Distribution. There are several species with similar appearance and biology in the Sphenophorus billbug complex. Collectively, they occur throughout the
United States and southern Canada. They are most destructive, however, in the eastern half of the continent. The most important species are maize billbug, Sphenophorus maidis Chittenden; and southern corn billbug, S. callosus (Oliver). Maize billbug is found in the midwestern and southern states. Southern corn billbug is found in the southern half of the United States, extending as far west as Arizona and north to most of the midwestern states and New England. Other billbugs occasionally mentioned as vegetable pests are bluegrass billbug, S. parvulus Gyllenhal; corn billbug, S. zeae (Walsh); claycolored billbug, S. aequalis Gyllenhal; cattail billbug, S. pertinax (Oliver); nutgrass billbug, S. cariosus (Olivier); and hunting billbug, S. venatus Chittenden. Apparently these are native species.
Host Plants. These billbugs feed mostly on grasses. Corn is the only vegetable crop damaged, and the principal cultivated host. For most species, the adult is the stage attacking corn, but in the case of maize billbug and southern corn billbug, both adults and larvae damage corn. Feeding also has been reported on barley, chufa, oat, peanut, rice, wheat, and timothy. Wild plants attacked include bentgrass, Agrostis; bluegrass, Poa sp.; jointgrass, Paspalum sp.; crabgrass, Digitaria sp.; nutgrass, Cyperus spp.; rushes, Juncus spp.; reeds, Phragmites sp.; and cattails, Typha spp.
Natural Enemies. The billbugs seem to have a few natural enemies. Ants are thought to gather the eggs. Several parasitoids have been reared from billbugs, including Bracon sphenophori (Muesebeck) (Hymenop-tera: Braconidae) from S. callosus; Bracon analcidis Ash-mead (Hymenoptera: Braconidae) and Myiophasia sp. (Diptera: Tachinidae) from S. parvulus; Vipio belfragei (Cresson) (Hymenoptera: Braconidae) from S. cariosus; and Gambrus bituminosus (Cushman) (Hymenoptera: Ichneumonidae) from S. pertinax. Severe mortality of pupae and adults has been observed in South Carolina, caused by the fungus Beauveria bassiana.
Life Cycle and Description. The billbugs are quite similar in form and biology, and except where noted, the following description applies mostly to maize billbug. The overwintering stage of these insects is generally the adult form, though in southern states more than one stage may be present during the winter months. There usually is a single generation, with adults ovipositing in May and June, larvae present through August, and pupation occurring in late summer and autumn. Some beetles emerge from the pupal stage and feed before overwintering, whereas others remain, as adults, in the pupal cell until spring.
Egg. The eggs are deposited singly in the stem of the host plant within a cavity created by the mouth-
parts of the female. The egg is white and bean-shaped. It typically measures 1.5-3.0 mm long and about 0.9 mm wide. The incubation period of the eggs is 4-15 days, usually averaging about seven days.
Adult. The adults are elongate-oval, and vary in color from brown to black. Beetles range in size from about 5-19 mm. The head is prolonged into an elongate snout that curves strongly downward and measures about one-third the length of the body. The thorax usually bears deep pits, and the elytra distinct grooves. Adults are long-lived, surviving not only the winter but also well into the summer. They become active in the spring when the soil temperature exceeds to 18°C. Mating can occur any time after emergence, and oviposition begins about 10 days after mating. Oviposition can occur for 1-3 months, and though insectary records document an average of only about 40 eggs per female, some records suggest that up to 200 eggs can be produced by a female. The adults rarely fly and disperse instead by walking.
A synopsis of billbugs was provided by Sat-terthwait (1919), and good treatments of maize billbug were given by Hayes (1920), Cartwright (1929), and Kirk (1957a). Detailed treatment of southern corn billbug was presented by Metcalf (1917). Forbes (1904) pictured and described briefly several billbugs attacking corn in Illinois.
Prior to the availability of modern persistent insecticides, billbugs were the most serious pest of southern corn crops. They can cause severe reduction in stand density, though they principally attack field corn rather than sweet corn. The adults feed just above or below the soil surface, where they puncture the stem, creating large cavities in the plant tissue. When leaves later unfurl, the plant displays even rows of round holes. Extensive feeding causes distortion and death of the plant. Beetles sometimes feed on germinating seed and roots. The adults also create cavities in which they deposit their eggs. Larvae, upon hatching, burrow upward in the stem, causing wilting and excessive lateral growth. Corn rarely produces ears when supporting larvae.
Low-lying and wet areas are more prone to billbug problems owing to the presence of alternate hosts such as cattails and rushes. Seedlings can be protected with liquid or granular insecticide applied at planting (Kirk, 1957b; Durant, 1974). Tillage and destruction of crop residue in the autumn can damage overwintering beetles, but the level of control attained by such practices is slight. Early planting allows the plants to gain enough size to tolerate considerable feeding, so it is sometimes recommended. Crop rotation is beneficial if crops other than grains can be grown profitably.
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