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Blister beetle in alfalfa

Gary Stone
Nebraska Extension Educator
Species of blister beetle, from left: ash-gray (UNL Entomology); black (UNL Entomology); three-striped (UNL Entomology); and spotted (photo by Terry Thormin).

Last year in another state, 14 horses died and another 100 were sickened from hay that contained blister beetles. Usually blister beetles are not a problem, but growers should be aware of the insect and what to scout for in their fields.

The blister beetle (scientific name Epicauta spp., order Coleoptera, family Meloidae) includes several species: ash-gray blister beetle (Epicauta fabricii), black blister beetle (Epicauta pennsylvania), three-striped blister beetle (Epicauta vittata), and spotted blister beetle (Epicauta maculata).

The blister beetle complete metamorphosis life cycle is unusual, complex and contains several immature forms. Blister beetles overwinter in a pseudo pupae stage in the soil. Final pupation occurs after an increase in temperature and moisture in the spring, from which adults emerge, usually in June and July. The adults congregate and feed on pollen, flowers and leaves of the host plant. The next generation begins when the adults mate and there is one generation per year. Two to three weeks after mating, the female adult beetles deposit up to six clutches of eggs, each containing 100 eggs in the soil near the host plants. After about two weeks, the eggs hatch into tiny mobile larvae called triungulins with legs designed for active movement in the soil. These larvae then search for food, primarily grasshopper eggs and solitary bee larval cells. As these larvae feed, they proceed through three more instars, gradually losing their legs and becoming sedentary. This final instar stage then transforms into the pseudo pupae form to overwinter in the soil to start the process again the next spring.

The adult blister beetles are the stage of the insect that causes damage to the host plant, usually alfalfa. Blister beetle adults range from 3/8 to 1 inch in length. The adults have long, cylindrical, soft bodies with heads wider than the pronotum, with the later giving the appearance of a neck. Color and size of the blister beetle adults found in Nebraska and Wyoming vary by species. The antennae and legs are generally moderately long in the adults.

The ash-gray blister beetle, Epicauta fabricii, is approximately ½ inch in length and gray in color. The black blister beetle, Epicauta pennsylvania, is also about ½ inch in length and is completely black. The three-striped blister beetle, Epicauta vittata, is approximately 5/8 inch in length with either two or three stripes down each of the elytra. The spotted blister beetle, Epicauta maculata, is similar in size and is black in color, covered with small white-colored hairs, except where the small black spots show through on the elytra.

The damage consists of feeding on the pollen, flowers and leaves of the host plant. Other crops, such as garden vegetables, potatoes, sugar beets, canola, soybeans and certain weeds can also be attacked. Damage to these plants can be extensive, although localized, and the real danger from this insect is to livestock.

The blister beetles contain a chemical substance called cantharidin. This stable chemical compound is an irritant and toxin that causes blisters on the external and internal/gastrointestinal body tissues of the livestock and to humans if they are exposed to the compound. Horses are particularly sensitive to this compound, followed by cattle and sheep, and the toxin has caused the death of these animals.

The cantharidin content concentration will vary by species, size and sex of the blister beetle. Female blister beetles will have less cantharidin concentration in their bodies than males. During haying operations, the blister beetles are trapped in the harvested crop and whether they are alive or dead, the cantharidin still remains in the insect body parts or is released to contaminate the hay and can be ingested by the livestock, causing the damage or death of the animal. The table shows the extent of blister beetle damage to horses.

There are no economic or treatment thresholds for blister beetles. If the hay crop being harvested and marketed as horse feed/forage, the producer should scout the fields prior to harvest to determine if the beetles are present. Then the producer can decide if a course of action is warranted, whether chemical and/or cultural.

MANAGEMENT OPTIONS

• Biocontrol: There are no known biocontrol options at this time.

