NDSU Extension - Dickey County


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Blister Beetle Watch

Breana Kiser, Agriculture and Natural Resource Agent

Since pests tend to travel north during a growing season, there have been reports of blister beetles in SD. When cutting your alfalfa this season keep a look out for blister beetles.

Blister beetles are an infrequent pest of several crops including alfalfa, sweet clover, potatoes, beans, and sugar beets. They normally don’t cause much plant damage but when ingested by horses or other livestock they can cause illness of even death.

There are several species found in the Great Plains during the summer months. Adults range from ½ to 1 inch long and have a characteristic narrow, elongate, soft body. As their name implies, striped blister beetles have black stripes that run the length of their yellow/orange abdomens. The flexible wing covers are rounded over the abdomen and the color varies from black to gray to brown.

Blister Beetle 1The common blister beetle in the Great Plains is the striped blister beetle. Striped blister beetles have an orange head with two oval markings that are separated by an orange line. The thorax, which is narrower than both the head and abdomen, is grey with two black lines on it. They have black antennae and gray/black legs. All blister beetles have elytra that are soft, and generally do not cover the end of their abdomen.


Blister Beetle 2

Another blister beetle that can be found is the Ash-gray blister beetle. Ash-gray blister beetles are similar to striped body shape. The thorax, which is narrower than both the head and abdomen, is grey with two black lines on it. They have black antennae and gray/black legs. Unlike the striped, the ash-gray blister beetle has light gray on its head, wings, and thorax.

All species of blister beetles produce a toxic substance called cantharidin. This toxin is a well-known vesicant that is quickly absorbed upon contact and causes inflammation and blistering on internal and external body tissues. The amount of toxin produced varies considerably between species. The ash-gray, black, striped, and margined blister beetles are a few of the more common species which produce levels of cantharidian capable of poisoning livestock. Livestock come in contact with blister beetles when they consume infested alfalfa hay. Horses are most susceptible to the toxin, while sheep and cattle are more tolerant. The reaction to the toxin depends upon the relative dose; enough ingeste4d beetles can be lethal to any animal.  

Several management options are available which can reduce the number of blister beetles found in forage crops but none will eliminate the problem. Adjust harvest dates and maintain weed free alfalfa. Since blister beetles are readily attracted to flowering plants, controlling the number of flowering weeds in the field and cutting alfalfa prior to bloom stage will reduce the potential for infestation.

Check hay for blister beetles prior to cutting. Blister beetles are gregarious and are often found in high numbers in localized areas of the field. Prior to harvest, growers should be aware of potential infestations, and if blister beetles are present in the field, the harvest should be delayed for several days. In many instances, the beetles will move. However, they may move to another part of the field, so a careful inspection is necessary. If beetles are present in the field at the time of harvest, avoid using hay conditioners or crimpers. These implements may kill the beetles and prevent them from moving out of the hay as it dries. A self-propelled harvester which has wide-set wheels and no conditioner or crimping equipment can be used to windrow the hay, resulting in fewer dead blister beetles in the hay.

Apply an insecticide for beetle control prior to harvest. Fields suspected of being infested should be thoroughly scouted (concentrating near the field edges) prior to harvest, and if blister beetles are present, an insecticide may be applied for control. Beetles killed by the insecticide will most likely fall to the ground and should not be picked up by the harvesting equipment.

Fields should be rechecked 24 hours prior to cutting to ensure that new swarms of blister beetles have not re-infested the fields. Several insecticides registered for use on alfalfa, dry beans, soybeans, potatoes and sugar beets show good activity on blister beetles. Observe label directions for rates, pre-harvest intervals, restrictions and precautions. Fields should not be treated at peak bloom to avoid bee kill. In all cases, hay suspected of being infested with blister beetles should be checked for beetles prior to feeding.

Contaminated hay should not be fed to horses or other livestock; removal of the beetles from the hay will not make it safe. It is to the grower’s advantage to minimize harvest operations which kill blister beetles, thereby minimizing the possibility of feed contamination. Management practices can only reduce the number of blister beetles present and the potential risk of cantharidin poisoning.


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