Two perennial pests in alfalfa in the western Corn Belt are alfalfa weevil and potato leafhopper. While these two insects are present in many of the same fields during the growing season, they have very different growth cycles, timing and type of crop damage.
Alfalfa weevils overwinter as adults and in some cases as eggs laid in the fall. Warm temperatures in the spring initiate egg laying and larval hatching. The damage to alfalfa is from larval feeding (Figure 1). In the southern parts of Kansas and Missouri, this can occur as early as the last week of March or early April, depending on spring temperatures. Scouting for alfalfa weevil should begin as soon as alfalfa begins to grow in the spring. Alfalfa weevil can reduce crop yields dramatically in a short period of time if left untreated. Harvest is an excellent control method if threshold levels are present when the crop is approaching bud stage; however, if the crop is several weeks away from harvest, a labeled insecticide application should be considered. Alfalfa weevils are less of a threat in late summer.
Adult potato leafhoppers (Figure 2) overwinter on the gulf coast and are carried north with spring storms. These pests can infest alfalfa in midsummer to late summer and thrive in warm temperatures. They feed by piercing alfalfa stems to suck out plant tissues. In the process of feeding, they inject toxins that can reduce plant growth. This damage can be confused with drought damage or in some cases a fertilizer deficiency. Yellow triangles appear on leaf tips (Figure 3) and the crop can have a stunted yellow appearance. Potato leafhoppers often attack small alfalfa stems after cutting, thus restricting regrowth. An insecticide application may be used to control potato leaf hopper; however, if the infestation is severe regardless of plant height, harvest may be needed to remove the infected stems and allow for new regrowth. Damaged stems will not regrow after they have been infected even if the hoppers have been controlled with an insecticide.