Following the first cutting, alfalfa fields should be monitored for the presence of blister beetles. These beetles are usually considered beneficial insects. However, blister beetles can become an issue when large populations of them are present in alfalfa fields. Alfalfa that is infested with blister beetles can be toxic and cause health concerns for livestock, especially horses, when it is used for feed.
Profile & Behavior
Blister beetles are commonly found throughout South Dakota. They have a soft, elongate body and most are about 1 inch long. Coloration varies based on the species, but they are typically either black, striped, or grey (Figure 1). A unique characteristic of these beetles is that their thorax is narrower than both the head and abdomen.
Blister beetle larvae are predatory and feed on a variety of other insects, including grasshopper eggs. Adults feed on leaves and flowers, making them highly attracted to flowering plants. Blister beetles get their name because they contain a chemical called cantharidin that causes blisters. The beetles use this chemical for defending themselves from predators. Cantharidin is released if the beetles are crushed, which causes problems when feeding contaminated alfalfa to livestock. If an animal ingests enough blister beetles, it can lead to sickness or even death.
To manage blister beetles, it is important to check alfalfa fields prior to each cutting. Second or third cuttings are at higher risk of an infestation because blister beetles are usually more abundant by early to mid-summer. Blister beetle populations are more likely to be present in areas where large grasshopper populations were present the previous year. Consider management if large numbers of blister beetles are present in the field. Several insecticides are labeled for blister beetles and can be found in the current version of the South Dakota Pest Management Guide: Alfalfa. Alternative management is to allow cut alfalfa to dry fully before raking so that the beetles are able to leave the crop before it is bailed. Furthermore, harvesting alfalfa in the bud stage minimizes flowering and will reduce the field’s attractiveness to blister beetles.