Watch for Black Cutworm, Bean Leaf Beetle, and Cereal Leaf Beetle

This week, three pest species that deserve attention during scouting efforts: black cutworm, bean leaf beetle and cereal leaf beetle

Black Cutworm

Earlier in spring, our Black Cutworm Monitoring Network identified an elevated risk from black cutworms in the vicinity of Franklin County, and about two weeks ago I recommended that folks in Franklin and adjacent counties scout for cutting damage from caterpillars in their corn fields. Beyond that part of the state, if you are growing corn, I recommend getting out into your fields to look for cutting damage. Finding this cutting damage and applying rescue treatments if necessary is the most effective and economical way to control black cutworm populations. Continue scouting fields until corn plants reach the V5 growth stage, when cutworm damage is rare. See the Black Cutworm article for more details.

Bean Leaf Beetle

As soybeans emerge from the ground, it is useful to review the risk posed by the bean leaf beetle, one of the primary early-season pests of soybeans. Bean leaf beetle is active now in central and southern parts of the state and can be found feeding on cotyledons and the first leaves of soybean plants, particularly in the earliest emerging soybean fields in an area. Soybeans grown from seeds coated with neonicotinoid insecticides should be protected against bean leaf beetle, but early-season feeding damage (Figure 1) often looks worse than it is, and yield loss from early-season feeding is rare. Remember that soybeans are very good at tolerating damage, so hold off on that urge to spray unless the plants are looking horrible or appear to be dying. One of the published economic thresholds for bean leaf beetles recommends treating young soybeans if 20% of plants are defoliated, beetles cause gaps in rows of one foot or more, or beetles defoliate at least one seedling per foot of row.

Adult bean leaf beetles vary in color, but can range from yellowish-brown to green to red, and they usually have four black spots on their wing covers. They always have a black triangle just behind their thorax (Figure 2). They overwinter in leaf litter and become active in April, when they tend to move into alfalfa or other legumes to feed and begin mating. When soybean emerges, adults move into these fields, feeding on young seedlings (V1- or V2-stage) and laying eggs. Larvae then feed on soybean roots, but this feeding does not appear to be economically important. These larvae will develop into the second generation of adults, which emerge in midsummer and feed upon leaves and occasionally pods.

Scout your soybean fields to know whether these pests are active. There is usually no need to control them, but walk your fields to be sure.

Cereal Leaf Beetle

We have heard reports from south central and southeastern regions of Pennsylvania that cereal leaf beetle larvae are active in wheat and barley fields (oats and rye can also be infested but less frequently). Cereal leaf beetle infestations are difficult to predict and can be very patchy with some fields heavily infested, while others nearby have almost none. Scouting is the key to identifying local populations and populations that are large enough to be managed. If you scout fields and find populations that exceed the economic threshold (one larvae per flag leaf), then using an insecticide is likely to be profitable. If you do not have populations, then an insecticide application:

• is unlikely to be helpful,
• may do more harm than good by suppressing beneficial insect populations, and
• is likely to waste money on a product that you do not need.

Similarly, I would strongly recommend avoiding the practice of adding an insecticide to a tank of fungicide, for example, simply because a sprayer is making a pass across the field; unless economically damaging populations of insects are present, there is no benefit to be gained.

Older cereal leaf beetle larvae can be mistaken for slugs because they can look dark and shiny; they cover themselves with their own frass, which is the proper word for insect feces. If you look closely at these shiny animals and see six small legs, you will know you are dealing with cereal leaf beetle larvae and not slugs (Figure 3). Moreover, slugs are nocturnal, so slimy creatures active during the day in small grains have a good chance of being cereal leaf beetle larvae. If young larvae, which are orange, are detected at populations exceeding the economic threshold, cereal leaf beetle is easily controlled with insecticides, but larger larvae are more difficult to control and cause more damage, particularly if they are feeding on the flag leaf. Insecticide treatments are warranted if populations exceed the economic threshold of one larva over an eighth-of-an-inch long per stem over a field or a portion of a field. The Cereal Leaf Beetle article 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.