Bean leaf beetles return--with a vengeance

We started detecting bean leaf beetles on April 19 this year as part of our annual bean leaf beetle monitoring program in central Iowa. As noted last week, bean leaf beetle mortality was predicted to be low, and based on last year's low numbers, we expected a slight increase in their numbers this year. However, we were surprised by the numbers we have found this past week!

We are finding about 340 bean leaf beetles per 50 sweeps in alfalfa. This average is only 84 beetles less than the early-season high (in alfalfa) from 2002 (our most abundant year to date). With these numbers, some growers may exceed the early-season economic threshold for bean leaf beetle injury (see tables). Palle Pedersen, extension soybean specialist, reported a 400-acre field near Grand Junction that averaged 3 beetles per seedling plant, so large populations can be found in Iowa this spring.

Bean leaf beetle population, May 2006
Bean leaf beetle sweeps show high early population levels.

So what can farmers do now? Follow our current recommendations (see accompanying flowchart (PDF)) for soybean management and choose the approach that best fits the end use of the soybeans (see graph).

Delayed planting

Planting from this date on could be considered as delayed, but planting in mid to late May still could yield near optimum yields without the beetle pressure. With high beetle pressure, late April and early May planting dates are at risk for sustaining large bean leaf beetle populations, pod damage, and poor seed quality.

Consider an early-season insecticide

For managing bean pod mottle virus, which is the pathogen transmitted by bean leaf beetles, our studies indicate that an early-season, foliar insecticide is critical for suppressing virus incidence under high bean leaf beetle pressure. It is important to apply this insecticide as soon as beetles are present in your soybean field.

Consider a mid-season insecticide

Additionally, our studies indicate that although a mid-season insecticide by itself can not prevent an increase in bean pod mottle virus, it seems necessary to improve seed quality. Furthermore, if combined with an early-season insecticide, the two treatments give an added positive effect on yield and seed quality.

Scout 1st generation beetles

Finally, continue to scout your field 1 week following the predicted 1st generation emergence (1212 degree days, base 46 °F) to determine if your field is at risk for 2nd generation beetle damage. More information will be published in Integrated Crop Management later this summer regarding these management recommendations. If you are already following a virus-management plan, bean leaf beetles may not rebound in your field and a third insecticide may not be necessary; however, scouting would be good insurance.

We will continue to keep you informed regarding the progress of the bean leaf beetle population this summer.

Table 1. Economic threshold of overwintered bean leaf beetles in early-stage soybean (beetles per plant). This chart does NOT consider the impact of bean pod mottle virus on soybean quality and yield.
Market Value
Growth Stage/Cost of Treatment ($/Acre)
VC V1 V2
6 8 10 6 8 10 6 8 10
5.00 2.4 3.2 4.0 3.7 5.0 6.2 5.9 7.8 9.8
6.00 2.0 2.7 3.4 3.1 4.1 5.2 4.9 6.5 8.1
Table 2. Economic threshold of 1st generation bean leaf beetles in soybean (beetles per 20 sweeps). This chart does NOT consider the impact of bean pod mottle virus on soybean quality and yield.
Market Value
Cost of Treatment ($/Acre)
7 8 10 12 15
5.00 23.0 26.2 32.6 39.0 48.6
6.00 19.3 22.0 27.3 32.6 40.6
8.00 14.6 16.6 20.6 24.6 30.6
10.00 11.8 13.4 16.6 19.8 24.6
13.00 9.2 10.5 12.9 15.4 19.1

This article originally appeared on pages 120-121 of the IC-496(11) -- May 15, 2006 issue.

Updated 11/09/2006 - 10:04am