not flowering. This is important to remember as the recommended cutting time for alfalfa is 10 to 20 per cent bloom.
Larvae primarily feed from late May to July. Peak feeding activity can occur from mid-June to mid-July. After larvae have completed several development stages, larvae move down the plant to ground level and pupate. Adult weevils emerge one to two weeks later and continue feeding through late summer.
To determine weevil populations, field scouting is necessary. Field monitoring should begin in early June. Fields should be checked every three to four days. Pure alfalfa stands or fields with a high percentage of alfalfa will be impacted the most. Larvae are difficult to count on the stems, and often drop to the ground when plants are disturbed. Begin scouting by walking a W-shaped pattern in the field. Carefully grab a handful of stems, cut and slap on a hard surface to dislodge any larvae. Use of a sweep net is very effective, as the net is sweeping ahead of any disturbance.
For alfalfa crops grown for hay, the most effective method of control is early cutting. However, if early cutting is not feasible, and the estimated yield and quality loss will be substantial, an insecticide application is recommended. Yield and quality loss depends on the vigor of the stand, weevil population and life cycle stage and expected number of days until cutting. In areas where weevil damage has been extensive, larval populations have remained high following cutting. Damage to crop regrowth has been significant and chemical control was recommended.
For hay crops, economic thresholds for chemical control are based on number of larvae per stem and plant height. If plant height is less than 30 centimeters, the threshold is one larvae per stem. If plant height is less than 40 centimeters, the threshold is two larvae per stem. Three larvae per stem require immediate control regardless of plant height. For a complete list of products registered for chemical control of alfalfa weevil and more information on thresholds, please refer to the 2013 Guide to Crop Protection.
For more information on the alfalfa weevil and possible control options, contact the Regional Forage Specialist at (306) 867-5559, visit the Ministry of Agriculture website at www.agriculture.gov.sk.ca and watch the web video Monitoring Alfalfa Weevil or contact the Agriculture Knowledge Centre at 1-866-457-2377.