Melanotus longulus oregonensis LeConte Coleoptera Elateridae

Natural History

Distribution. The genus Melanotus is widespread in North America, but the western half of the continent has relatively few species. This is likely related to their preference for moist habitats. Corn wireworm is widespread in the eastern United States and occurs as far west as Nebraska and Texas. In Canada it is known from Ontario and Quebec. Oregon wireworm occurs in all states west of the Rocky Mountains and in British Columbia. Although these are the most common Mel-anotus spp. affecting vegetables, others including M. depressus (Melsheimer), M. verberans (LeConte), and M. cribulosus (LeConte) are sometimes reported to be damaging, particularly to corn. Apparently these are native species. (See color figure 126.)

Host Plants. Melanotus species are reported to damage vegetables such as cabbage, corn, escarole, lettuce, pepper, potato, sweet potato, and other crops such as field corn, sorghum, soybean, sugarcane, and wheat.

Natural Enemies. The natural enemies of wireworms are not well known, and generally seem to be unimportant. In Florida, M. communis is parasitized by the wasp Pristocera armifera (Say) (Hymenoptera: Bethylidae), but Hall (1982) reported that only about 4% of wireworm larvae were affected.

Life Cycle and Description. The biology of Melanotus is poorly studied relative to other genera of wireworms, which is surprising considering the frequency of association with corn. In Iowa, they are reported to have a five-year life cycle, with egg deposition occurring in June, egg hatching in July, and larvae requiring four additional summers to complete their development. In southern Florida, however, the life cycle apparently is reduced to 2-3 years (D. Seal, personal communication). Pupation occurs in the autumn. Although Fenton (1926) suggested that adults emerge from the soil to overwinter under the bark of trees, Hyslop (1915) documented overwintering by adults in the soil in pupal cells. Adults reportedly feed on pollen (Fenton, 1926).

Studies in Florida showed that most flight activity occurred in May and June. Adults fly in the evening hours. Females contain 50-100 eggs within their ovaries, indicating a fairly high fecundity (Cherry and Hall, 1986). A strong association of larvae with cool and moist soil is evident, though warmer soil may be tolerated if it is moist (Shepard, 1973b). Adults are reddish brown to dark brown, and measure 10-13 mm long. Larval morphology is fairly typical of wireworms. They are yellow to yellow-brown, shiny, and elongate. The head, thoracic plate, and anal plate are darker. The most distinctive feature about Melanotus larvae is the lack of a notch at the tip of the abdomen in combination with a flattened abdominal tip. A key to distinguish Melanotus 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). Keys for the identification of adult Melanotus were provided by Quate and Thompson (1967). A key for Mela-notus larvae of mid-western corn fields was developed by Riley and Keaster (1979). The ecology and management of wireworms was reviewed by Thomas (1940).

Damage

The larvae of Melanotus feed below-ground on seeds, roots, tubers, and other plant tissue. They will kill young plants and deface the surface of potato

Corn wireworm larva.

Corn wireworm larva.

Elateridae Figure
Terminal abdominal segment (dorsal view) of corn wireworm larva.
Limonius Larvae Key
Terminal abdominal segment (dorsal view) of Oregon wireworm larva.
Melanotus Longulus
Adult corn wireworm.

and sweet potato tubers. Melanotus spp. prefer high moisture, and the heavier, wet portions of fields are most likely to experience damage. Damage is exacerbated by dry soil conditions, however. Larvae tend to move toward the soil surface in the spring as the soil warms, where they feed until the soil attains about 21° C; they then move deeper in the soil where it is cooler. Larvae again return to the upper layer of the soil in the autumn when the soil surface is cool, and feed until it becomes cold, then they return to a position deep in the soil until the soil warms in the spring (Fisher et al., 1975).

Management

Sampling. Considerable effort has been made to develop baits for wireworm sampling. Growers are advised to assess the population densities of wire-worm larvae before planting in the spring. Baiting is accomplished by burying whole wheat, corn, sorghum, or other attractive food sources in the soil at a depth of 10-15 cm and counting the number of wireworm larvae attracted (Apablaza et al., 1977, Jansson and Lecrone, 1989). As few as 12 bait stations per hectare are often recommended. Soaking the bait for one day increases its attractiveness. Baiting does not work well if the soil is less than 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 (Ward and Keaster, 1977; Kirfman et al., 1986). Larvae can also be separated from the soil by sifting; a screen with 6-7 meshes per centimeter is desirable. Soil sampling followed by screening is an accurate, but labor intensive, means to predict wireworm problems before planting. Spatial distribution of larvae is variable, but sometimes aggregated, requiring extensive sampling. A mobile, self-propelled sampling device has been developed (Smith et al., 1981). Simmons et al. (1998) compared several methods of sampling and reported that corn/wheat baits were the most accurate, precise, and cost effective.

The adult populations can also be sampled. Phero-mones of the tufted-apple budmoth, Platynota idaeusa-lis Walker (Lepidoptera: Tortricidae), are attractive to some Melanotus spp. adults (Brown and Keaster, 1983, Keaster et al., 1987). This suggests that a sex phero-mone of Melanotus spp. could be identified and synthesized for population monitoring, as has been done for Pacific Coast wireworm, Limonius canus LeConte (Butler et al., 1975). Pitfall traps and sticky traps are useful for sampling adult populations on the soil surface and in flight, respectively (Brown and Keaster, 1986). Genung (1972) noted that click beetle catches on sticky traps and in light traps were not completely equivalent, and that both types of trap should be used in population studies. In southern Florida, blacklight traps are sometimes used to predict wireworm population densities during the next growing season.

Insecticides. Insecticide is often applied to prevent susceptible crops from being injured by wireworms. The most common approach to wireworm suppression is to apply persistent liquid or granular formulations at planting, and to incorporate the insecticide into the soil near the seeds or seedlings. Preplant applications generally, but not always, are more effec tive than postplant treatments (Linduska, 1979; Chalfant et al., 1992b). Sometimes insecticides are applied broadcast over the entire field, but more commonly they are applied in bands over or along the rows. For long-duration crops, the planting-time application may be followed by a mid-season application. Insecticide also may be incorporated into the irrigation water (Chalfant et al., 1992a). Effectiveness varies considerably among insecticides (Day and Crosby, 1972), and some insecticide resistance has been reported. Soils with high organic matter content sometimes interfere with insecticide efficacy (Campbell et al., 1971).

Cultural Practices. Cultural practices are the most important approach to population management. Crop rotation and fallowing are particularly effective. Grasses are particularly attractive to Melanotus wireworms, and a survey of wireworm problems in Missouri corn fields established a relationship between wireworms and previous growth of sod (Belcher, 1989). The type and temporal occurrence of cover crops can affect subsequent damage by wireworms. Grass cover crops such as sorghum-sudan are highly attractive to Melanotus spp., and it is important to have the fields free of such grasses during peak flights (Jans-son and Lecrone, 1991). Melanotus spp., unlike some other wireworms, will inhabit forested areas, and crops planted on recently cleared land may be damaged (Cheshire and Riley, 1988). Where crops can be flooded, six weeks or two four-week periods separated by two weeks of drying can eliminate Mela-notus wireworms (Genung 1970).

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