Figure 1. Adult, overwintering bean leaf beetle feeding on a soybean seedling.
Overwintering bean leaf beetle adults will soon start emerging as they do each spring. This overwintering generation is capable of causing serious defoliation injury to seedling soybean plants (Figure 1). However, the population density of overwintering bean leaf beetles is negatively affected when winter air temperatures get too cold. Because of this, an estimate of the emerging populations can be made 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 during such temperatures. Using this model, the predicted mortality rates for the 2017-2018 overwintering bean leaf beetle populations in Eastern South Dakota appeared to be 100% at every location (Figure 2). This is a dramatic increase from the 2015-2016 winter where the mortality ranged from 66-95% and 2016-2017 winter where the mortality ranged from 73-100%. Although mortality is expected to be high, there is still the potential for bean leaf beetles to emerge if they had adequate 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.
Figure 2. Predicted bean leaf beetle mortality in South Dakota based on temperatures from the 2017-2018 winter.
Scouting & Management
Based on 2017 bean leaf beetle populations, farmers in Southeast South Dakota should scout early in the 2018 season as it is likely that some populations will be observed there again. 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 effectively managed using insecticide seed treatments or foliar insecticides.