Description and Biology
Several species of wireworms attack planted sorghum seed. True wireworms are immature stages of click beetles, family Elateridae. False wireworms are immature darkling beetles, family Tenebrionidae. Wireworms are shiny, slender, cylindrical, smooth, and hard-bodied larvae ranging in color from white to yellow or brown, and are about 25 mm long when grown (Color Plate 1). The last segment of the larva usually is ornamented.
All life stages of wireworms except the adult develop in the soil. Depending on the species, a generation is completed in one, two, or more years. Wireworms usually overwinter in the damaging larval stage and are present in the soil at the time sorghum seed are planted in the spring.
Symptoms and Damage
Wireworms feed on and damage planted sorghum seed. Wireworms feed less on seedling roots. Seeds hollowed out by larvae do not germinate, thus reducing plant stand. Evidence of wireworm damage to sorghum is a nonuniform plant stand with stunted, weak seedlings.
Soil in fields should be inspected for wireworms before sorghum is planted. However, visually locating wireworms in soil is difficult. Wireworm abundance can be assessed by placing about 300 g of nontreated sorghum seed in a 12-cm-wide hole at least 10 cm deep, deep enough to reach moist soil. After covering the hole with soil, the baited trap should be covered with a 1-m2 sheet of black plastic that warms the soil and attracts wireworms to the warmed soil and grain. The trap should be marked with a stake. One trap for each 2.5 to 5 ha of field should be installed at least two weeks before planting. Grain in the trap should be examined two weeks after placement and wireworms counted. Two or more wireworm larvae per trap are sufficient to cause stand loss and justify the use of insecticide, especially if conditions are not good for rapid seed germination.
The amount of damage to sorghum depends on the number of wireworms in the field at the time of planting and the length of time seed are in the soil before they germinate. The seedbed should be prepared properly and seed planted at a time to ensure rapid seed germination. Sorghum should be planted in a field where a nongrass crop was grown the previous year. Also, tillage before planting reduces noncrop plants in the field. Insecticide applied to seed or in the planter-box usually is effective in controlling wireworms. Seed may be purchased already treated with a systemic insecticide that protects against wireworms. In-furrow insecticide applications also control wireworms. When wireworms are abundant, seed treatment should be supplemented with a granular or liquid insecticide applied to soil at planting.