The bean leaf beetle overwinters as an adult that feeds on young soybeans in May and June. They lay eggs that produce the first generation, which attacks beans in July. The subsequent generation, the second, is of most concern when protecting the plant from the beetles feeding because it attacks the pods and beans as well as foliage. That explanation comes from Jon Tollefson and Marlin Rice, Iowa State University Extension entomologists.
The first generation adult bean leaf beetles began emerging from the soil during week of July 7 in central Iowa. To determine if the second generation will reach economic numbers and to avoid injury to beans as the populations increase, the first generation can be sampled now.
Now is time to scout for first generation
If number of beetles present exceeds the economic threshold, the farmer should be ready to treat the field when second generation begins to emerge in late August from the soil and the presence of new adults, which are soft and gray in the field is confirmed. The scouting methods explained by the ISU specialists sample the first generation, which is occurring now, to predict the size of the second generation, which will occur in late August and September. For photos of this pest, along with economic thresholds to help you decide when to spray, and tips on how to scout are on ISU's Crop News Web site at www.extension.iastate.edu/CropNews/2008/bean+leaf+beetles+coming.htm. The treatment thresholds are for prevention of economic injury to the beans by the adult bean leaf beetle. They do not include prevention of disease.