Expect lots of bean leaf beetles in 2000

If you read the bean leaf beetle prediction in the May 1 ICM newsletter, you'll know that winter mortality of this soybean pest is expected to be at a 12-year low. The result will be that we should have more bean leaf beetles this spring than during an average year. In fact, our predictions, based on summer population increases during the last several years coupled with the mild winter, are for the populations to be very large. Sweep samples taken from alfalfa fields in Ames on May 2 indicate that the beetles are extremely abundant; they should move to soybeans as soon as the plants germinate.

Bean leaf beetles look much like black-spotted yellow, brown, or brick red lady beetles. Unlike lady beetles, bean leaf beetles have a prominent black triangle at the front of the wing covers that points backward and most, but not all, have a series of rectangular spots on the wing covers. Adults occur in two color phases, red and yellow. Bean leaf beetles have the noticeable habit of abruptly dropping to the ground and scurrying into cracks and crevices when disturbed. Crop scouts need to approach infested plants carefully and watch for this behavior to estimate population size effectively.

The yellow phase of the bean leaf beetle is the most common color variety.
Some bean leaf beetles are red instead of yellow.

Bean leaf beetles are commonly found on alfalfa after emerging from their winter habitat. The beetles quickly move to soybeans; sometimes as soon as the bean plants crack the ground. This colonization of soybean is typical in Iowa, but because the overwintering bean leaf beetle population is usually low, beetles often are not obvious on young soybean plants. Bean leaf beetles feed on young, new tissue and can quickly cause noticeable defoliation on seedlings. But noticeable defoliation and economically significant defoliation are different. Early-season damage does not gain economic significance until cotyledons are lost and regrowth is suppressed by feeding activity.

Populations will probably be larger in 2000 than last year's populations. However, this does not mean that early-season economic damage will result. Economic damage requires huge populations of feeding adults, and treatments are not often justified. Table 1 shows the number of beetles per plant (or foot of row) needed to justify insecticide treatment (Table 2). Three or more beetles per plant rarely are documented but may be possible in local areas. Because of the high initial populations, late-season pod feeding may be a concern, but several environmental factors will play a role in the actual population density present by midsummer.

Table 1. Early-season bean leaf beetle economic thresholds in soybeans (beetles per plant).a

Cost of Treatment ($/Acre)

Value ($/bu)
Growth Stage VC Growth Stage V1 Growth Stage V2
$6 $7 $8 $9 $10 $11 $6 $7 $8 $9 $10 $11 $6 $7 $8 $9 $10 $11
$5.00 2.4 2.8 3.2 3.6 4.0 4.4 3.7 4.4 5.0 5.6 6.2 6.8 5.9 6.8 7.8 8.8 9.8 10.7
$6.00 2.0 2.3 2.7 3.0 3.4 3.7 3.1 3.6 4.1 4.7 5.2 5.7 4.9 5.7 6.5 7.3 8.1 8.9

a For beetles per row-foot, multiply number by 7.6.

Table 2. Insecticides labeled for bean leaf beetle in soybeans.

Insecticide Amount/Acre Harvest

Interval (days)
Ambush 2EC* 3.2-6.4 ounces 60
Asana XL* 4.8-9.6 ounces 21
Lorsban 4E 1-2 pints 28
Penncap-M* 2-3 pints 20
Pounce 3.2EC* 2-4 ounces 60
Sevin XLR Plus 1-2 pints 0
Warrior T* 1.92-3.2 ounces 45

* Restricted use insecticide.

This article originally appeared on pages 71-72 of the IC-484 (9) -- May 15, 2000 issue.

Updated 05/14/2000 - 1:00pm