University of Illinois

No. 9/May 22, 1998

Insect Activity in Alfalfa

Development of alfalfa weevils has progressed enough in most areas of Illinois that maps revealing heat-unit accumulations no longer are needed. Larvae are feeding on alfalfa throughout the state, but reports of economic damage are few and far between. If such is the case, the likelihood of injury to regrowing buds after a cutting is minimal. However, for your information, we remind you that control may be warranted after a cutting when alfalfa weevil larvae and adults are feeding on more than 50 percent of the crowns and regrowth is prevented for 3 to 6 days.

Information from entomologists in Kentucky reveals that potato leafhoppers are active in alfalfa fields in some areas of the their state. We know that potato leafhoppers have arrived in Illinois (remember, they do not overwinter here), and it's just a matter of time before their densities build to threatening levels. Adult leafhoppers arrive in Illinois in the spring of the year, borne on prevailing winds from the southern states. As soon as the adults "settle in" to our alfalfa fields, they feed a little, and females begin laying eggs to start a new generation. In most fields, densities of leafhoppers do not increase to threatening levels until after the first cutting. However, as soon as the first cutting is made, feeding by leafhoppers on regrowing alfalfa can be quite damaging. We recommend that scouting for potato leafhoppers in alfalfa should commence almost immediately after the first cutting. It only takes 0.2 leafhopper per sweep of a sweep net to cause economic damage to regrowing alfalfa plants (0 to 3 inches tall).

Also, if you are scouting alfalfa that was planted this spring, be especially watchful for potato leafhoppers. Spring-seeded alfalfa is especially susceptible to injury caused by potato leafhoppers. As more develops with this important insect pest, we'll keep you informed.

Kevin Steffey (, Extension Entomology, (217)333-6652