Integrated Crop Management

Bean leaf beetle mortality in winter: 2003 vs. 2004

Bean leaf beetle populations have shot upward during the last several years and reached an historical high during 2002, but declined during the summer of 2003. These large populations have been partially due to favorable winter conditions, such as mild temperatures or snow cover in previous winters.

Bean leaf beetles feeding on early soybean, including the cotyledon.

The second generation of bean leaf beetles hibernate mostly in Iowa woodlands. About 80 percent of the adults overwinter here while the remainder hides in crop residue, alfalfa fields, and grassy areas. Beetles begin to emerge in late April and early May. Often, they first fly to alfalfa or wild hosts (such as trefoil or clover) before flying into soybean fields.

Research at Iowa State University has shown that low winter temperatures have a great impact on the survival of overwintering bean leaf beetles. An estimate of the mortality rates (%) of overwintering populations can be predicted by accumulating the daily average subfreezing temperatures (those below 32°F), from October 1 to April 15.

The map shows the estimated beetle mortality in Iowa during the 2003 and the 2004 winters. Mortality in crop reporting districts (CDRs) 1 and 2 was very high in 2004, although similar to 2003. Last winter, however, the percent mortality was lower in all other districts, indicating greater beetle survival compared to 2003. This suggests that soybean producers, especially those in central and southern Iowa, should expect to find bean leaf beetles in their fields. The earliest emerging fields, especially the first ones to emerge in an area, typically will attract the highest beetle populations.

Bean leaf beetle map

Estimated mortality (%) of overwintering bean leaf beetles from nine crop reporting districts in Iowa (2004 compared to 2003).

This article originally appeared on pages 29-30 of the IC-492 (6) -- May 3, 2004 issue.

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