Management of forage insect pests is important to achieve high yields of high-quality forage. The primary insect problems in Ohio are the alfalfa weevil and the potato leafhopper in alfalfa. The alfalfa weevil is primarily active in the spring. The potato leafhopper is active during the summer months and can cause severe yield and quality losses in alfalfa. New alfalfa seedings are especially vulnerable to potato leafhopper damage.
When pest populations reach or exceed action thresholds, it is economically justifiable to either harvest the crop, provided it is near the harvestable stage; or treat the stand with an insecticide to control the pest in question. Producers should scout fields and determine if the action threshold has been exceeded.
A general threshold for potato leafhoppers in alfalfa is as follows: if alfalfa is more than seven days from a harvest for plants under normal stress, treat when the average number of leafhoppers in a single sample (10 sweeps) equals or is greater than the height of the alfalfa. For example, if the alfalfa is 8 inches tall and the average number of leafhoppers per sample is eight or higher, treatment is warranted. If the average is seven or lower, the grower should come back within a few days to see if the population is higher or lower. Vigorous alfalfa can tolerate higher numbers, and stressed alfalfa can tolerate fewer. The threshold should be lowered when the alfalfa is under stress, especially for new seedings. Potato leafhopper-resistant alfalfa varieties offer an excellent tool for managing this insect pest in Ohio. The action threshold for leafhopper resistant varieties is about three times the normal threshold for susceptible alfalfa.
The alfalfa weevil is a small, brown, snout-nosed beetle approximately 3/16 inch in length with a wide dark stripe down its back. The larva is green with a black head and a white stripe down its back. Both the adult and larvae feed on alfalfa foliage. Foliar feeding injury by the adult is not significant. Foliar injury by young larvae is primarily confined to the growing tips. Older larvae may extensively defoliate alfalfa when abundant. In general, foliar injury by alfalfa weevil occurs on the first cutting of alfalfa. During periods of heavy weevil activity, early growth of the second cutting may be affected.
Over the past few decades, populations of alfalfa weevil have seldom reached economic levels of abundance due to biological control by a complex of three parasitic wasps and a fungal pathogen. Occasionally, however, a rescue treatment of insecticide is warranted. Application of an insecticide to prevent excessive defoliation is justified when one or more late instar larvae are found feeding per stem and the stand cannot be harvested early. Because alfalfa weevil is usually controlled by beneficial wasps, which are susceptible to chemical treatments, it is important that treatments not be applied unless necessary. The yield impact of weevil feeding declines as alfalfa stand height increases, and decisions to treat alfalfa for weevil should be focused on an alfalfa stand when larvae can be readily found on alfalfa that is 12 inches or less in height. Once alfalfa is 16 inches or taller, early cutting is a preferred option for reduction of weevil impact.
More information about potato leafhopper and weevil management in alfalfa can be found in fact sheets and videos at forages.osu.edu/forage-management/pestsdiseases and at aginsects.osu.edu.