As the growing season for soybeans gets going, it is useful to review the risk posed by bean leaf beetle, one of the crop’s primary early-season pests.
Tooker reports that 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 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 1 foot or more, or beetles have defoliated at least one seedling per foot of row.
Adult bean leaf beetles vary in color, ranging 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.
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 soybeans emerge, 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.