Potato leafhoppers are on the increase in alfalfa fields across the state. Now is the time to scout alfalfa on a weekly basis for these pests. If leafhoppers are abundant, there will be losses in yield. The most obvious signs of injury are the reduced stem height and yellowing of the leaves, which is called hopperburn. Damaged plants are severely stunted, often to the point of providing opportunities for increased weed growth. Losses in harvest tonnage are usually attributed to this stem-shortening response.
The effect of feeding on forage quality is seen in the leaves, where the blockage of fluid results in leaves that have about half the normal amount of crude protein. Digestibility and energy availability are not significantly reduced in damaged leaves. In most situations, even damaged leaves have enough protein to fulfill nutritional requirements of ruminants. However, in dairy production crude protein may be limiting, so control of hopperburn is necessary.
Severe hopperburn on alfalfa.
An additional impact of potato leafhopper feeding on alfalfa is the delay in maturity of the crop. In some situations, the delay may be extreme enough to eliminate a final harvest in the fall.
To scout for leafhoppers, begin checking regrowth after the first cutting and continue to check every 5 to 7 days. Do not wait for hopperburn to appear on the leaves before scouting. Use a sweep net and take 20 sweeps in 5 different areas of the field. Consider an insecticide application if the number of leafhoppers exceeds the following thresholds:
- If the hay is less than 10 inches tall, consider spraying if you collect more than 0.1 leafhopper per sweep for each inch of plant height. For example, if the hay is 7 inches tall, spray only if the average number of leafhoppers exceeds 0.7 per sweep.
- If the hay is taller than 10 inches, 2 or more leafhoppers per sweep is an acceptable threshold. Some of the insecticides labeled for leafhoppers are listed below. The minimum label rate and the preharvest interval are given. Always check the label for additional cautions and remarks.
Insecticides labeled for leafhoppers
This article originally appeared on pages 116-117 of the IC-476(17) -- July 8, 1996 issue.