Lesser cornstalk borer
The larvae of lesser cornstalk borers (Elasmopalpus lignosellus) attack roots and bore into the stems of young plants of peanuts, corn, sorghum, and other crops. Damaging infestations of this insect rarely occur in sorghum. The larvae are light bluish green with prominent transverse reddish-brown bands (Fig. 33). They feed in silken tunnels covered with soil particles. The larvae pupate in silken cocoons under crop debris.
Infestations of lesser cornstalk borers usually are more severe during dry periods and in sandy soils. To discourage the insect, adopt cultural practices that preserve moisture and increase organic matter in the soil. Early planting and rotation with nonhost crops help avoid damage from lesser cornstalk borer. Insecticidal control rarely is justified, although formulations of lambda- cyhalothrin, gamma-cyhalothrin, deltamethrin and chlorpyrifos are labeled for control of lesser cornstalk borer in grain sorghum.
The sugarcane borer (Diatraea saccharalis), southwestern corn borer (Diatraea grandiosella), European corn borer, (Ostrina nubilalis), Mexican rice borer (Eoreuma loftini), and neotropical borer (Diatraea lineolata) are closely related insects that tunnel in the stalks of sorghum, corn, and other crops.
The biology of these four species is similar. The moths are white to buff colored, and the females deposit clusters of flattened, elliptical to oval eggs that overlap like fish scales in a shingle-like arrangement on the host plant leaves. The eggs hatch in 3 to 7 days.
The larval stage lasts about 25 days and the pupal stage about 10. There are two to three generations a year. The larvae are creamy white and about 1 inch long when fully grown. Most of the body segments have conspicuous round brown or black spots (Fig. 34). The spots on mature overwintering larvae are lighter or absent. Most of these borers pass the winter as fully grown larvae in cells inside the stalks that remain after the crop is harvested.
Young larvae feed for a few days on the leaves or leaf axes. Older larvae tunnel into the sorghum stalks; larvae bore up and down the pith of the stalk. Borer-infested stalks may be reduced in diameter and yield less. Larval tunneling just below the grain head can cause it to break and the grain head to fall. Injury by borers increases sorghum susceptibility to stalk rot diseases and lodging.
Plant sorghum early because borers are typically more abundant in late-planted sorghum. In northern Texas regions, shredding stalks very close to the ground or plowing and disking stubble destroys overwintering larvae of the southwestern corn borer by exposing them to cold temperatures. This practice reduces borer abundance the next year.
Check the plants carefully for stem borers. Look for small holes near the leaf axis, which indicate that a larva has entered the stalk. Once the larvae have entered the stalk, it must be split to see them. Inspect the leaves carefully—the eggs are hard to find. Clusters containing 10 to 20 individual eggs may be on the top or underside of leaves, depending on the borer species. Assess the abundance of eggs and small larvae before the larvae bore into stalks. Insecticidal control is effective only if applied before larvae bore into stalks.
Sugarcane rootstock weevil
The sugarcane rootstock weevil, Apinocis (Anacentrinus) deplanatus, infests sorghum sporadically, especially during dry years and in fields where johnsongrass is abundant. The adult weevil is dark brown or black, about 1/8 inch long and 1/16 inch wide (Fig. 35). It overwinters beneath plant residues on the ground. In early spring, the weevils infest wild grasses, such as johnsongrass, and later move to sorghum. The female uses its mouthparts to make a small puncture at the base of the plant, where the egg is deposited and concealed. It lays about 16 eggs, which hatch in 6 days. When fully grown, the larvae are white, legless grubs about 1/5 inch long. A generation is completed in about 40 days.
Adult weevils feed on young sorghum plants and create pinpoint holes in the leaves. The larvae cause the most damage as they tunnel into the sorghum stalk just above or below the soil surface. The larvae are often found at nodes and near the outer surfaces of the stalk. As a result of larval feeding, the plants appear drought stressed and may lodge. Pathogens can invade the plant through feeding tunnels. Although locally damaging populations may occur, economic thresholds for this pest have not been established, and control has usually not been required.