In 2002, bean leaf beetle populations in Iowa reached their highest levels in 14 years (figure, left). In Iowa, this increase in beetle populations has been partly fueled by weather conditions that favor winter survival, such as mild temperatures (2001-2002: second mildest winter on record) or snow cover (2000-2001: snow cover for 99 consecutive days in central Iowa). The increase in beetle populations has followed the trend for warmer weather during the previous six winters (figure, right). High numbers of bean leaf beetles have provoked concern about direct losses from beetles plus losses related to the disease they transmit: bean pod mottle virus.
|The yellow phase of the bean leaf beetle is the most common color variety.
The bean leaf beetle normally has three populations throughout the year: overwintered, first generation, and second generation. The abundance of the previous beetle generation and environmental factors can influence the timing and duration of the following generation. For bean pod mottle virus, the most important times are during the emergence of overwintered and first-generation beetles. These early- (May) and mid-season (late June and July) populations cause the initial introduction and spread, respectively, of this pathogen into soybean fields. The second generation of beetles is not important for transmission of bean pod mottle virus; however, these beetles will cause feeding injury to soybean pods. Both bean pod mottle virus and pod feeding cause economic damage to soybean yield and seed quality.
In 2002, bean leaf beetle numbers were high throughout most of Iowa; however, depending on your location, you may have seen fewer second-generation beetles than expected. Based on research trials that we conducted last summer, we observed three different patterns of beetle emergence near Ames, Nashua, and Sutherland (figure, right). At Nashua, the beetle populations seemed to be influenced by mid-season rainfall. For example, 12.9 inches of rainfall was recorded near Nashua from July 10 through August 24. This heavy rainfall probably reduced the second-generation in that area because the larvae may have drowned in the rain-soaked soils. However, populations remained well over economic threshold for late-season injury.
Next month, we will present information on how the current winter may have affected the overwintering bean leaf beetle populations.
This article originally appeared on pages 20-21 of the IC-490 (3) -- March 17, 2003 issue.