Stalk Borers: Sugarcane Borer, Diatraea saccharalis (Fabricius); Neotropical Borer, D. lineolata (Walker); Southwestern Corn Borer, D. grandiosella Dyar

Description and Biology: Sorghum in the United States usually is not seriously infested by stalk borers. However, stalk borers in sorghum in other areas of the world are often key insect pests. Larvae of these stalk borer species resemble each other in appearance and have similar biologies. A fully-grown larva is about 25 mm long, with a brown head and thorax, and white- to yellow-colored body. Most body segments have conspicuous round, brown or black spots. Spots are lighter colored or absent on more mature overwintering larvae. The insects pass the winter as fully-grown larvae in cells inside stalks or root crowns that remain after the crop is harvested. They pupate in the spring and adults emerge a few weeks later. On leaves of host plants, the buff-colored moths lay clusters of elliptical to oval-shaped, flattened eggs that overlap like fish scales in a shingle-like arrangement. Eggs hatch in three to seven days. The larval stage lasts about 25 days and the pupal stage about 10. There is one to three generations a year.

Symptoms and Damage: Young larvae feed for a few days on leaves or in leaf axis. Older larvae tunnel into stalks and bore up and down the pith of the stalk. Borer-infested stalks are reduced in diameter, and may lodge. Boring by larvae in the peduncle of sorghum often causes the peduncle to break and the panicle to fall. Injury by borers makes the plant more susceptible to stalk rot diseases.

Monitoring: Determining the presence of stem borers requires careful examination of sorghum plants. Small holes near the leaf axis indicate a larva entered the stalk. Once larvae have entered the stalk, stalks must be split to see the larvae. Eggs on the leaves are difficult to find. Clusters containing 12 to 20 small individual eggs may be on the top or underside of leaves, depending on the borer species. Abundance of eggs and small larvae must be assessed before larvae bore into stalks if insecticidal control is to be effective.

Sugarcane Borer

Sugarcane Borer

Neotropical Borer

Neotropical Borer

Southwestern Corn Borer

Southwestern Corn Borer

 

Management: Planting sorghum early is an important management tactic because borers typically are more abundant in late-planted sorghum. Plowing and disking stubble to bury crop residues soon after harvest destroys overwintering larvae by exposing them to cold temperatures. Chemical control rarely is justified. For insecticides to be effective, they must be applied to control larvae before they bore into stalks.

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