No. 10/May 30, 1997
What Insects Are in Alfalfa
The alfalfa weevil seems to be leaving us with a whimper this year. We have received very few reports of alfalfa weevil activity that requires much attention this spring. Also, enough heat units have accumulated throughout the state that maps predicting their development no longer seem necessary. However, because surviving larvae and adults can feed on regrowing buds after the alfalfa has been cut, growers should watch the fields closely after cutting to make certain the alfalfa greens up nicely.
The other key insect in alfalfa is the potato leafhopper. We reported its presence as early as mid-April this year, and folks scouting alfalfa fields are beginning to find leafhoppers with relative ease. At least one person reported numbers of leafhoppers above the economic threshold in western Illinois. Remember that the "static" economic thresholds for potato leafhoppers vary with the height of alfalfa. An insecticide application might be warranted if the following combinations of leafhopper numbers and alfalfa height are reached or exceeded: 0.2 per sweep in alfalfa 0 to 3 inches tall; 0.5 per sweep in alfalfa 3 to 6 inches tall; 1 per sweep in alfalfa 6 to 12 inches tall; and 2 per sweep in alfalfa 12 inches or taller.
The next few weeks will be a critical time to monitor the development of potato leafhopper populations in alfalfa fields. Numbers of leafhoppers tend to increase around the time the first cutting of alfalfa is made. Tender alfalfa that is regrowing after a cutting is particularly susceptible to leafhopper injury. Although current weather conditions are not ideal for rapid buildup of leafhopper populations, the arrival of warmer weather will accelerate their development.
Scout for leafhoppers by using a 15-inch-diameter sweep net. As indicated previously, economic thresholds are based on numbers of leafhoppers per sweep of a sweep net, not on the appearance of injury. In fact, when you see symptoms of leafhopper injury (see image below), initially a V-shaped yellowing at the tips of the leaflets, some yield loss and reduction in hay quality already has occurred. Don't let potato leafhoppers get ahead of you this year, as they did in some fields of alfalfa in 1996.
|Potato leafhoppers and injury to alfalfa|
Some observers are finding aphids and spittlebugs in alfalfa fields. The aphids are probably pea aphids, and unless their numbers become extremely large, they cause no economic damage to alfalfa. Dave Feltes, IPM Educator at the Quad Cities Extension Center, and Jim Morrison, Crop Systems Educator at the Freeport Extension Center, scouted some alfalfa fields in northwestern Illinois during the week of May 19. They found very few alfalfa weevils and noted the presence of aphids and spittlebugs. Ellen Phillips, Crop Systems Educator at the DeKalb Extension Center, also observed spittlebugs in some alfalfa fields in DeKalb County. These spottings make it worthwhile to discuss spittlebugs.
The spittle masses found in alfalfa fields were created by the nymphs, which require a moist environment to survive. If you tease apart a spittle mass, you should be able to locate one or more orange-yellow to yellow-green or green nymph (color varies with size) inside the spittle. These insects have piercing-sucking mouthparts with which they extract fluids from the plants. Nymphs typically are present for 5 to 8 weeks, depending on the temperature.
Although spittle masses may seem prevalent in some fields, don't overreact to their presence. Spittlebugs infrequently cause economic damage in alfalfa. However, if their numbers exceed one per plant, which is an exceptionally large population, they can cause yield losses.
Kevin Steffey, Extension Entomology, (217)333-6652