Written collaboratively by Patrick Wagner, Philip Rozeboom, Brad McManus, and Adam Varenhorst.
Large populations of blister beetles were observed while scouting alfalfa fields this past week. Blister beetles feed on leaves and blossoms and contain a toxic chemical called cantharidin. This chemical poses a health threat to livestock that consume bales of alfalfa containing whole or partial blister beetle bodies. In South Dakota, these beetles don’t typically appear until late June or early July. The first cutting of alfalfa is underway or occurring soon in many parts of the state. Although blister beetles usually aren’t an issue until after the first cutting, their early activity this year means growers should monitor alfalfa fields prior to harvest from now through the remainder of the season.
Profile
Blister beetles have one generation per year. Adults can be up to 1 inch long, have a soft, elongate body, and vary in color and size based on species. A unique characteristic of blister beetles is that their thorax (middle body segment) is narrower than both the head and abdomen. The most common species and the first to be observed this year is the ashgray blister beetle (Figure 1).
Blister beetles get their name because their blood contains cantharidin, which causes blisters if it comes in contact with skin. The beetles release this toxic chemical through “reactive bleeding” to defend themselves from predators. Cantharidin is also released if the beetles are crushed and becomes a problem when feeding infested hay to livestock. If an animal ingests enough blister beetles, it can lead to mouth sores, digestive issues, kidney damage, and, in severe cases, death.
Management
It is important to scout alfalfa fields for blister beetles prior to each cutting. Second and later cuttings are at higher risk of infestation because blister beetles are more abundant by mid-summer. Due to their predation on grasshopper eggs as larvae, blister beetles are more likely to be present in areas where large grasshopper populations were observed during the previous year.
Insecticide applications are not recommended as this kills the beetles but does not remove them from the hay. Dead blister beetles remain toxic, and spraying may increase the number of beetles per bale.
An alternative is to cut alfalfa prior to peak bloom as blister beetles are attracted to the blossoms. The cut alfalfa should be allowed to dry fully before raking so that the beetles have time to vacate the plants prior to baling. If alfalfa is infested, it is also important to mow or windrow the hay and not crimp it. Crimping increases the likelihood of crushed beetles remaining in the hay. Based on personal observations, many blister beetles will escape from mowed alfalfa within the first 3-6 hours.
When selling hay, ensure that blister beetles aren’t present or notify the buyer of any infested bales. Horses have a particularly high sensitivity to cantharidin, so it is important that they don’t consume infested hay. In addition, avoid feeding hay from field edges to horses because blister beetle numbers are typically highest in those areas.
