Distribution. These native species are cool weather wireworms, and principally western in distribution. They cause damage in the United States from Oregon and Washington east to the north-central states. In Canada, C. aeripennis is destructive in British Columbia, the Prairie Provinces, and north into the
Northwest and Yukon Territories. C. glauca is damaging in British Columbia and the Prairie Provinces, but C. pruinina is not known from Canada. Unlike some other wireworms, Ctenicera spp. tolerate dry soil and are damaging to dryland crops.
Host Plants. These wireworms are best known as wheat pests, the principal crop in the arid regions where they are commonly found. However, they have a wide host range. The larva, or wireworm stage, attacks many vegetables, including bean, carrot, corn, lettuce, lima bean, onion, potato, tomato, and probably many others. Additional crops injured, in addition to wheat, are barley, canola, flax, oats, rye, sugarbeet, and sunflower. Ctenicera spp. also feed on other insects, such as fly larvae, if provided with the opportunity (Zacharuk, 1963).
Natural Enemies. As is the case with other wireworms, Ctenicera spp. are relatively free of important natural enemies. Birds often feed on the adults in the spring when they emerge from the soil. The horned lark, Otocoris alpestris alpestris, is a species that is especially effective at gleaning adults. When the soil is tilled and larvae are exposed, a number of birds will take advantage of this temporary food source, but probably only a small proportion of larvae are ever exposed long enough to be captured by avian predators. Predatory insects such as ground beetles (Coleoptera: Carabidae) and stiletto flies (Diptera: Therevidae) are associated with Great Basin wireworm. Mortality due to the fungi Beauveria and Metarhizium, and unspecified bacteria, have been noted during rearing studies, but these pathogens seem of little significance in nature (Zacharuk, 1962a).
Life Cycle and Description. These are long-lived species, normally requiring a minimum of three years for development, and as much as 10 years under unfavorable conditions. Adults, which are also called click beetles, deposit eggs in the spring. Larvae complete their development after a minimum of three summers, and this is followed by pupation in the autumn, and overwintering by the adult. Following are brief descriptions of two of the more important species: Great Basin wireworm and prairie grain wireworm.
The eggs of Great Basin wireworm are white, and elliptical, measuring only slightly longer than wide. They are covered with viscous, sticky secretion that causes soil particles to adhere. The female normally deposits about 350 eggs over a 21-day period, often at a depth of 10-15 cm. She apparently seeks moist soil for oviposition. Eggs must absorb water from the soil in order to complete embryonic development, and they absorb water more rapidly at higher temperatures (Doane, 1969a). Duration of the egg stage is about 30 days, with hatching occurring in June. Young larvae are white, but soon turn yellow. The head, thorax, and anal plate are yellowish brown. When larvae hatch they measure about 1.5 mm long, but eventually attain a length of about 18-20 mm. As is the case with most wireworms, they are shiny in appearance and the thoracic legs are not pronounced. Larvae feed on rootlets and roots, usually molting twice per year with a range of 1-5 molts per season. Pupation occurs in the soil at a depth of 15-25 cm, in a small cell formed by the larva. The pupal stage is initially white but turns yellow with time. In form, the pupa greatly resembles the adult, but the elytra are not completely developed and are twisted ventrally. Duration of the pupal stage is only about 10 days, but beetles remain in the cell until the following spring when they dig to the surface. The adult is brown in color. It measures about 13-15 mm long. The elytra are marked with numerous parallel furrows. The beetles become active in the spring, usually May, when the soil attains a temperature of at least 13°C and air temperature reaches 18-20°C. Adults are relatively short lived, usually perishing after 3-4 weeks. The adults seem not to feed, or if they do, they feed sparingly. The adults of dryland wire-worm are quite similar to Great Basin wireworm in appearance except that they average smaller in size, about 7-10 mm.
Prairie grain wireworm and Puget Sound wire-worm are similar in morphology and ecology, and recently they have been treated as a single species. However, the work of Glen et al. (1943) suggested that they are different species. Eggs are white and oval, measuring about 0.48 mm long and 0.34 mm wide. The color is white, and eggs are coated with a sticky, viscous material that causes oil particles to adhere. Duration of embryonic development is 11.5 days at 27°C and 20 days at 21°C. Larvae are elongate and yellowish, as is typical of wireworms. Young larvae seem to feed principally on root hairs and fungal mycelia; it is only after the first two instars that larvae begin to feed on seeds. Apparently these larvae do not descend deeply into the soil during the winter months, but remain near the surface. They become active as the soil warms above 7°C. The number of larval instars varies from 9-11 and requires from one to over two years under laboratory conditions. Under field conditions, larval development time is thought to average three years. Larvae of C. a. aeripennis attain a length of 28 mm at maturity, but C. a. destructor rarely exceeds 22 mm (Wilkinson, 1963). As larvae attain maturity they form a pupal cell in the soil, usually at a depth of 5-10 cm. The pupae resemble the adult in form, although the elytra are not fully developed. Duration of the pupal stage is 2-3 weeks at 20°C. Adults remain in the pupal chambers until spring, when they dig to the soil surface. They typically emerge in April or May. The adults have a black pronotum and bronze or greenish elytra. They measure about 14 mm long. Females mate soon after emergence. About six days after mating the female commences oviposition. She usually deposits from 400-1400 eggs over a period of about 20 days. Females that have access to food live longer and tend to produce more eggs. Doane (1963) noted that this was an unusually high rate of fecundity for a wireworm.
