After overwintering, bean leaf beetle adults emerge in the spring and they can cause serious defoliation injury to seedling soybean plants (Figure 1). However, the populations of the overwintering bean leaf beetle are negatively affected by cold air temperatures, and an estimate of the emerging populations can be made based on how cold the winter was.
Determining mortality rates
When the air temperature drops below 14° F (-10° C) bean leaf beetles freeze and subsequently die. They can survive cold temperatures under plant debris and loose soil, especially in wooded areas, but 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 overwintering bean leaf beetle populations last winter in Eastern South Dakota ranged from 66-95% (Figure 2).
Figure 2. Predicted bean leaf beetle mortality in South Dakota for the 2015-2016 winter. Climate data provided by the South Dakota Mesonet.
Scouting for overwintering adults
For the 2016 growing season, soybean farmers in areas with decreased bean leaf beetle mortality should focus on early season scouting for emerging overwintering adults. Scouting should begin at soybean emergence and continue through the early vegetative stages. This is especially true for soybeans that are planted early, and/or have had past issues with bean leaf beetle defoliation or Bean pod mottle virus transmission.