By Ryan Flaming, County Extension Agent, Agriculture & Natural Resources
“Headworms” are beginning to show up in some sorghum fields and Kansas State University scientist Jeff Whitworth says scouting for the worms while they are still small is important.“They’re showing up on sorghum heads in some south central and southeast Kansas fields,” said Jeff Whitworth, entomologist with K-State Research and Extension. “Sorghum headworm infestations are often not noticed until the worms are mature and fairly large. That means that most of the feeding – and thus damage – has already been done.”
Whitworth is encouraging sorghum producers to scout their fields now while the worms are small, to detect any infestations as they are starting. The most common “worms” associated with sorghum-head feeding are larvae of the corn earworm and fall armyworm, he said. Feeding damage is similar for both insects, although their biology is somewhat different. “Corn earworm moths seem to prefer to lay their eggs in corn. As the corn plants mature, however, the moths will lay eggs in sorghum or soybeans,” Whitworth said. Sorghum typically is vulnerable to that larval feeding damage from the time the crop starts blooming through its milk stage.
Early detection of earworm infestations is important, before the larvae mature and cause the most damage. Generally, the larvae can cause about a 5 percent yield loss per larva per head, he said. Because these larvae are in the head and not protected by foliage, treatment often provides good control. Still, the bases for treatment decisions should be expected yield, crop value, and treatment cost.
Fall armyworms generally migrate into Kansas in July and lay eggs on corn, sorghum, soybeans, and other crops, Whitworth said. Damage to sorghum can continue until frost. Leaf feeding may be especially evident in late-planted sorghum. Fall armyworm larvae cause rather large and thus highly visible holes while feeding in the whorls, the entomologist said. This leaf feeding is usually of little consequence, as sorghum is fairly resilient. In any case, the larvae are not vulnerable to insecticide treatment while they are protected inside the whorl. “However, head feeding by fall armyworm larvae may cause considerable yield loss,” Whitworth said. “Feeding and damage potential of fall armyworm larvae is comparable to that of the corn earworm.”
Treatment recommendations are therefore the same: Growers can expect about a 5 percent yield loss per larva per head from bloom through the milk stage. They can achieve good control, the scientist said, but should initiate treatment, if justified, while the worms are small.
While scouting for corn earworms and fall armyworms, southern Kansas growers also should be on the watch for sorghum webworms, he added. Unlike corn earworms or fall armyworms, sorghum webworms are small, striped worms and very fuzzy in appearance. They do more damage in late-planted fields from August to October, and growers should consider treating for them if a field average of five webworms per head is detected in sorghum heads that have not yet passed the milk stage.