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Agriculture Series
F@stSheet Ent-2006
Blister beetles get their common name from the irritating
reaction of their body fluids with animal skin or delicate membranes. Although some
species of blister beetles are considered a beneficial insect because the larvae eat
grasshopper eggs, the adults are considered an important pest because, in addition to
eating foliage, they cause health problems to livestock when ingested in contaminated hay.
Some of the blister beetles that may be found include the ashgray blister beetle (Epicauta
fabricii (LeConte)), the spotted blister beetle (E. maculata (Say)), the black
blister beetle (E. pennsylvanica (DeGeer)) and the striped blister beetle (E. vittata
(Fabricius)).
BIOLOGY
Blister beetles are found throughout the entire United States; however, heaviest populations occur west of the Mississippi River. Blister beetles vary in size and shape but can be recognized by the first portion of the thorax, just behind the head, which is narrower than either the head or first pair of wings. Also, the first pair of wings are soft and flexible.
Adults emerge in late May and June and feed on leaves in the tops of a plant, including alfalfa, but are especially attracted to flowers where they feed on nectar and pollen. They gather in groups, so large numbers can occur in concentrated clusters in a field. Female blister beetles lay clusters of eggs in the soil in late summer. The small, active larvae (called triungulins) that hatch from these eggs crawl over the soil surface entering cracks in search for grasshopper egg pods which are deposited in the soil. After finding the egg mass, blister beetle larvae become immobile and spend the rest of their developmental time as legless grubs. The following summer, the larvae transform into the pupal stage and soon emerge in the adult stage. This is why blister beetle numbers often increase dramatically following high grasshopper populations.
DAMAGE
Cantharidin is the poisonous substance in blister beetles. Although horses are
considered to be very susceptible, comparable doses can poison cattle or sheep. Very small
amounts of cantharidin can cause colic in horses. The substance is very stable and remains
toxic in dead beetles. Animals may be poisoned by ingesting dead beetles in cured hay.
Cantharidin can cause severe skin inflammation and blisters. It is absorbed through the
intestine and can cause symptoms such as inflammation, colic, straining, elevated
temperature, depression, increased heart rate and respiration, dehydration, sweating, and
diarrhea. There is frequent urination during the first 24 hours after ingestion,
accompanied by inflammation of the urinary tract. This irritation may also result in
secondary infection and bleeding. In addition, calcium levels in horses may be drastically
lowered and heart muscle tissues destroyed. Since animals can die within 72 hours, it is
imperative to contact a veterinarian as soon as blister beetle poisoning is suspected.
Toxicity to horses has not been definitely determined, but the estimated minimum lethal
dose is l milligram of cantharidin per kilogram of body weight of horse. Many factors such
as horse age, breed and health will also determine the individual animal's susceptibility.
The number of beetles necessary to provide a lethal dose depends on the species of beetle
and how much cantharidin it stores. Tables 1 provides data of three common blister beetles
found in alfalfa in Minnesota. Table 2 provides data on lethal doses of cantharidin
depending on the amount in the beetle and horse's weight.
Table 1. Cantharidin levels present in common species of blister beetles1.
| Blister beetle species |
Sex |
milligrams cantharidin / beetle |
| Average |
Range |
| Black |
Male |
0.40 |
0.03 - 0.70 |
| Black |
Female |
0.08 |
0.07 - 0.30 |
| Ashgray |
Male |
1.25 |
0.06 - 3.38 |
| Ashgray |
Female |
0.49 |
0.14 - 0.75 |
| Striped |
Male |
5.21 |
1.43 - 11.13 |
| Striped |
Female |
4.52 |
2.18 - 8.50 |
1From "Blister Beetles in Alfalfa: Management
Options to Minimize Poisoning in Horses" Minnesota Extension Service,
AG-FO-5510-D, 1990.
