Pest description and crop damage Adults are slender, oblong beetles with a relatively large head and an extenuated neck, often with yellow stripes in some form. Eggs are oblong and yellow. Larvae usually are found in several stages. All stages have three upper body legs. The pupa is a stage of darkening, starting with the eyes. The beetles get their name from a toxin they contain called cantharidin. Even dead insects can cause blisters if handled.
Blister beetles are mostly foliage feeders but are not considered a major pest of vegetable crops. While feeding, they leave behind a distinctive black stringy substance. The beetles tend to move in swarms and can cause a great deal of localized defoliation. They generally do not stay in one area for very long.
Biology and life history Blister beetles pass the winter in a pseudopupa stage and then go through a final molt in the spring. After a short period of activity, the larva enters the true pupal stage. Adults emerge in midsummer. There is a single generation each year. Blister beetles tend to stay in weedy areas, especially where there is an abundance of grasshopper eggs. Larvae are considered beneficial, as they can consume large amounts of grasshopper eggs. Only the adult stage damages the crop.
Management-chemical control: HOME USE
- azadirachtin (as a mix with pyrethrins)-Some formulations are OMRI-listed for organic use.
- bifenthrin-Not registered for home use in WA on Swiss chard.
- pyrethrins-Some formulations are OMRI-listed for organic use.
Management-chemical control: COMMERCIAL USE
- malathion (Malathion 8) at 1 lb ai/A. PHI 14 days. REI 12 hr. Retreatment interval 7 days, 2 lb/A per year. Blister beetles are not specifically cited in Swiss chard on the label, however Malathion has been effective in treating blister beetle in other crops. Washington and Oregon only.
Note: Blister beetles are controlled easily with insecticides.