ENID, Okla. —
Army worm infestations are being observed in some parts of northwest Oklahoma.
This caterpillar will measure about 11⁄4 inches in length when mature and has a dark brown to grayish body with two pale yellow bands extending down the back. The early signs of an infestation include leaves with ragged margins that have been chewed on. You may also find frass, i.e. the excrement from the army worm caterpillars, around the base of the wheat plant. First observations show up around waterways, areas of lush growth or areas with lodged wheat plants. These should be checked first to determine the size of the infestations in your fields.
Yield loss from army worm feeding can occur in two ways. First, they can feed on the flag leaf and the awns, which causes physiological yield loss because the grain head cannot fully mature and usually will shrivel the grain. These worms also can cause direct yield loss by clipping the heads off the wheat stems.
Fortunately, head clipping is rare, but has occurred in northwest Oklahoma. Head clipping I normally would see if we have heavy enough infestations is limited to secondary tillers with small, green heads that likely would not contribute much to yield.
In general, if wheat is past the soft dough stage, control of caterpillars is not warranted unless obvious head clipping can be seen and the caterpillars still are present and continuing to feed.
Worms feeding on the awns when plants are past the soft dough stage of growth will not cause enough yield loss to justify the expense of a plant protection application.
To scout for army worms, select several locations and search the ground and plant material for army worms. The caterpillars tend to feed at night, so a good strategy is to bring a flashlight and look at the fields after dusk when they are feeding.
Army worms have a number of natural enemies that help keep populations in check, if given a chance. In particular, parasitic wasps and flies will attach to them. Parasitized army worms often can be recognized by the presence of small white eggs attached behind the caterpillar’s neck.
The eggs are about the size of a period at the end of each sentence in this article.
The suggested thresholds for army worms are four to five unparasitized caterpillars per foot of row. If control is needed, there are a number of products registered for use. A producer must consider at this time the harvest restriction period that must be observed prior to harvest.
Your Oklahoma Cooperative Extension Service ag educator, one of which is located in each county in northwest Oklahoma, has OCES Extension Fact Sheet CR-7194, “Management of Insects and Mite Pests in Small Grains,” available for you.
Gribble is Oklahoma Cooperative Extension Service northwest area agronomist.