Table 2. Estimated number of beetles necessary to provide a lethal dose to horses (estimated 1 mg/Kg lethal dose).1
Cantharidin content / beetle
(mg) |
Horse Weight (pounds) |
| 275 |
555 |
835 |
1200 |
| 1.0 |
125 |
250 |
375 |
545 |
| 2.0 |
63 |
125 |
188 |
273 |
| 3.0 |
41 |
83 |
124 |
182 |
| 4.0 |
31 |
63 |
94 |
136 |
| 5.0 |
25 |
50 |
75 |
109 |
1From "Blister Beetles in Alfalfa: Management
Options to Minimize Poisoning in Horses" Minnesota Extension Service,
AG-FO-5510-D, 1990.
CONTROL
Control of blister beetles requires vigilant scouting and good management skills. The
critical problem of blister beetle poisoning develops when large swarms of adult beetles
are incorporated in the hay during the harvesting process. Adults are very mobile and may
move from one area of the field to another in a short period of time.
The major step is to cut on a schedule that keeps alfalfa and weeds from producing the
flowers that attract beetles and keep them in the field. Cutting hay at or before 5
percent bloom greatly reduces the risk of blister beetle contamination. Scouting should be
conducted one or two days before harvest to determine the presence or absence of this
pest. Generally, blister beetles are found within 50 yards of the field border. If large
concentrations are found, producers should avoid harvesting the area until the adults have
moved out of the field.
Research has shown that dead blister beetles on the ground can be incorporated into the
hay during raking and baling. Therefore, growers should avoid killing blister
beetles with insecticides in the field. Cantharidin is very stable and remains toxic
in dead beetles. Animals may be poisoned by ingesting beetles in cured hay.
Crimping and conditioning hay crushes and retains beetles that otherwise would walk out
of the swathed hay before baling. Producers can modify a mower/windrower by removing the
crimped rollers and placing the cut swath in a narrow band or windrow.
Horse owners should become familiar with their alfalfa hay supplier. Check with the hay
producer to make sure the producer is aware of blister beetle poisoning. If
possible, horse owners should request that hay come from the first cutting since the
chances of blister beetles infesting the alfalfa hay are small. There is no sampling
method that can detect toxic levels of blister beetles in cured hay. Examining a
bale takes several hours and the horse owner must be familiar with the different species
of blister beetles.
FOR MORE INFORMATION
For more information about blister beetles refer to Extension
fact sheet B-1401, Integrated Pest Management Guide for Texas Forage Crops, or to the
following web sites:
http://www.agweb.okstate.edu/pearl/plantsoil/crops/f-2072.pdf
http://www.uky.edu/Agriculture/Entomology/entfacts/fldcrops/ef102.htm
http://www.oznet.ksu.edu/pr_forage/pubs/97notebook/fora32.pdf
These publications, and help with additional questions about insect or other pest
problems, can be obtained by contacting your local county Extension office.
Authors:
Chris Sansone, Ph.D., Entomologist, Texas AgriLife Extension Service
Reviewed by Allen Knutson, Ph.D., Extension Entomologist, Texas Agricultural
Extension Service
Publication information:
This publication is part of the House & Landscape Pest Series
produced by the Department of Entomology, Texas A&M University, College Station, TX
77843-2475. The most recent update can be found at: http://165.95.73.17/FastSheets/Ent-2006.html . Series Editor: M. Merchant. For more information
about arthropods, check out the Texas A&M Entomology Website at http://insects.tamu.edu
Last Revised: 5/29/02
The information given herein is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by the Texas AgriLife Extension Service is implied. Additional, or updated copies of this fact sheet may be obtained by contacting the author(s) at the Texas AgriLife Extension Svc., 17360 Coit Road, Dallas, Texas 75252-6599. Extension programs serve people of all ages regardless of socioeconomic level, race, color, sex, religion, disability or national origin. The Texas A&M University System, U.S. Department of Agriculture, and the County Commissioners Courts of Texas Cooperating.
All content and images Copyright © 2000, Texas AgriLife Extension Service, unless otherwise noted. All rights reserved.
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