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