Watch for Black Cutworm and Bean Leaf Beetle

This week, two pest species deserve attention: black cutworm and bean leaf beetle.

Black Cutworm

Earlier in spring, our Black Cutworm Monitoring Network identified elevated risks from black cutworms, and in previous weeks, I recommended that folks in Lebanon and Lancaster Counties scout for cutting damage from caterpillars in their corn fields.  Now, degree-day accumulations suggest that this weekend, corn growers in Bedford County should scout for cutting damage from black cutworm caterpillars. So, if you are growing corn in south central PA, 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, which rarely receives cutworm damage. See our fact sheet on black cutworm 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.