Although potato leafhoppers have been present in alfalfa fields for weeks, their numbers thus far have not generated much concern. Jim Morrison, University of Illinois Extension crop systems educator in Rockford, found an average of 0.2 and 0.25 leafhopper per sweep in Stephenson County fields of 6-inch and 12-inch alfalfa, respectively, on June 17. These numbers are relatively low. However, as wet weather subsides and more typical hot, dry weather prevails, densities of potato leafhoppers could increase rather quickly. Although leafhoppers do not have the reproductive capacity of aphids, they can complete a generation in about 20 days, depending on temperatures. Potato leafhoppers complete several generations in a year in Illinois, and when generations begin to overlap, densities can double in size in about 8.5 days.
Potato leafhopper nymph (left) and adult (right). (Photo courtesy of Marlin Rice, Iowa State University.)
In issue no. 9 (May 23, 2003) of the Bulletin, Kelly Cook included an article about potato leafhoppers and discussed their appearance, symptoms of injury they cause to alfalfa, monitoring guidelines, and static thresholds. If you are interested in more dynamic thresholds, use the ones developed by entomologists at Iowa State University, who have indicated that economic thresholds need not depend on plant height. Table 2 provides some economic thresholds for your consideration based on crop value, control costs, and leafhopper densities. In general, these thresholds are less conservative than those (based on plant height) mentioned previously for shorter plants and more conservative for taller stands (12 inches or more). Refer to the aforementioned article for insecticides suggested for leafhopper control in alfalfa and the preharvest intervals for all products.--Kevin Steffey