• Chemical control: There are no currently labeled seed treatments or planting products available for blister beetles. Foliar applications, whether by ground or air, include carbaryl, pyrethrins, chloratraniliprole + lambdacyhalothrin, gamma-cyhalothrin, chlorpyrifos + gamma-cyhalothrin, chlorpyrifos + lambdacyhalothrin, lambdacyhalothrin. These provide limited control as they are short-lived and timing of application is critical for maximum efficacy against the target pest(s). Blister beetles are mobile and can move into the crop at any time. Application of insecticides will kill the beetle, but the bodies and therefore the cantharidin will potentially remain in the hay crop to be ingested by livestock.

Preharvest interval is very important when using insecticides for this pest. Ground application carrier rates vary by product label, but in general are a minimum of 5 gallons per acre (GPA) to 20 GPA or “adequate for coverage.” Aerial application carrier rates also vary by product label, but in general are a minimum of 1 GPA to 5 GPA.

There are many different product label names that contain the same or similar active ingredients and formulations, but may not have the site/crop listed on the label. Use of products without the site/crop listed is a violation of federal law. Not all common name products, active ingredients and their methods of application are listed and are not limited to these shown. Chemigation may be a method of application on some product labels and additional information such as application set-backs/buffer zones from well heads, bodies of water, drift, etc. should be reviewed.

Chemical controls are part of an IPM program and all options should be considered when implementing one. When considering a chemical option, the producer must read, understand and follow all label directions. Producers who have any questions should consult the chemical representative and/or local agricultural chemical dealer or applicator.

Some items to review when considering a chemical treatment option or options are the preharvest interval and the replant interval to other crops after application. Other items include tank mixes, tank mix compatibility, tank mix order of mixing, and timing of application when considering bees and other pollinators. The product(s) may be a restricted use pesticide, application records may need to be kept, the field may have to be posted prior to and after application. The producer should also consult the local agricultural chemical dealer, applicator, chemical representative and /or local extension personnel to determine the efficacy history and any known pest resistance. Most product labels now contain and are classified by “insecticide mode of action groups” to help manage insect resistance.

• Cultural control: Methods and types of haying will help reduce the number of blister beetles in harvested forage. Use of haying equipment without conditioners has shown to reduce the number of dead beetles. If the hay is harvested in this manner and the hay is allowed to dry in the windrows, the majority of blister beetles can move out of the windrows before the hay is baled.

The use of sicklebar mowers has shown an increase in blister beetles in the harvested crop because the blister beetles are crushed driving over the cut hay and then picked up when raked into windrows and baled. Hay harvested early in mid-May and later in the season, usually after early September, are less likely to have blister beetle in it.

• Physical control: No physical controls are available for the management of blister beetles in field crop situations.

• Plant-resistant controls: No known plant-resistant blister beetle varieties of alfalfa are available at this time. ❖

Article sources

• Bauernfeind, R. J., et al.; “Blister Beetles in Alfalfa”; Kansas State University, Entomology 111, Publish Date Unknown

• Crop Management Data Systems Inc., Pesticide Labels and MSDS Sheets Reference, http://www.cdms.net/LabelsMsds/LMDefault.aspx?t=

• Blodgett, Sue, (revision) and Brewer. M. J. et al.; “Blister Beetle — Alfalfa — High Plains IPM Guide”; University of Wyoming, University of Nebraska, Colorado State University and Montana State University; 2006

• Blodgett, S., et al.; “Blister Beetles of Montana”; Montana State University Extension, MT200209AG, 2010

• Brewer, M. J.; “Blister Beetles”; University of Wyoming, #B-1013.3, 1995

• Campbell, J. B.; “Management of Blister Beetles in Alfalfa”; University of Nebraska, NebGuide G1645, 2006

• Mulder, Phillip; “Blister Beetles and Alfalfa”; Oklahoma State University, Cooperative Extension Service #EPP2072, Publish Date Unknown

• “Blister Beetle”; Texas A&M University, Texas AgriLife Extension, http://insects.tamu.edu/fieldguide/bimg167.html


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