Distribution. The Limonius spp. are principally western in distribution, occurring from the Rocky Mountains to the Pacific Ocean, and from British Columbia to southern California. In addition to sugar-beet wireworm and Pacific Coast wireworm, other western species that sometimes damage crops are western field wireworm, L. infuscatus Motschulsky; and Columbia Basin wireworm, L. subauratus LeConte. In the eastern United States the eastern field wireworm, L. agonus Say, is an occasional pest. Apparently these species are native to North America.
Host Plants. The larvae of these beetles, called wireworms, damage cantaloupe, cauliflower, corn, lettuce, lima bean, mustard, potato, tomato and other crops with tubers or a substantial root system. They are most frequently noted as pests of potato and to a lesser degree of sugarbeet and alfalfa. Weeds are suitable for larval development, and they have been observed to feed on dock, Rumex sp.; Johnsongrass, Sorghum halepense; nettle, Urtica sp.; pigweed, Amar-anthus retroflexus; wild beet, Beta sp.; and others. Limo-nius spp. larvae are active, and aggressive toward each other, sometime displaying cannibalistic tendencies. Adults, which are called click beetles, have been observed to feed on a variety of foods, including beet roots, foliage of several plants, and pollen.
Natural Enemies. Natural enemies of wireworms are few, and often relatively ineffective. Birds are often observed to follow tractors as soil is tilled, and to con sume large numbers of wireworm larvae. However, they glean only a small proportion of the larvae. Insectivorous birds also prey on adults, as do predatory ground beetles (Coleoptera: Carabidae). Perhaps the most important natural enemy is the stiletto fly Psiloce-phala frontalis Cole (Diptera: Therevidae). The larvae of this predatory species, which resemble wireworms and are sometimes mistaken for beetles, occur in the soil. They puncture the integument of wireworm larvae and feed upon them.
Life Cycle and Description. Among the Limonius spp. the biology of sugarbeet wireworm is well-studied. The following description applies to sugarbeet wireworm except where noted. However, the biologies of the several Limonius spp. seem to be similar.
Duration of the sugarbeet wireworm life cycle is highly variable, ranging from 1-5 years. In California, 70% of the insects complete their development in two years, but further north 3-4 years may be required for development. Adults emerge in the spring, lay eggs, and larvae live for various lengths of time before pupating. Pupation occurs in the autumn, with adults overwintering in the soil prior to emergence.
Sugarbeet wireworm larva.
Terminal abdominal segment (dorsal view) of Pacific Coast wireworm larva.
but soon turns yellowish. In shape, the pupa greatly resembles the adult except that the abdomen is slightly more elongate than in the adult, and the elytra are reduced in size and twisted ventrally. Pupae measure about 11.5 mm long and 3.6 mm wide. Duration of the pupal stage is 7-9 days.
Adult. The adult is 8.5-12.0 mm long. It generally is dark brown to black, but sometimes it is orangish brown or reddish brown. The elytra are marked with numerous parallel channels. Females are more likely to have four and five year life cycles than males; males are more likely to have a one year life cycle than females. Adults mate immediately upon emerging from the soil. After a preoviposition period of about six days, females commence egg production. Duration of the oviposition period is about 25-35 days. Their length of life is given at only 30 days for males and 45 days for females; this seems short and may be an artifact of caged conditions.
Detailed biology of sugarbeet wireworm was provided by Graf (1914) and Stone (1941). A key for important Limonius spp. was published by Lanchester (1946). A key to distinguish Limonius from the other common vegetable-attacking genera of wireworms can be found in Appendix A. Keys for the identification of the genera of adult Elateridae can be found in Arnett (1968), and of larvae in Wilkinson (1963) and Becker and Dogger (1991). Wilkinson (1963) provided excellent keys to distinguish among the western wire-worm species, as well as their biology and history of damage in British Columbia. Good reviews of wire-worm biology and damage can be found in Thomas (1940) and Glen et al. (1943).
These wireworms are damaging in irrigated crops. They normally are not abundant in dryland production or in grasslands. Damage to seed or to germinating seedlings normally occurs soon after planting. Such plant material is easily destroyed by the feeding of a single wireworm larva, and results in reduction in stand. Early symptoms of infestation are poor germination, and wilting or death of seedlings. In older plants considerable root injury can occur before signs
Sugarbeet wireworm larva.
of wireworm feeding are evident. In crops such as potato, however, where the appearance of the tuber is important, even superficial damage can be costly. Wireworms often tunnel deeply into potato tubers, which provides entry for soil-borne plant pathogens. Wireworms are the most serious soil pests of potato in the Pacific Northwest (Toba, 1987). Onsager (1975) noted that Pacific Coast wireworm was more damaging to potato than to southern potato wireworm, Conoderus falli Lane; any tubers fed upon by Pacific Coast wireworm qualify for rejection according to current standards. Small tubers are subject to more injury than are large tubers (Toba and Turner, 1981b).
Sampling. Sex pheromones for Pacific Coast wire-worm have been tentatively identified and may prove useful for population monitoring (Butler et al., 1975). Presently, population monitoring depends on larval assessment, and fields should be sampled before susceptible crops such as potatoes are planted. Larval populations tend to be aggregated (Williams et al., 1992). 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. Soaking the bait for one day increases its attractiveness. Baiting does not work well if the soil is under 8°C, so a sheet of 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. Wireworm counts of one or more per bait station suggest that wireworms perhaps will be a problem and insecticides may be needed (Bynum and Archer, 1987). At larval densities of more than four per bait station it is not advisable to plant potatoes even if insecticides are to be used. Simmons et al. (1998) compared several methods of sampling and reported that corn/wheat baits were the most accurate, precise, and cost effective.
For soil sampling, 15 x 15 cm soil samples, including soil to a depth of 30-40 cm, are removed and checked for wireworms. This is normally done by sieving with coarse and fine screen to separate insects from the soil. If wireworm counts exceed 2 in 40 samples, insecticide treatment is warranted.
Insecticides. Insecticides in granular or liquid form are often applied broadcast before planting, and incorported into the soil to kill wireworms. Also, fumigation of soil is practiced in many areas with a history of wireworm problems. Alternatively, insecti cides can be applied with the seed at planting, or adjacent to the plants after planting. Wireworms are fairly mobile, with sugarbeet wireworms often moving 45 cm in a two-week period (Toba, 1985b); this can make band treatments risky. Broadcast applications are most desirable when wireworm densities are high, but in-row applications are adequate at low to moderate densities (Wilkinson et al., 1977; Toba and Turner, 1979; 1981a).
Plant-produced toxicants such as isothiocyanates, which apparently function as biofumigant allelochem-icals, have had limited testing for suppression of such insects as sugarbeet wireworm. Incorporation of rape-seed seed meal into soil demonstrated the potential to kill wireworm larvae, but available varieties of rape are too low in isothiocyanates to provide practical suppression (Elberson et al., 1996).
Cultural Practices. Crop rotation can be beneficial in alleviating wireworm problems. Some crops, particularly alfalfa, tend to dry the soil, which is deleterious to wireworms. Fallowing of cropland can also disrupt wireworm populations by affecting soil moisture and by depriving larvae of food. Thus, it is beneficial to alternate such practices with production of wire-worm-susceptible crops.
Biological Control. Toba et al. (1983) evaluated the effectiveness of Steinernema spp. (Nematoda: Stei-nernematidae) for suppression of sugarbeet wireworm in Washington. Application of nematodes to the soil surface at high rates demonstrated some infection, but high levels of inoculum are needed to provide good suppression of wireworms.
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