Heavy populations of “headworms” have been observed in some northeast Texas grain sorghum fields. The two species of caterpillars that make up these populations are corn earworms (Helicoverpa zea) and the fall armyworm (Spodoptera frugiperda).
The corn earworm and fall armyworm moths lay eggs on leaves or grain heads of the developing milo crop.
Newly hatched corn earworm larvae are pale in color and only on-eighths of an inch long. They grow rapidly and become variously colored, ranging from pink, green, or yellow to almost black.
Many have conspicuous stripes.
Fully-grown larvae are robust and one and a half to two inches long. Young fall armyworm larvae are greenish with black heads. Mature larvae range from greenish to grayish brown and have a light colored, inverted, Y-shaped suture on the front of the head and dorsal lines lengthwise on the body.
Larvae of both species feed on developing grain heads. Small larvae feed on flowering parts of the grain head first, and then hollow out kernels.
Larger larvae consume more kernels and cause most of the damage. In fact, the last two larval stages cause about 80 percent of the damage. Frass can be commonly found in infested grain heads, on the tops of the upper leaves, and on the ground under the plants. Feeding damage under wet and humid conditions may cause mold problems with the grain.
Natural mortality of both species is very high. Adult moths can lay several hundred eggs on sorghum grain heads before or during flowering, but only a few larvae survive. Natural factors suppressing these insects include predators, parasites, pathogens, and cannibalism among the larvae.
Why are populations so heavy this year? We usually see more headworms following insecticide applications to control sorghum midge. This is because the insecticides used to control the midge also destroy most of the beneficial arthropods on the heads.
What is puzzling about this year is that there have been very few insecticide applications to control midge this year. Based on reports from the southern parts of Texas, we are experiencing larger-than-normal populations of caterpillars this year in both cotton and grain sorghum.
We suspect the cooler, wet weather has been favorable for moth emergence and survival and these moths have laid an unusually high number of eggs in the area grain sorghum crop. Even though we had beneficials “in place,” there were probably too many eggs laid at one time for the beneficials to handle.
How do I sample my crop to determine if I need to spray?
Begin inspecting sorghum grain heads soon after flowering and continue at five-day intervals until the kernels are in the hard dough stage. To examine grain heads for larvae, shake randomly selected grain heads into a 5 gallon bucket, where the larvae can be seen and easily counted. This “beat bucket” technique permits detection of even the smaller larvae commonly overlooked during the visual inspection of the grain head.
Inspect at least 30 grain heads from a field to ensure reasonable reliability of sample size. The economic injury level is about one to two larvae per head.
Which insecticides are suggested to control this pest?
There are a number of insecticides that are labeled to control these pests.
These materials will generally do an acceptable job of controlling both corn earworm and fall armyworm larvae. The most economical treatments are probably the pyrethroid insecticides.
Since the populations we have seen consist of small, medium, and large worms, we suggest that growers use a “mid rate” of an insecticide rather than the lowest labeled rate.
Be sure to read the label of your chosen material to be certain it is labeled for grain sorghum before applying it.