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Native to the United States and Canada, the blister beetle is also referred to as the oil beetle. Despite their wide range in size and color, adult blister beetles may be identified by their elongated, thin, cylindrical bodies and soft, flexible wings.
Blister beetles produce a chemical (cantharidin) that blisters animals and human tissue. The female beetle applies the chemical received from her mating partner to her eggs to protect them from predators. Their most distinctive feature is the narrow thorax and wider head.
Let’s look at some common blister beetles, their unique life cycle, and how to get rid of them. By the end of this article, you will know how to stop striped blister beetles in their tracks
Accidentally crushing this member of the Meloidae family against the skin can result in painful blisters, the basis of the insect’s common name. Blister beetles have three-quarter inch to an inch and a quarter long narrow bodies, broad heads, and antennae that are about a third the length of their entire bodies.
The front wings are soft and flexible in contrast to the hard front wings of most beetles. There are about 410 species of blister beetles in North America north of Mexico and about 4,000 worldwide.
Vegetable crops such as bean, beet, carrot, cabbage, Chinese cabbage, corn, eggplant, melon, mustard, pea, pepper, potato, radish, spinach, squash, sweet potato, Swiss chard, tomato, and turnip are targets for blister beetles.
This article highlights the striped blister beetle (Epicauta vittata) but also references other blister beetles commonly found in the United States.
The striped blister beetle (Epicauta vittata) often damages alfalfa, beet, potato, tomato, and other crops by defoliation.
They feed on a wide variety of plants, including Amaranthaceae (beets, chards, spinach), Solanaceae (potato, tomato, eggplant), and Fabaceae (peas, beans). They are attracted to lights.
Description and Life Cycle of the Striped Blister Beetle
Generally, blister beetles overwinter as larvae. Adult blister beetles emerge in mid-spring to early summer after about two weeks of pupation in early spring.
Adults feed, reproduce, and females start depositing egg clusters on the ground.
The small, movable triungulin larvae that emerge from the eggs use bees and grasshoppers as hosts.
Before pupating in the spring, the larvae begin to resemble grubs as they search for hosts on whom to feed. There is typically just one generation each year for most blister beetle species.
The striped blister beetle has two generations per year. From May to June, early-emerging adults produce another generation that will emerge in September.
However, the maximum emergence of adults occurs in July, and few of the later-emerging adults contribute to the second generation.
The generations overlap, resulting in an abundance of adult striped blister beetles in the field from late May to late October.
The adults are black and yellow and range in length from a third to two-thirds of an inch. Geographically, the color pattern varies, but typically, the head has two black spots, the thorax has two black stripes, and the elytra each has two or three black stripes.
On each elytron, northern populations frequently only have two black stripes, but southern populations typically have three.
The lemniscate race is the name given to the southern coastal plain race, which has three stripes on each elytron.
The adult blister beetle is elongated and thin in shape, with a thorax that is narrower than the head and abdomen and relatively long legs and antennae.
The body is covered with short hair and has several little punctures.
The elytra are lengthy and cover the abdomen, but they are divided or divergent at the ends. Transparent wings cover the back.
The adults are most active early and late in the day, seeking cover from the sun in the middle of the day. In very hot and dry areas, they don’t move around during the day and only become active at night.
They are highly startled and quickly descend off the plant before hiding or running away.
The striped blister beetle’s preovipositional period is around 20 days, with a 10-day gap between egg mass production.
The striped blister beetle lays white eggs that average 0.07 inches in length and 0.03 inches in breadth. They have rounded ends and are long and oval-shaped.
The eggs can be found in the soil within a tubular chamber at a depth of an inch to an inch and a half.
After oviposition, the female covers the eggs and hatch in 10 to 16 days. In captivity, females are said to lay several hundred eggs; however, this estimate may be low given several closely related species lay 2000 to 3000 eggs during their 20 to 50-day lives.
As is typical with blister beetles, the larva initially has long legs and is quite mobile. The legs become smaller in size as the larva develops, and instars 6 and 7 (if present) have small head capsules and do not feed.
Newly hatched larvae are whitish but soon turn reddish-brown with dark brown bands on the thorax and abdomen.
