Although bean leaf beetles won’t be emerging for a while yet, it is important to be prepared for potential early season defoliation. Once soybean are planted and begin emerging, the overwintering population of bean leaf beetle adults will move into soybean fields and begin feeding on the seedlings. This overwintering generation is capable of causing serious defoliation injury, especially when large populations successfully overwinter (Figure 1). However, the population density of overwintering bean leaf beetles is negatively affected when winter air temperatures get too cold. This allows us to estimate the emerging populations based on how cold the winter was. The first step in calculating bean leaf beetle mortality is recording the air temperatures that drop below 14 °F (-10 °C). At such temperatures, bean leaf beetle adults freeze and subsequently die. Although the adults can survive cold temperatures by hiding under plant debris and loose soil, especially in wooded areas, they cannot survive sustained sub-zero temperatures.
Lam and Pedigo (2000) developed a survival model for the bean leaf beetle to predict overwintering population mortality. This model functions by accounting for the accumulation of sub-freezing temperatures (14 °F or colder), and the known response of overwintering bean leaf beetles in both exposed (under crop residue in a field) and protected (residue in wooded areas) environments to such temperatures. Using this model, the predicted mortality rates for the 2019-2020 overwintering bean leaf beetle populations in Eastern South Dakota appeared to range from 61-100% (Figure 2). This is a change from the 2018-2019 predictions where mortality was 100% for all of the South Dakota locations. Although mortality was high at several locations, there is still the potential for bean leaf beetles to emerge in those areas if they had adequate snow cover during the cold temperatures. Areas that had snow cover during weeks with sub-zero temperatures still have a likelihood of experiencing early season bean leaf beetle populations. In areas where the population mortality was below 100%, there is the potential for large populations of overwintering bean leaf beetles, especially in areas where snow cover was present through periods of sub-zero temperatures.
Scouting and Management
Based on the 2020 bean leaf beetle populations, farmers in Southeast South Dakota and the Highmore area should scout early in the season for bean leaf beetle activity. Early season bean leaf beetle scouting should begin at soybean emergence and continue through the early vegetative stages. The overwintering generation of bean leaf beetles can be managed effectively using insecticide seed treatments or foliar insecticides. View the 2020 South Dakota Pest Management Guide - Soybeans for a list of seed treatments and foliar insecticides that are labeled for bean leaf beetle management in South Dakota.