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First cutting of alfalfa marks the beginning of the potato leafhopper infestations. Because of the late spring we’ve had this year the crop progress is somewhat behind average, especially in eastern SD, but once you pass the first cutting it is a good idea to keep potato leafhoppers in mind. Scouting is critical: once you see symptoms of their damage, known as hopper burn, it is too late. Hopper burn, the characteristic V-shaped yellowing of the leaf tips, is caused by the toxic properties of the saliva of these leafhoppers, which have piercing-sucking mouthparts. Potato leafhoppers have multiple generations per year, and while the first cutting of alfalfa is usually not affected, all later cuttings are at risk. They are especially damaging to new seedlings. Heavy populations of potato leafhoppers can stunt the plants and significantly reduce the yield.
Fields should be scouted every week following the first cutting of alfalfa. Use a 15” sweep net, and sample when the plants are dry. Take 10 sweeps at 10 random locations within the field by walking in a ‘U’ pattern, avoiding field margins. After taking 10 sweeps, swing the net a few more times to force the insects to the bottom of the net, grasp the net about 10 inches from the bottom and slowly open the net. Count the potato leafhoppers while slowly opening the net: they are going to be jumping out fast but if you unfold the net slowly, you will be able to count them. Count only the pale green or yellow leafhoppers; there may be other leafhoppers and jumping insect that are captured in the net. Average the number of leafhoppers you captured per 10 sweeps and use the threshold tables (Potato Leafhopper Scouting In Alfalfa Is Critical Now) to make a management decision based on height of alfalfa and cost of insecticide applications.
In addition to the potato leafhoppers that are about to start occupying our scouting calendar, alfalfa weevils (Spring is Here: Watch out for alfalfa weevils) should remain on everyone’s radar. They may be a little slower this year but we have gotten reports of activity in the state already, and scouting should continue. For both potato leafhoppers and alfalfa weevils early cutting is one of the recommended management strategies, if possible, and if populations are high watch for slow regrowth following the cutting.
Aphids are also virtually always present in alfalfa but their impact on the crop is not significant unless their numbers are very high. Below are some examples of aphid species you may find in alfalfa and their thresholds. Pea aphids are probably the most common in alfalfa in SD and tend to get a lot of people nervous because they are pretty large aphids. However, you need over 40 pea aphids per stem before they affect the yield! Remember that scouting for aphids should be done by examining individual stems rather than sweeping. Take a random sample of about ten stems in five areas of the field and average the number of aphids per stem for the field. Aphids in alfalfa rarely require insecticide management.
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