Photo: North Dakota State University

There are lots of insects that cause alfalfa plant damage and yield loss. The list is much shorter for those insects that have negative impacts on animals that actually consume the alfalfa — and blister beetles top that list.

In the Northern Plains, blister beetle sightings are already being reported in alfalfa fields, according to James Rogers, an extension forage crops production specialist with North Dakota State University (NDSU).

Blister beetles produce a naturally occurring toxin called cantharidin. If the beetle is crushed, this toxin is released and causes blistering on the skin. Horses are especially sensitive to hay that is contaminated with crushed blister beetles. Several years ago, a Wisconsin horse ranch reported losing 17 animals to blister beetle-infested hay that they had purchased from a Western hay producer.

The potential for lethal consequences that blister beetles pose to horses and other livestock has been widely known and chronicled for many years. Actual occurrences are rare, but that provides little solace to the unknowing horse owner who experiences a catastrophic loss.

Adult blister beetles are attracted to blooming alfalfa fields and weeds such as goldenrod and dandelion. They feed on nectar and pollen and can also devour leaves, stems, and flowers. Blister beetles produce one generation per year and will be active from June to September, laying eggs in the soil from late summer to early fall. There are several species of blister beetles, including black, ash gray, and striped.

“Cantharidin from blister beetles can cause severe inflammation and even death in horses,” says Rogers. “Depression, inflammation and ulceration of the mouth, irritation of the gastrointestinal tract, and painful urination are common symptoms of sublethal doses of cantharidin. Cattle and sheep are much more tolerant of cantharidin ingestion,” he adds.

The forage specialist notes that blister beetle toxicity levels are higher in males but also vary by species and by the region in which the beetles are found. The 1/2- to 1-inch-long adult beetles are easily identified among other beetles as the “neck” region located behind the head is narrower than the head when viewed from above.

“Blister beetles are mobile and tend to swarm in large numbers in small areas of a field,” says Miranda Meehan, a NDSU extension livestock environmental stewardship specialist. “They can move into alfalfa fields from field edges that have a host plant such as sweetclover, which blooms earlier than alfalfa. Once alfalfa begins to bloom, they may move into a field and feed for a short period of time before migrating to other areas of the field or to new fields.”

The NDSU specialists recommend the following practices to reduce the risk of crushed blister beetles contaminating hay:

1. Use equipment such as a disc mower without hay conditioners or crimpers that may crush blister beetles and release toxin into the hay. This also encourages beetles to move out of the hayfield.

2. Allow cut hay to fully dry before raking to allow beetles to move out of the hay. Raking may dislodge dead beetles from hay; however, the potential still exists for cantharidin in the hay.

3. Control blooming weed hosts near or in alfalfa fields.

4. Cut alfalfa at less than 10% bloom.

5. Check fields 24 hours prior to cutting to ensure that new swarms of blister beetles have not reinfested the field.

6. Scout harvested hay and underneath windrows closely for blister beetles and allow beetles to move out of the drying hay before baling. Turning the windrows may be helpful to get blister beetles to move out.

7. If large numbers of blister beetles are observed in spots during baling, quit harvesting and allow blister beetles to move out or harvest around them.

8. If blister beetles are suspected in harvested hay, do not feed it to horses.

9. If blister beetle toxicity is suspected, a lab test of the animal’s plasma or urine can be done to confirm the diagnosis.