True armyworm (Pseudaletia unipuncta) overwinters in the southern U.S. and adult moths migrate northward in April and May. Females lay eggs in grassy fields including rye cover crops, and the young caterpillars feed there, typically attacking corn from early may through June. Corn planted into rye cover is at greater risk for early season armyworm feeding because the caterpillars may already be in the field and move to the corn after the rye is killed. Armyworm can also move into corn from other fields such as wheat, in which case infestation usually occurs along field edges. Though some growers include an insecticide in their rye burndown herbicide, this prophylactic application is not recommended because in many years the armyworm populations will not be sufficient to warrant it or its cost. Foliar insecticides work well as a rescue treatment and can be applied in years when scouting indicates it will help. Corn fields planted into rye cover or into other no-till grassy habitats should be scouted beginning in early to mid May in southern Ohio and mid to late May moving further north.
Armyworms take shelter during the day in corn whorls or under debris so it can be difficult to find them. Their feeding damage is more obvious, with ragged edges that progress towards the midrib. When 15 to 20% of the stand has feeding damage the field should be re-checked within a few days to determine if defoliation is increasing. Rescue treatments in corn may be needed if stand infestation is greater than 50% and larvae are not yet mature. If defoliation remains less than 50% and the new growth shows minimal feeding injury, the stand will likely recover with minimal impact on yield. Early scouting is important because the caterpillars are easier to kill when small, and also because larvae nearing maturity have already done most of their feeding.
A number of labeled insecticides are available for armyworm (http://www.oardc.ohio-state.edu/ag/images/Corn_2013_ArW.pdf), and certain Bt trait packages are also labeled for true armyworm control (http://www.msuent.com/assets/pdf/28BtTraitTable2016.pdf).
Photo credit: James Kalisch, University of Nebraska, Bugwood.org