Blister Beetles and Root Curculio in Alfalfa

Look Out for Blister Beetles and Clover Root Curculio in Alfalfa

Add two more pests to the list of beetles that alfalfa producers should watch for.  The blister beetle  does not reduce yield, but can be harmful to livestock in cut hay, and the clover root curculio—an often overlooked pest—could be reducing alfalfa yield and stand life in Utah. 

Blister Beetle

Alfalfa growers and livestock owners should always be concerned with blister beetles.  These beetles belong to the family Meloidae and produce cantharidin, a chemical toxic to people and animals. Smashing one of these beetles against the skin can lead to painful blisters and swelling.  A recent incident with alfalfa hay infested with blister beetles resulted in the death of a horse, which are particularly sensitive to this beetle’s toxin.  When livestock eat hay containing cantharidin the gastrointestinal and urinary tracts become irritated and complications can lead to death.  If blister beetle poisoning is suspected (symptoms include blisters and colic among others) contact a veterinarian immediately.

Blister beetles lay eggs in the soil and first instar larvae actively search for grasshopper egg cases to complete their juvenile life stages.  The spotted blister beetle is one of the most common ones found in Utah, but many species are present and vary in color.  These beetles are frequently associated with grasshopper outbreaks.  There are other species that actively search for bee eggs.  Adults are soft bodied and emerge from the soil throughout the growing season.  Typically, the first-cut hay escapes being contaminated by blister beetles because peak activity occurs in the middle of the season.

Minimize alfalfa hay contamination.

  • Check alfalfa for blister beetles before cutting. These beetles tend to cluster in masses near field edges.
  • Manage weed problems and harvest alfalfa prior to bloom. Blister beetles prefer blooming plants.
  • Harvest practices such as hay conditioning, crimping, and other practices that kill or crush blister beetles exposing the cantharidin toxin to plants can increase contamination.
  • Although insecticides are available for blister beetle management, some entomologists have recommended against using insecticides because beetles often leave the field after a short time and treatment would keep the beetles and their toxin in the field.  The toxin lasts even after the beetle dies.

Clover Root Curculio

Clover root curculio (left) looks similar to alfalfa weevils but smaller and without distinct patterns on the wings. Larvae are small white grubs that scar roots. (right)

In a recent article in Hay and Forage Grower magazine, our neighbors to the north at the University of Idaho Extension brought up concern over another weevil in alfalfa that could be impacting yield and stand longevity.  Unlike alfalfa weevil whose larvae feed on foliage, the clover root curculio has larvae that feed below ground on the roots of alfalfa.

Clover root curculio root damage.

The clover root curculio is a weevil that is smaller than the alfalfa weevil.  The adult beetles do feed on foliage but the larvae are the major culprits of alfalfa damage.  Larvae feed on the roots and first target small roots and nodules eventually moving to the tap root.  By pruning lateral roots and scarring taproots the larvae can expose alfalfa to other disease problems.  Symptoms from larval feeding may be mistaken for nutrient deficiencies or diseases that may come secondarily.  Slow green-up and growth, and yellowing, are typical symptoms.

To diagnose these beetles, examine the roots to spot larvae and root damage.  University of Idaho Extension recommends washing roots before examining them to clearly see feeding scars on the taproots.

Management is limited because no insecticides are registered in Utah for these beetles in alfalfa.  Crop rotation to grasses or row crops other than soybeans may reduce populations. More research is required to determine the economic impact and develop management tactics for clover root curculio in the West.

-Ricardo Ramirez, Entomologist