Natural History

Distribution. Epicauta species and other blister beetles are native to North America, and though many species have been reported to feed on crops, few are commonly destructive. Black blister beetle, Epicauta pensylvanica De Geer, is usually the most common species, and occurs everywhere in the eastern United States and southern Canada west to the Rocky Mountains. Immaculate blister beetle, E. immaculata (Say); spotted blister beetle, E. maculata (Say); and striped blister beetle, E. vittata (Fabricius), are the other common vegetable-feeding blister beetles, and are treated separately.

Other common but less damaging Epicauta spp. include ashgray blister beetle, E. fabricii (LeConte) in eastern North America west to the Rocky Mountains but absent from the southeast; E. cinerea (Förster) in the northeastern and midwestern states; and margined blister beetle, E. pestifera Werner, which is most abundant in the northeastern and midwestern United States and Canada. The taxonomy of blister beetles is difficult and many of the old records are suspect, or host associations are reported but little information on crop damage is given, so the economic importance of many species is questionable.

Other genera of Meloidae occasionally affect crops, but incidents of damage are isolated. Among such occasional pests are Linsleya sphaericollis (Say) in the Rocky Mountain region and west to the Pacific Ocean; Nuttall blister beetle, Lytta nuttalli Say, throughout the west and east to Nebraska; and Meloe niger Kirby, which occurs throughout the United States and southern Canada except for the southeastern states.

Host Plants. Blister beetle adults are found on a number of vegetable crops, including asparagus, beet, broad bean, cabbage, carrot, celery, chicory, Chinese cabbage, corn, eggplant, lima bean, mustard, okra, onion, pea, pepper, potato, pumpkin, radish, snap bean, spinach, squash, sweet potato, Swiss chard, tomato, and turnip. Vegetable crops specifically known to be damaged by black blister beetle include bean, beet, cabbage, potato, onion, pea, radish, squash, pumpkin, and tomato, though blossom feeding is greatly preferred over foliage consumption. Other crops such as alfalfa, clover, soybean, and sugarbeet, as well as numerous flower crops, fruit trees, and broadleaf weeds are occasionally eaten. Adults often collect on goldenrod flowers, Solidago spp., in the autumn.

Natural Enemies. Surprisingly little is known concerning the natural enemies of blister beetles, reflecting their minor status as crop pests and the subterranean habits of larvae. Undoubtedly starvation of first instars is a very important factor during most seasons, and cannibalism is prevalent among larvae. Antlike flower beetles (Coleoptera: Anthicidae), false antlike flower beetles (Coleoptera: Pedilidae), and some plant bugs (Hemiptera: Miridae) have been implicated as mortality agents of blister beetles. The larva of the blister beetle Epicauta atrata (Fabricius) has also been shown as predatory on eggs of E. pensylvanica, and it is possible that other species within the genus are predatory (Selander, 1981; 1982). See also the discussion of natural enemies under striped blister beetle.

Life Cycle and Description. There is a single generation per year, with overwintering occurring as any of the larval instars. In Arkansas, pupation typically occurs in July or August, adults are found in August-November, eggs are found in SeptemberNovember, and new larvae are present beginning in October.

  1. The eggs of black blister beetle are elliptical, measuring about 1.3 mm long (range 1.0-1.6 mm), and 0.6-0.7 mm wide. The eggs are pearly white initially, but become darker as the embryo develops. The eggs are deposited within a cavity in the soil, in clusters of 100-200 eggs. The soil cavities are tubular structures about 20-30 mm deep and only 3-4 mm in diameter, and the entrance is closed by the female after she completes the act of oviposition. Eggs normally hatch in 14-15 days.
  2. The larval instars are quite varied in appearance, reflecting the unusual biology of the insect. Unlike most insects, the ovipositing female does not locate a food source for her offspring, apparently depositing her eggs randomly. Thus, when young larvae hatch they must dig to the surface and disperse to find a grasshopper egg pod on which to feed. The first instar is thus quite mobile, and equipped with long legs with which to disperse. First instars explore cracks, crevices, and depressions in the soil as they search to find an egg pod. Once locating a pod, they pierce an egg and consume the liquid contents. Usually a single egg is adequate for complete development of the first instar. The number of instars is normally 6-7. Larvae are creamy white or yellowish white, with brown head capsules. After the first instar the larva moves little, and the legs become relatively smaller and smaller. The sixth instar does not feed, instead digging 2-3 cm into the soil and preparing a cell. The sixth instar may be followed by another non-feeding instar, or by the pupal stage. The sixth and seventh instar bear only minute legs, and the head capsule is reduced in size and retracted into the body. Black blister beetle consumes 21-27 eggs of Melanoplus differentialis Thomas during its larval development. This grasshopper, and some other Melanoplus spp., produce 100 or more eggs, allowing more than one blister beetle to develop. If blister beetle larvae encounter one another, however, they fight and only one survives. Many grasshoppers produce small egg pods, with less than 25 eggs, thereby limiting the ability of blister beetles to develop. The mean head capsule widths of the larval instars are 0.41, 0.57, 0.85, 1.28, 1.77, 1.55 and (if present) 1.77 mm, respectively, for instars 1-7. Mean development time is 8.5, 2.1, 2.4, 2.8,12.5, 7.3, and 7.6 days, respectively, for a total larval development time of about 43 days unless the larva enters a period of arrested development for the winter.
Epicauta Linsleya
Black blister beetle, first instar.
Epicauta Linsleya
Black blister beetle, fourth instar.
Black blister beetle, seventh instar.

