Blister Beetles of Montana

By Hayes Goosey, Forage Specialist; Tiziana Oppedisano, Agronomic Entomologist; Kaleena Miller, Madison-Jefferson County Agent; Allison Kosto, Broadwater County Agent; Elizabeth D’Imperio, Gallatin County Agent; and Gabrielle Sexton, Dawson County Agent; all MSU Extension

Blister beetles (meloid beetles) are leaf-feeding insects that secrete a liquid that can cause blisters, and can be toxic when accidentally fed to livestock in forage. Of nine species observed in Montana, only one has been reported to have damaged crops. This fact sheet describes the species found in Montana and discusses the risks of crop damage and livestock poisoning.

BLISTER BEETLES DERIVE THEIR NAME FROM A POTENT toxin known as cantharidin, present in their bodily fluids. When these beetles are disturbed, they exude a fluid containing this toxin from their leg joints, which can cause blistering upon contact with the skin.

While beetles inadvertently killed during alfalfa harvest and subsequently incorporated into baled alfalfa can pose a toxicity risk to livestock, extensive surveys conducted over multiple years in Montana have determined that blister beetle species and their seasonal occurrences are generally of minor concern to alfalfa producers and individuals who feed hay produced in the state.

Damage and risk posed by blister beetles

Numerous species of blister beetles are present across the United States, including Montana, where their prevalence varies by region, season, abundance during the growing period, and toxicity levels, all of which determine the severity of their threat as pests. Blister beetles commonly target leaves of crops such as potatoes, sugarbeets, soybeans, and alfalfa. In Montana, they are most commonly found in alfalfa hay and canola fields, with occasional reports in potatoes and other broadleaf crops.

Blister beetles can exhibit both direct and indirect pest behavior depending on the crop and the potential for damage. While they are not typically considered direct pests in alfalfa forage, their adult feeding activities may lead to localized defoliation. The primary concern with blister beetles in alfalfa lies in the risk of contaminating the forage with cantharidin toxin. This contamination occurs when beetles are inadvertently killed during alfalfa harvest and their dried remains, still containing the toxin, are integrated into baled alfalfa.

In crops like canola, potatoes, or sugarbeets, adult blister beetles can cause significant defoliation and damage to flowers or buds, resulting in adverse effects on yield and quality. Although blister beetles have been observed feeding on canola flowers and green seedpods in Montana, the full extent of their economic impact remains unknown.

Despite their potential for damage, the predominant blister beetle genus in Montana, Epicauta, also offers a beneficial aspect as their immature stages feed on grasshopper eggs, thus aiding in pest control.

Description

The blister beetle adult is distinguished by the long cylindrical soft body, with the tip of the abdomen extending beyond the end of the wing covers (elytra), chewing mouthparts, and a thorax (neck) narrower than both the head and the abdomen. Species range in size from ¼ inch to almost 1¾ inches. Individual species can be characterized by distinctive coloration and antennal characteristics.

Life Cycle

Blister beetles lay eggs in clusters of up to 100 in the soil. The eggs hatch within 14 days into tiny, mobile larvae, called triungulins, which move about searching for food. The immature beetles feed on grasshopper egg pods or the larval cells of solitary bees, becoming increasingly sedentary as they feed. The larvae overwinter in the soil. Pupation from larvae to adult beetles occurs in spring in response to increasing temperatures. There is one generation per year.

The adults emerge, usually from late May until mid-July, and begin to feed and lay eggs. Adults of each blister beetle species emerge successively during the growing season. Various blister beetle species may be found in Montana from May until mid-September (Table 1).

Table 1. Seasonal distribution of nine blister beetle species in Montana. Data obtained from museum and field collected species.*

*Data collected by Sue Blodgett, former MSU Extension IPM Specialist; Patricia M. Denke, former Research Associate, MSU Department of Entomology; and Virginia Knerr, Broadwater County MSU Extension Agent

Crop damage and economic impact

SUGARBEETS

Several species of blister beetles feed on sugarbeet foliage and may cause isolated patches of defoliation in the crop. Black, ash-gray and spotted beetle species have been documented on sugarbeets in Montana. However, economic infestations are rare.

POTATOES

Blister beetles can occur in mid-summer on potatoes and may cause extensive foliar damage. Typically, blister beetles may cause localized areas of foliage damage, but widespread economic damage is rarely encountered.

Although adult beetles feed on alfalfa foliage, their primary impact is not due to defoliation damage. Instead, blister beetles are of concern because they contain a toxin in their body fluids called cantharidin, a very stable compound. Blister beetles can be killed during hay harvest and incorporated into baled forage.