The biology of Great Basin wireworm was given by Lane (1931), prairie grain wireworm by Zacharuk (1962a,b), and dryland wireworm by Hyslop (1915). Keys for the identification of the genera of adult Elater-idae can be found in Arnett (1968), and of larvae in Becker and Dogger (1991). Wilkinson (1963) provided an excellent key to distinguish among Ctenicera spp., and other wireworms found in British Columbia, and includes information on biology and economic impact. Thomas (1940) provided a review of wire-worm biology and management, and King (1928a) and Glen et al. (1943) gave useful information on wire-worm ecology and damage in Canada. A key to distinguish Ctenicera from the other common vegetable-attacking genera of wireworms can be found in Appendix A.
Dryland wireworm larva.
Dryland wireworm larva.
The larvae feed on seeds and below-ground portions of plants, often killing several plants during the course of their development. In southern portions of their range or dry climates the damage usually occurs in the spring months, and the larvae descend deep into the soil during the summer as the soil dries and warms (Lane, 1931). In northern climates larval feeding occurs throughout the summer, with Burrage (1963) reporting maximum damage to potato tubers in Saskatchewan during June-August. Damage was positively correlated with soil temperature as long as soil moisture levels were adequate.
Sampling. The statistical distribution of Ctenicera spp. varies with the stage sampled. Eggs and young larvae are aggregated, but the aggregation dissipates as the insects become more mature and mobile (Doane, 1977). Doane (1969b) presented a technique for separation of eggs from the soil. Population monitoring depends primarily on assessment of larval populations, and fields should be sampled before susceptible crops such as potatoes are planted. Larval populations are sampled by baiting or soil sampling.
Baiting is accomplished by burying whole wheat, corn, potato, carrot or other attractive food sources in the soil at a depth of 10-15 cm and counting the number of wireworm larvae attracted (Doane, 1981). For Ctenicera spp., grains seem to be more attractive than vegetable matter (Toba and Turner, 1983). Carbon dioxide emanates from such bait, and larvae follow the carbon dioxide gradient to the food source (Doane et al., 1975). Soaking the bait for one day increases its attractiveness. Baiting does not work well if the soil is cold, so a sheet of black or transparent plastic is sometimes placed over the section of soil containing bait; this warms the soil and increases the mobility of wireworm larvae, allowing more accurate counts. Simmons et al. (1998) compared several methods of sampling and reported that corn/wheat baits were the most accurate, precise, and cost effective.
Soil sampling requires 15 x 15 cm soil samples, including soil to a depth of 30-40 cm, that are removed and checked for wireworms. This is normally done by sieving soil with coarse and fine screens to separate insects from the soil. Although accurate estimates of wireworm population densities are possible with soil sampling, this approach is very labor intensive; thus, baiting is more commonly used.
Insecticides. Insecticides are often applied for wireworm control when susceptible crops such as potato are planted. Insecticides may be applied broadcast before planting, but more commonly at the time of planting in the seed furrow, over the row, or broadcast. At low wireworm densities, applications limited to the rows are adequate (Toba and Turner, 1979, 1981a; Toba and Powell, 1986), but at moderate wire-worm densities broadcast applications are often more effective. At high densities susceptible crops should not be planted.
Cultural Practices. Ctenicera spp. have a wide host range and crop rotation generally is not practical. Summer fallow sometimes can be a useful practice, however, because if weed control is strictly practiced during the fallow period the larvae are deprived of food. Although the larger larvae are extremely tolerant of starvation, sometimes surviving two years without food, the small larvae suffer from food deprivation. Thus, summer fallow alone is not dependable for protection against these wireworms. Special caution should be taken if crops follow grass sod, a favored habitat.
Shallow cultivation is sometimes recommended where wireworms are a problem. By cultivating shal-lowly, the lower layers of soil remains compact, and the beetles are unable to burrow deeply as they prepare to oviposit. Shallow oviposition results in desiccation of the eggs.
The method of seeding also influences wireworm damage potential. Shallow seeding is desirable if adequate moisture is available, because the warm soil speeds germination and growth, providing the plant the opportunity to outgrow insect feeding damage. It also is advantageous to avoid early or late planting because cool or dry soil inhibits plant growth and increases the likelihood of significant injury.
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