Black blister beetle pupa.

  1. The pupa resembles the adult beetle, though the legs and wings are folded against the underside of the body, and the color initially is creamy white, becoming black at maturity. The pupa measures about 10 mm long, and is found within a cell in the soil. Mean duration of the pupal stage is about 14 days, with a range of 11-28 days, and there appears to be no tendency for diapause in this stage.
  2. The adult digs to the soil surface after pupation. The adult is elongate and slender in form, black in color, and measures 7-15 mm long. As is typical with this family, the thorax is narrower than the head and abdomen, and the legs and antennae are moderately long. The body bears numerous small punctures, and is sparsely clothed with short hairs. The elytra are long, covering the abdomen but separated or divergent at the tips. The hind wings are transparent. (See color figure 99.)

Unfortunately, E. pensylvanica is not the only black species, though few other black Epicauta spp. are common or damaging. Black species of Lytta and Meloe, including those mentioned previously, occasionally are injurious, though these species are purplish or greenish black (Lytta) or bear abbreviated elytra and are broad rather than slender in form (Meloe).

Black blister beetle was treated in detail by Horsfall (1941), and also was included in the publications of Milliken (1921) and Horsfall (1943). Werner (1945) and Pinto (1991) included this species in keys to North American Epicauta. Horsfall (1943) is a good source of information on all the common Epicauta spp. and described a method of rearing blister beetles. Downie and Arnett (1996) provided keys to the eastern species of blister beetles, Werner et al. (1966) to those in the southwest. Information on Lytta spp. can be found in Selander (1960) and Church and Gerber (1977b); on Meloe spp. in Pinto and Selander (1970) and Mayer

Adult black blister beetle.

and Johansen (1978); and on Linsleya spp. in Selander (1955) and Church and Gerber (1977a).

Damage

The adult blister beetles defoliate plants, though the pattern of injury is irregular. Beetles tend to aggregate, apparently in mating swarms, so damage can be severe in relatively small areas of a crop and absent or trivial elsewhere. Although adult black blister beetles inflict damage to vegetable crops primarily through consumption of foliage, in some instances they prefer blossoms.

Preference of black blister beetle and other blister beetle species for blossoms is most noticeable and potentially damaging in alfalfa, where beetles congregate mostly during periods of bloom. Aggregations of beetles can be incorporated into alfalfa hay when it is baled, particularly if the stems are crushed as part of the harvesting process. Crushing, or crimping, aids in the drying and preservation of hay, but also can result in death of aggregations of blister beetles, and incorporation of their bodies into the hay. Blister beetles, even dead individuals, contain a vesicating substance called cantharidin which, when ingested, damages the digestive tract of animals. Cantharidin also causes blisters to form on the skin of sensitive humans who come into contact with crushed beetles, and in formation of blisters in the mouths of livestock, particularly horses. This blistering action is the basis for the common name of the beetles. Cantharidin content varies among species, ranging from 5% in E. immaculata to about 1% in E. pensylvanica (Capinera et al., 1985). Despite the relatively low toxin content in each beetle, large numbers can be incorporated into hay, and horses can ingest enough beetles to cause death (Blodgett et al., 1991). Cantharidin likely serves as a feeding deterrent to most predators, thereby protecting blister beetles and their eggs from consumption. However, some insects are attracted to cantharidin, and this compound is involved in the chemical communication among blister beetles (Young, 1984a,b; Klahn, 1987).

The damage caused by Epicauta spp. blister beetles is offset, at least during periods of relatively low beetle density, by the predatory behavior of blister beetle larvae. Epicauta spp. larvae feed on the eggs of grasshoppers, including many crop-damaging Melanoplus spp. During periods of grasshopper abundance the number of blister beetles tends to increase substantially. Studies of egg pod destruction in western states during a period of grasshopper abundance (Parker and Wakeland, 1957), for example, documented that 8.8% of pods were damaged by blister beetles. Although the blister beetles eventually contribute materially to the suppression of grasshopper population outbreaks, the higher numbers of blister beetles often cause greater crop injury during, and immediately after, the periods of grasshopper abundance. The larvae of blister beetles other than Epicauta spp., however, seem to feed principally on ground-nesting bees (Hymenop-tera: Andrenidae, Halictidae, perhaps others) and the bee's nest provisions. Their abundance fluctuates less, and they provide no known agricultural benefits.

Management

Blister beetles infrequently are vegetable crop pests, though they may become quite abundant during and following long-term grasshopper population increases, particularly populations of Melanoplus differential and M. bivittatus (Say). Suppression of grasshoppers indirectly suppresses blister beetles by eliminating the food supply of the blister beetle larvae. Direct suppression of blister beetles usually does not occur in conjunction with chemical treatment of grasshopper populations, because the grasshoppers occur earlier in the season, when blister beetles are still in the soil. Blister beetles are easily controlled by application of common insecticides to crop foliage, and small plantings can be protected with row covers or screening. Because black blister beetle is highly attracted to alfalfa, especially during periods of bloom, large numbers of blister beetles may disperse to nearby crops following alfalfa harvest.

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