When hay containing blister beetles is ingested by sensitive livestock, blistering of the esophagus and stomach can occur, kidney and heart function can be impaired – and in severe cases, death can result. Most cases of poisonings are documented with horses, however, all livestock are prone to blister beetle poisoning, which occurs when they feed on dried, baled forage containing the dried beetles.

The risk of blister beetle poisoning depends on how much cantharidin is ingested. Several factors may affect the cantharidin concentration in the baled forage including the species of blister beetle, the beetle population, and its distribution or density within the field. Some species occur in very dense aggregations similar to that of a honeybee swarm and these species have been most frequently implicated in horse poisoning cases. Aggregations of blister beetles occur in localized areas of 100 to 200 sq. ft. In this case, only a few flakes from a bale of hay may contain blister beetles. However, the ‘Striped’ or Vittata species which cluster in swarms have not been found in Montana. Therefore, the risk of blister beetles to forage production is low in Montana.

Major blister beetle species that occur in Montana.

1. Black blister beetle; Epicauta pensylvanica. Photo: ©Judy Gallagher, 2017

2. Spotted blister beetle; Epicauta maculata. Photo: ©Lynette Elliot, 2006. bugguide.net

3. Ash gray blister beetle; Epicauta fabricii. Photo: ©Brent Guinn, 2021

4. Rust-colored blister beetle; Epicauta ferruginea. Photo: ©Catherine Galley, 2020

5. Dark blister beetle; Epicauta normalis. Photo: ©Lon Brehmer and Enriqueta Flores-Guevara, 2011. bugguide.net

6. Nutall’s blister beetle; Lytta nutalli. Photo: ©Steve McKechnie, 2019. bugguide.net

How many blister beetles can kill a horse

Several factors contribute to the severity of blister beetle poisoning, including concentration of the toxin cantharidin, size and health of the horse, and the dosage (or number of beetles) consumed. John Capinera (formerly at Colorado State University) investigated the dosage of the toxin, cantharidin (contained in blister beetles), needed to kill horses of different sizes. Table 2 summarizes work that he published.

Although all blister beetle species contain some level of the toxin, cantharidin, only those species belonging to the ‘Striped’ group, that form dense aggregations have been documented to kill horses. These species have not been found to occur in Montana.

Table 2. Cantharidin toxicity in horses (study by Capinera, Colorado State University)

Major species that occur in Montana

A survey of blister beetles in alfalfa was conducted during a four year study at Montana State University. Nine species were found to occur in Montana alfalfa (Table 3). Blister beetle collection information was also gathered from the museum collection, Montana Entomology Collection and used where host plant data are indicated. Epicauta normalis was not found in field collections, but specimens are present in the Montana Entomology Collection.

Table 3. Summary of blister beetle research observations.*

*Data collected by Sue Blodgett, former MSU Extesnion IPM Specialist; Patricia M. Denke, former Research Associate, MSU Department of Entomology; and Virgina Knerr, Broadwater County MSU Extension Agent

Management

It is important to monitor blister beetle numbers and species to assess the potential for livestock injury. When blister beetles are collected at harvest, using equipment that allows them to escape from mowed and swathed forage is essential since the type of equipment and its operation directly affects blister beetle mortality. Research conducted at Kansas State University indicates that swathers equipped with windrowing attachments but without conditioning rollers are safer options compared to mower conditioners and sickle-bar mowers.

Although sickle-bar mowers have traditionally been recommended for minimizing blister beetle incorporation in baled forage, they were found to cause considerable mortality if recently cut hay was driven over. Additionally, wheel traffic on freshly mowed alfalfa hay led to mortality among beetles remaining in the cut forage. Allowing beetles to disperse after mowing and before baling decreased the number of beetles incorporated into baled hay.

While insecticides can be used to reduce blister beetle populations, their effectiveness may be limited due to the mobility of blister beetles and their ability to infiltrate crops at any stage. The residual activity of registered insecticides may not be adequate to control blister beetles leading up to harvest.

An alternative strategy involves treating immediately before harvest; however, insecticides with zero-day preharvest intervals are not approved for blister beetle control. Furthermore, Kansas State University discourages the use of insecticides due to the resultant beetle mortality, which may result in beetles remaining within the forage.

Contact the local MSU Extension office or the MSU Extension Forage Specialist for additional management options.

Summary

The species that have been detected in Montana are not regarded as a significant problem because numbers are relatively low and occur infrequently. The species most frequently cited as the cause of horse deaths have not been found to occur in Montana.

Acknowledgment

In recognition, the original authors of this MontGuide are: Sue Blodgett, former MSU Extension IPM Specialist; Patricia M. Denke, former Research Associate, MSU Dept. of Entomology; and Virginia Knerr, former Broadwater County MSU Extension Agent.

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