When blister beetles are in their larval form, they feed either on the juvenile stages of solitary bees or on the eggs of short-horned grasshoppers, a common grasshopper found in meadows and grasslands.
In the dirt, you can find the pupal stage. Although the wings and legs are closely pulled to the ventral surface, the pupa’s shape is similar to that of the adult.
The pupa is initially pale in appearance but eventually turns darker. The pupal stage lasts for nine to thirteen days. From May through August, pupae can be found.
Striped Blister Beetle Host Plants
The striped blister beetle (Epicauta vittata) often damages alfalfa, bean, beet, carrot, cabbage, Chinese cabbage, corn, eggplant, melon, mustard, pea, pepper, potato, radish, spinach, squash, sweet potato, tomato, and turnip crop by defoliation.
As indicated, they feed on a wide variety of plants, including Amaranthaceae (beets, chards, spinach), Solanaceae (potato, tomato, eggplant), and Fabaceae (peas, beans). They are attracted to lights.
Damage Cause by Striped Blister Beetles
In places where it occurs, the striped blister beetle is one of the blister insects that causes the most harm to vegetable crops. This is a result of its eating habits, which include:
- Compared to other blister beetles, the striped blister beetle has a preference for leaves across several common crops
- The striped blister beetle’s preference for solanaceous plant fruits as food
- The comparatively large size and ravenous hunger of the striped blister beetle
- Due to the striped blister beetle’s high degree of dispersiveness and strong desire to aggregate into big mating and feeding swarms, massive swarms of beetles may suddenly develop.
The bean pod mottle virus spread to soybean has also been attributed to the striped blister beetle.
Blister beetle larvae, which feed on the eggs of grasshoppers, including many crop-damaging Melanoplus spp., counteract the damage caused by Epicauta spp. blister beetles, at least during times of relatively low insect population.
Blister beetles become much more prevalent when grasshoppers are in great numbers. Studies of egg pod destruction in western states during a grasshopper boom, for instance, revealed that blister beetles caused damage to 8.8% of the pods.
The higher density of blister beetles generally results in more crop damage during and soon after periods of high grasshopper abundance, even if they eventually contribute significantly to reducing grasshopper population breakouts.
However, the larvae of blister beetles other than the striped blister beetle (Epicauta spp.) appear to eat ground-nesting bees (Hymenoptera: Andrenidae, Halictidae), and the food they gather for their nests primarily.
They are less abundant and have no recognized agricultural advantages.
Striped Blister Beetle Natural Enemies
The robber flies (Diptera: Asilidae) and avian predators, such as the meadowlark, bluebird, and scissor-tailed flycatcher, are the only known natural enemies of the striped blister beetle.
Additionally, there are accounts of the predatory blister beetle (Epicauta atrata) eating striped blister beetle eggs.
Striped Blister Beetle Management
Blister beetle populations can be efficiently decreased by applying insecticides topically to leaves. Beetles tend to congregate, resulting in severe defoliation in some fields while performing little to no damage elsewhere.
According to reports, the two striped grasshoppers (Melanoplus bivittatus) and differential grasshopper (Melanoplus differentialis) are very closely related to the striped blister beetle populations because the former two generate large egg pods.
Management For Alfalfa Producers
- Alfalfa fields should be monitored from June through September for the presence of blister beetles. Cuttings before June are at a lower risk of being contaminated by blister beetles. Monitoring can be done quickly and efficiently with a sweep net.
- Blooming fields are very attractive to blister beetles, so fields should be managed such that cutting occurs at early (5-10%) bloom.
- Areas where observed clusters can be left unharvested or treated with an insecticide as a “spot treatment” because certain species of blister beetles, notably the striped blister beetle, gather or form large clusters in a limited area of the field. Be mindful of the pesticide application times before alfalfa harvest.
The striped blister beetle is an antidote to the grasshopper threat but presents a threat of its own for leafy and nightshade crops.
In addition, they are dangerous for livestock if eaten in feed, dead or alive. The best management methods seem to be insecticidal, but do it with care and only where beetle populations justify it.
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