BLISTER BEETLES IN ALFALFA

This time of year blister beetles become a major consideration to hay buyers and producers. Adult beetles are active from mid-May to October; however, striped blister beetles (important one to alfalfa growers or buyers) concentrate in high numbers in June and July. An estimated 75% of the total numbers of blister beetles occurring in Oklahoma alfalfa are striped blister beetles.  

The primary problem with blister beetles is related to the toxin, cantharidin, present in the insect's body fluids. Cantharidin provides the beetles protection, but may also effect livestock health when beetles are inadvertently incorporated into baled alfalfa during harvest. When livestock, particularly horses, are fed forage containing bodies or fragments of these insects, illness or death can result. Cattle and sheep also have died from cantharidin poisoning. The biggest sub-lethal effects on ruminants have been reduced milk production and weight gains. Horse poisoning has been recognized in Texas, Oklahoma, Illinois, and Colorado, just to name a few. If the buyer knows when the hay was cut, buying Oklahoma alfalfa should warrant no more additional risk than buying hay elsewhere. In Oklahoma, we suggest using first or last cuttings for horse feed. These cuttings have the least risk of blister beetle contamination.  

Growers can reduce the risk of contaminated hay by following these suggestions: carefully monitor hay before cutting, cut at 10% bloom or less, spray infested areas and/or field borders with low residual insecticides, do not use a crimper when swathing hay and do not drive over cut hay while cutting the next swath. If the crimper is removed and the windrow is straddled by subsequent tire traffic, blister beetles are allowed to crawl away from infested hay. Unfortunately, this practice greatly increases drying time for the hay.  

Short residual insecticides that provide an alternative include Malathion, permethrin, and Sevin^® . The efficacy of the first two products on blister beetles in alfalfa has not been thoroughly tested. In addition, their labels do not list blister beetles as a target pest. Sevin^® insecticide is cleared for this use, however, there is a 7 day waiting period before harvest. Beetle swarms from adjacent areas can move into fields treat with Sevin^® before the alfalfa can be legally harvested. 

FOR MORE INFORMATION:  Blister Beetles and Alfalfa. OSU Extension Facts F-2072.