Epicauta immaculata Say Coleoptera Meloidae

Natural History

Distribution. This native species is found widely in the midwestern states and Great Plains region of the

United States from Kentucky west to Colorado and New Mexico.

Host Plants. Immaculate blister beetle has a fairly broad host range, and though it is especially often recorded as a pest of potatoes, it also attacks such vegetables as bean, beet, cabbage, onion, pea, pumpkin, radish, squash, and tomato. Post-bloom lima bean also is suitable, though pre-bloom plants are reportedly toxic. Other crops and flowers including alfalfa, sweet clover, hollyhock, gaillardia, and marigold can serve as hosts. Weeds including sunflower, Helianthus spp.; cactus blossoms, Opuntia sp.; wild lettuce, Lactuca sp.; Russian thistle, Salsola kali; and mullein, Verbascum sp.; also are consumed.

Natural Enemies. The natural enemies of immaculate blister beetle are not precisely defined, but undoubtedly are the same or similar to those affecting black blister beetle, Epicauta pensylvanica De Geer, and striped blister beetle, Epicauta vittata (Fabricius).

Life Cycle and Description. There is a single generation annually. In South Dakota, adults are present from June to August, but are most abundant in July. Eggs are present in July and August, and early instars through November. Overwintering occurs in instar six, with the final instar and pupation in May.

  1. The eggs are deposited in a bell-shaped cavity near the surface of the soil, at a depth of perhaps 3 cm. The cavity is plugged with soil by the female after she completes oviposition. Several hundred eggs reportedly can be placed in a single cavity. The egg is elongate oval, with rounded ends. It measures 1.31.5 mm long and 0.5-0.6 mm wide. It is yellowish white initially, becoming darker with age. Duration of the egg stage is 11-18 days.
  2. The young larva is yellowish white, with a brown head. It is long-legged and mobile, and after remaining in the egg cavity for 1-2 days, forages actively for grasshopper eggs. Young larvae are active on the soil surface but also crawl into cracks and crevices in search of grasshopper eggs. Upon locating a supply of food the larva commences feeding and progresses through four additional instars while continuing to feed. Duration of the first instar is 3-5 days; instars 2-4 require 1-4 days each, and the fifth is 713 days in duration. On average the first five instars require about 18 days. Because the larvae are not required to search for additional food, their legs are not needed and they become smaller with each molt. The fifth instar burrows deep into the soil, molts to the sixth instar, stops feeding, and overwinters. The head is reduced in this and the subsequent stage, which also is a period that is free of feeding. The final, or seventh, instar digs closer to the surface in preparation for the molt to the pupal stage and constructs a small chamber. Duration of the seventh instar is 8-15 days.
  3. Pupation normally occurs at a depth of 2-5 cm. The pupa is yellowish-white, about 17 mm long, and similar in form to the adult, though the wings and legs are pulled closely to the ventral surface of the developing beetle. Duration of the pupal stage is 10-14 days.
  4. This is one of the largest species of Epicauta, measuring 13-23 mm long. The background color is black, but the body is covered with short dense pubescence of various colors, imparting an overall gray, tan, or reddish brown appearance. The adult is elongate and slender and as is normal with this family, the thorax is narrower than the head and abdomen, and the legs and antennae are moderately long. The elytra are long, covering the abdomen but separated or flared at the tips. The hind wings are transparent. New adults fed voraciously for 7-10 days after emerging, and then after another week commence oviposition. Periods of feeding are interspersed by acts of ovi-position. (See color figure 101.)

Immaculate blister beetle and its biology were described by Milliken (1921) and Gilbertson and Horsfall (1940). Werner (1945) and Pinto (1991) included this species in keys to North American Epicauta. Horsfall (1943) described a method of rearing blister beetles. Downie and Arnett (1996) provided keys to the eastern species of blister beetles, though it was derived from Werner's key.

Damage

Immaculate blister beetles are large and can cause extensive defoliation. Also, adults are quite gregarious and sometimes assemble in very large numbers.

Adult immaculate blister beetle.

Such aggregations can cause severe localized injury, though many nearby areas may escape with no visible damage. In most respects, the injury caused by immaculate blister beetle is equivalent to damage by other meloids. (See the discussion of damage in the section on black blister beetle for additional information.)

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 differential grasshopper, Melanoplus differentialis Thomas, and twostriped grasshopper, Melanoplus bivittatus (Say). As immaculate blister beetle is a large species and requires considerable food, the larvae develop successfully only in the presence of grasshoppers which produce large egg pods, such as differential grasshopper and twostriped grasshopper. 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.

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