by Doug Johnson, Kentucky Pest News Number 1035

Grain sorghum is beginning to find a larger place in our grain production system. One reason is the lack of insect pests when the crop is grown during its optimum planting time. However, like any of our other grain crops, when grain sorghum is planted out of its optimum time period it is subject to insect infestation. Grain sorghum can be planted too early and too late. Obviously, at this time of year we are interested in the too late part.

If your grain sorghum crop is already in the hard dough stage you are probably by most of the problems.
However, if your crop is still the milk to soft dough stages, you need to watch out for the "head worm" complex. This is a group of caterpillars that will feed in the heads as long as the grain is soft enough for them to eat.

The likely first "head worm" will be the corn earworm, aka soybean pod worm. As corn and soybean fields begin to mature, the adult moths begin Young (typically double crop timed) grain sorghum can be just the right food source.

The corn earworm larvae vary in color from light green to black, with lighter stripes running the length of the body. When larval development is complete earworms may reach 1 ½" in length. Larvae have three pair of true legs near the head, four pair of fleshy legs near the center of the body and one pair of fleshy legs near the rear end. These larvae may be confused with fall armyworm; however, earworms do not have an inverted "Y" on their heads. (Corn earworms and fall armyworms may be found in mixed populations. If both species are feeding on the heads, it makes little difference which species they are.) Both species are "naked" worms in that they have no, or few, obvious hairs.

The later arriving pest and the most typical one for September is the sorghum webworm. In addition to feeding on the grain, this pest makes a mess of the head. They spin a small web which tends to hold moisture and to collect their feces, so the whole area is contaminated. Sorghum webworm is smaller than the corn earworm (or fall armyworm), reaching ½" in length when mature. Webworms are green and "bristly".

Neither of these pests (and they may occur together) is hard to find or count on heads. To scout, examine twenty heads in each location for the presence of the worms. If you find an average of two worms or more per head of either or both species then you need to consider an insecticidal control.

If insecticidal control is warranted you may check in ENT- 24, Insecticide Recommendations for Grain Sorghum (Milo), to find products and use suggestions. This publication may be found on the web at: http://www.uky.edu/Agriculture/PAT/recs/rechome.htm or in print from your County Extension office.

You can obtain a grain sorghum scouting manual by going to the IPM Web page at: http://www.uky.edu/Agriculture/IPM/ipm.htm then "click" on "Manuals & FactSheets".