Scout early emerging soybeans for bean leaf beetles
Bean leaf beetle Accurately predicting spring bean leaf beetle infestations in Nebraska is difficult. You would think that the uncommonly warm winter we had would favor bean leaf beetle survival; however, persistent snow cover and moderate winter temperatures are most favorable for beetle survival.
Extremely warm, open winters can cause beetles to become active prematurely, resulting in increased metabolism and reduction of fat reserves, starvation, desiccation, or simply exposure to inclement weather. The best thing to do when we have very warm winters is to be vigilant and expect the unexpected.
Bean Leaf Beetle Biology
Bean leaf beetles have two generations a year in Nebraska. Because they overwinter as adults, three periods of beetle activity are seen in the growing season:
- Overwintering colonizers,
- F1 generation (offspring of the colonizers, the true first generation), and
- F2 generation (the generation that will overwinter).
Bean leaf beetles overwinter as adults in leaf litter (woodlots) and soybean residue. They become active fairly early in the year (April-May), and often can be found in alfalfa prior to soybean emergence. As soybeans emerge, the beetles quickly move to the seedling plants, feeding on cotyledons and expanding leaf tissue. These overwintered beetles, called colonizers, mate and begin laying eggs. Females live about 40 days and lay from 125 to 250 eggs. After egg-laying is complete, the colonizing population dwindles as the beetles die. A new generation of beetles (F1) will begin to emerge in late June to early July. The F1 beetles mate and produce a second generation of beetles (F2) that begin to emerge in mid- to late August.
Bean leaf beetles vary in color, but are usually reddish- to yellowish-tan. They are about ¼ inch long and commonly have two black spots and a black border on the outside of each wing cover. These spots may be missing, but in all cases there is a small black triangle at the base of the wings near the thorax.
Because they move to soybean fields so soon after seedling emergence, early-planted fields will usually have more beetles and suffer the most injury, particularly if they are the only beans up and available for the beetles to move into. Although the defoliation the beetles cause can appear quite severe, research in Nebraska and elsewhere has shown that it usually does not result in economic damage. Soybean plants can compensate for a large amount of early tissue loss, so it takes a considerable amount of beetle feeding to impact yield. Generally, soybeans planted during the normal soybean planting window in Nebraska are not colonized by enough beetles to cause economic injury.