By Glenn Studebaker, Extension Entomologist
We have quite a bit of conventional field corn in Arkansas this year and have been getting questions on when to spray for fall armyworm (Fig. 1), corn earworm (Fig. 2), and southwestern corn borer (Fig. 3). First, I will address corn earworm and fall armyworm.
Treatment is warranted for these two pests when we find 3 to 6 larvae per plant during the whorl stage. Let me repeat that. We only treat for corn earworm and/or fall armyworm during in whorl stage in corn. Currently, most, if not all, of our field corn is past this stage.
Treating for corn earworm infestations in the ear on field corn is not economical or necessary. We conducted research several years ago looking at potential yield loss from corn earworm in field corn and could not measure any significant loss in yield from corn earworm infestations in the ear (Figure 4). Therefore, it is not necessary to treat for corn earworm infestations in the ear, no matter how effective the insecticide may be.
Southwestern corn borers are a different animal. They bore into the stem and can cause significant yield loss when infestations reach a level of 25% of the plants infested. Because larvae bore into the stem, they must be controlled before entering the stem. It is usually the second generation corn borers that can cause yield loss. In Arkansas, this generation usually shows up around the end of June or first of July. We have been monitoring fields across the state with pheromone traps and have not seen high numbers yet, but this could change in the coming week. Some fields I have looked at in Northeast Arkansas have between 5-10% infestation levels of southwestern corn borer (first generation) at this time. Many of the larvae in the stem are in the later instars and will be emerging soon, probably around the first week of July in North Arkansas. With corn borers, insecticides need to go out before young larva bore into the stem, so applying something a little early is better than being too late. Insecticides with a long residual such as Prevathon, Besiege, Belt or Intrepid would be good choices. There has also been some confusion on identification of southwestern corn borer larva compared to corn earworm and fall armyworm. Southwestern corn borers are cream colored with dark spots up and down the body. Falls are generally a light brown color while corn earworm vary in color from brownish to green with hairs scattered across the body.