By Robert Koch and Bruce Potter
We are finding bean leaf beetles colonizing fields and feeding on soybean leaves. Bean leaf beetles had been uncommon in Minnesota soybean since the mid-2000s. However, over the last couple years, we’ve seen an increase in the abundance of this insect in western Minnesota, with some fields reaching economically significant infestations. While beetles are present in 2022, it is still unclear what impact winter might have had on bean leaf beetle populations. Here we provide an overview of the biology, scouting and management of this pest.
Bean leaf beetles are about ¼-inch long and may be yellow, tan, orange or red in color. The beetle’s backs usually have black-rimmed elytra (hardened front wings) with four rectangular black spots, and a black triangle at the base of the wings. The number of spots can vary, but the black triangle at the base of the wings is always present.
Bean leaf beetles spend the winter under leaf litter in wooded areas or field margins. As spring temperatures warm into the 50s Fahrenheit, the overwintered beetles become active, mate, and feed on alfalfa and clovers, moving to soybean seedlings as the plants emerge. Bean leaf beetle feeding injury to soybean leaves is distinctive, consisting of small rounded holes. The eggs are laid in the soil near soybeans plants, hatching into small white larvae. The larvae are similar to corn rootworm larvae and feed on soybean roots and nodules of the plants, but this feeding by larvae is assumed to have minimal if any impact on yield. The larvae will eventually pupate in the soil and the next generation of adults will emerge to feed on the leaves and pods of these plants. Feeding on the surface of soybean pods can injure seeds and allow the entry of bacterial and fungal pathogens. In Minnesota, there is typically only one generation of bean leaf beetles per year. However, with warm temperatures and early planting, two generations are possible in southern Minnesota.
For soybean, the earliest planted fields are often most attractive to this pest. Neonicotinoid seed treatments usually provide good control of early season infestations of bean leaf beetle. Soybean plants are very tolerant to defoliation from adult bean leaf beetles, meaning plants can sustain considerable levels of loss of leaf area without impacting soybean yield. Treatment decisions for bean leaf beetles on early vegetative stage soybean can be based on a threshold of 30% defoliation (with the pest still present in the field). Thresholds based on counts of bean leaf beetle are also available, but the mobility and skittish nature of these beetles can make it difficult to get accurate counts. Bean leaf beetle can also vector bean pod mottle virus (BPMV). More information on BPMV can be found in the factsheet, Bean Pod Mottle Virus on Soybean. This disease is particularly important in seed production fields where bean leaf beetle may require more aggressive management.
Scouting for bean leaf beetle should continue through the growing season, because of feeding from the subsequent generation/s. Particular attention should be paid to early infested fields because populations may build over the season in those fields resulting in significant defoliation and pod feeding. In later growth stages of soybean, decisions to apply foliar insecticides can be based on levels of defoliation. The recommended thresholds for defoliation consider the combined feeding from bean leaf beetles and other defoliating insects such as caterpillars, grasshoppers, etc. For soybean prior to flowering a conservative threshold for treatment is 30% defoliation (with the pests still present in the field). For soybean at or after flowering this threshold decreases to 20% defoliation. Estimates of defoliation should include plants from multiple locations in the field and leaves from the top, middle and bottom of each plant to provide an overall estimate of canopy-level defoliation. Estimating defoliation can be deceptive with many people tending to overestimate the percent of leaf area removed. To calibrate your eye for estimating defoliation, tools like Disease Severity and Insect Defoliation Training can be used. If bean leaf beetle are feeding on pods, consider treating if pod injury reaches 10%. If the beetle begin clipping pods from the plants, the infestation should be treated more aggressively.
Source : umn.edu