Insecticide application may be justified if larval feeding reduces leaf area by more than 30 percent or is damaging the developing grain head or growing point within the whorl.
The fields where I have found the armyworms had less than 5% infested plants and the feeding damage was below the economic threshold. Often, I was finding two or three consecutive damaged plants but only one armyworm was found, indicating the worms are moving between plants.
All sorghum fields should be scouted weekly throughout the season for insect pests. The primary insects of concern are different aphids. Yellow sugarcane aphids can be damaging to sorghum until the plant has 5 leaves.
Yellow sugarcane aphids are usually found on the underside of the leaves, the aphid is lemon yellow but sometimes pale green. They are covered with small spines and have two double rows of dark spots on the back. (See Pic on Left)
Corn leaf aphids may also be found at this time but are usually found in the whorl of the plant. Corn leaf aphids don’t cause much damage to the plants and rarely require treatment. They are a food source to beneficial insects. Thus, the corn leaf aphid can actually be beneficial to your sorghum production.
The Sugarcane aphid has not been found in fields of the Mid Coast yet. This aphid can cause yield losses when it’s numbers exceed 50 aphids per leaf. Keep an eye out for sugarcane aphids throughout the season; especially if you begin to see honeydew on plants.