Conditions are right for heavy – and late – alfalfa weevil outbreaks this year, said a University of Missouri entomologist. Alfalfa growers are urged to scout their fields for the small green worms. Warm, summer-like days this spring encourage feeding by alfalfa weevil, said Wayne Bailey, extension entomologist. Weevil damage has been varied and mostly moderate across the state. Heaviest damage, requiring control sprays, is in southwest Missouri. At the MU South Farm near Columbia more than 1,000 weevil larvae were collected per 10 sweeps of an insect net in recent scouting trips.

“When we got above that level last spring, the weevil just ate us up,” Bailey said. The potential for damage is high because of the large number of eggs that were laid over winter and continue to be laid, Bailey said. “Warm dry weather last fall encouraged adult weevils to deposit lots of eggs. A mild winter did not control the weevil, and the adults are laying more eggs. “We’re seeing more egg laying late into the season. This is about three weeks later than usual.”

The late eggs indicate potential damage to second cutting alfalfa, Bailey said. Missouri farmers harvest alfalfa four or five times a season. “Usually, we spray once in the spring or harvest the first cutting of hay before the weevil larvae does much damage,” Bailey said. “This year there could be lots of freshly hatched adult weevils available when the crop tries to start re-growth,” after mowing. Adult weevils girdle the stems of the re-growth, causing death of the plant.

Wet weather confuses the weevil outlook, Bailey said. If weevils are there, and farmers can’t get a spray rig into the field, the insects can eat up a crop in about three days. Wet weather, however, could encourage a fungal pathogen that kills weevil larvae. “We haven’t found the fungus yet this year,” Bailey added. Farmers should check alfalfa fields every couple of days. The first sign of infestation will be small holes eaten in the leaves. To scout for larvae, which hide in the terminal buds, Bailey said 50 stems should be collected in a white bucket. When stems are shaken, the small green larvae can be counted on the bottom of the bucket. Care must be taken that the weevils are not shaken off before the stems are put in the bucket, Bailey cautioned.

The economic threshold for applying an insecticide is one larva per stem. Many insecticides are approved for weevil control, Bailey said. On almost-mature plants nearing the one-tenth-bloom stage, harvesting will reduce the weevil population by up to 98 percent. Cattle in management-intensive grazing systems will control about 90 percent of the weevils.