Bean leaf beetles have been at or near historically high populations during each of the last two summers. This has resulted in more plant damage, transmission of the bean pod mottle virus, and reduced yields. Winter survival of this insect pest may be important management information to consider because the number of insects surviving the winter is closely related to problems in soybeans.
Overall average mortality for bean leaf beetles in Iowa was 48 percent.
The bean leaf beetle has two generations a year in Iowa. Approximately 80 percent of the second generation adults (these are the beetles that fed on pods last fall) overwinter in leaf litter of woodlands. The other 20 percent hibernate in crop residue, alfalfa fields, and grassy areas. Beetles begin to emerge during late April and early May. Often they will first migrate to alfalfa or wild hosts. When soybeans emerge, many of them will fly to the seedling soybean plants.
Low winter temperatures have a great impact on the survival of overwintering bean leaf beetle populations. During the winters of 1996-1999, Frankie Lam at Iowa State University conducted a study on winter survival of bean leaf beetle in central Iowa. He found that he could predict the mortality of overwintering populations by accumulating the daily average subfreezing temperatures through the winter. The accumulated daily average subfreezing temperature can be obtained by using the daily average temperature (°F) minus 32 and accumulating only those temperatures that are negative through the winter. For example, in October 2000, after each daily average temperature minus 32, negative temperatures were obtained on 2 days; one was -10 and the other was -20. Thus, the accumulated daily average subfreezing temperature for October was -30. By accumulating the daily average subfreezing temperature from October 1-April 15 of the following year, the percentage of beetle mortality can be estimated.
The map shows estimated beetle mortality in the 9 crop reporting districts of Iowa during the winter of 2001-2002. The statewide average mortality is predicted to be 48 percent, suggesting that more than half of the beetles will survive the winter. During the past 13 years, the average beetle mortality in central Iowa was 71 percent (ranging from 41-95 percent). Last fall, bean leaf beetles were documented at their second highest population size ever in central Iowa. This factor, coupled with the high winter survival of beetles, strongly suggests that bean leaf beetles should be present in large numbers in soybean fields this spring throughout the state.
This article originally appeared on page 54 of the IC-488 (6) -- April 29, 2002 issue.