Insect Pests to Keep in Mind: Black Cutworm, Cereal Leaf Beetle, and True Armyworm

Posted: May 24, 2017

I have received a good number of phone calls from growers experiencing the usual suspects for this time of year: black cutworm, cereal leaf beetle, and slugs. And we would be foolish to ignore the threat from armyworm. Below, I review each of these pests and encourage folks to keep these pests in mind and plan their scouting around them.
Figure 1. Cereal leaf beetle larvae feeding on wheat, showing their damage and slimy appearance (Photo by Mike Fournier, Penn State Extension).

Figure 1. Cereal leaf beetle larvae feeding on wheat, showing their damage and slimy appearance (Photo by Mike Fournier, Penn State Extension).

We have been accumulating growing degree days to predict black cutworm cutting damage, and we are just starting to hear from a few farmers that have found cut plants in their corn fields. Perhaps you will recall that a few weeks ago now our Black Cutworm Monitoring Network detected “significant flights” of black cutworm moths in four locations: near Fayetteville (Franklin County), near Montoursville (Lycoming County), near Manheim (Lancaster County), and North Cornwall (Lebanon County).  Significant flights occur when we capture in an individual trap eight moths over the course of two nights, we can then predict that a significant population may develop and that cutting larvae will be active in that area in approximately 300 degree days. As of this morning, degree-day accumulations have exceeded 300 degree days for Fayetteville and Manheim, and are at 213 and 232 for Cornwall and Montoursville, respectively. I would recommend growers in the southern counties of PA to scout their corn fields now for cutting damage, and growers in the central part of the state to consider scouting for damage in the coming days. Remember that well-timed scouting and spot rescue treatments are usually the most economical strategy for managing black cutworm. For more details on black cutworm, its biology and management options see our fact sheet.

Cereal leaf beetle

The larvae are also active around the state, so it would be prudent to scout wheat and oat fields. This pest species is occasionally mistaken for slugs, but is easy to identify once you notice them (Figure 1). Larvae are most difficult to control when feeding on flag leaves, and are more easily controlled when infestations are detected early. As is often the case, outbreaks tend to occur in fields that are not scouted regularly. Insecticide treatments are warranted if populations exceed the economic threshold of one larva over 0.13 inches long per stem over a field or a portion of a field. Our fact sheet provides more details on their lifecycle and some images of adults, larvae, and their damage. For insecticide options, please consult Penn State’s Agronomy Guide.


The cool spring has made good conditions for slugs, and we are aware of corn and soybean fields suffering heavily from slug damage. At this point in the season only a few management tactics hold potential, and true relief will come in the form of warm, sunny weather that can dry out fields. If you have portions of fields that are heavily damaged, metaldehyde-based baits can be useful, but these baits can be ineffective at times, particularly when rain keeps coming (the rain will wash away much of the bait). Some growers have had luck spraying nitrogen solutions for slugs, but this tactic will burn plants and folks considering this idea should talk with others that have tried this approach.

Our research indicates that managing slugs takes an integrated approach that should be planned well before spring planting. The most diverse rotations tend to experience the fewest slugs, and cover crops can be part of the solution to increasing rotational diversity. Many farmers believe that cover crops tend to be part of the problem, but our research indicates that cover crops can be helpful in the fight against slugs. Other helpful tactics can include planting crops at appropriate soil temperatures and ensuring good furrow closure. Our research is also showing that strong populations of ground beetles can help suppress slug populations. These beetles can be suppressed by insecticide use, including seeds treated with neonicotinoid insecticides, so consider avoiding treated seed if you have fields that are perennially damaged by slugs. Our fact sheet on slugs describes scouting and management options.


Lastly, portions of the Midwest experienced strong flights of true armyworm, and our colleagues trapped a good number of armyworm moths at Penn State’s research farm in Centre County. To this point, I have not heard of any infestations of true armyworm, but they are an annual threat. Most true armyworm problems occur in corn, hay, and small grains. In corn, the damage is often associated with stands that were preceded by a rye cover crop. In hay fields, armyworm can be an unpleasant surprise. Of course, hay fields are not typically scouted, so populations can build slowly in these fields quietly and then start marching to adjacent fields. In wheat and other small grains, their damage using begins as leaf feeding and then progresses to head clipping in wheat. Remember that armyworms are nocturnal and will hide during the day. In corn, they will hide in the whorl, where their frass (i.e., feces) accumulates, providing a good indication of their presence. Armyworm feeding rarely crosses the midvein, so this sort of damage can be diagnostic as well. For more information, see our fact sheet.

Contact Information

John Tooker
  • Extension Specialist
Phone: 814-865-7082