Integrated Crop Management

Predicting first-generation bean leaf beetles

Bean leaf beetle feeding on soybean pods can lead to significant reductions in seed quality and yield. Management during the pod setting and filling stages can be frustrating because beetles may feed on pods for a couple of weeks before the population reaches the economic threshold. In this situation, some loss in seed quality and quantity occurs before an insecticide application can be economically justified. Several years ago, Larry Pedigo, a lecturer in the Department of Entomology, and his students developed research-based information to help make a management decision for second-generation bean leaf beetles based upon the population size of the first-generation bean leaf beetles.

The beetle has two generations a year in Iowa. The overwintered population (actually the second generation from last year) fed on soybean during May through June. Females from this overwintered population lay eggs that develop into first-generation beetles that emerge in July. First-generation adult populations usually peak in the early reproductive soybean stages, whereas the second-generation adults peak during the pod-fill stages. Feeding by first-generation beetles on soybean leaves seldom results in economic yield losses, but second-generation feeding on pods in late summer can be significant.

A degree-day model was developed to estimate the occurrence of first-generation adults. The degree days for the first-generation adults were estimated to be 1212 degree days with a developmental base threshold at 46°F. The overwintered female beetles usually begin to lay their eggs after colonizing the bean fields. The degree-day estimation for the first-generation adults is calculated by accumulating the temperature at the week of soybean emergence. Table 1 shows the accumulated degree days and predicted dates for the peak emergence of first-generation adults.

The first generation can be sampled to predict the size of the second generation. Tables 2 and 3 refer to the size of the bean leaf beetle population that is occurring in the field during mid-July. These are first-generation beetles. If this population exceeds the thresholds, the field should not be sprayed now, but instead sprayed later during the beginning of the second generation of beetles, which will appear sometime in mid-August. Sampling the fields now will help you predict the possibility of economic pod damage at the beginning of pod growth and development a month from now.

The new management concept is to sample the first-generation beetles and then to use this information to manage the second-generation beetles. Here is how it works:

  1. Determine what week your soybean plants emerged from the soil.
  2. Consult the left-hand column of Table 1 and find the dates that match your soybean emergence date.
  3. Determine which of the five Iowa locations is closest to your field.
  4. Where the date (row) and location (column) intersect represents the predicted date for peak first-generation beetle emergence.
  5. Sample your soybean fields one week after the predicted peak emergence. If the number of beetles reaches or exceeds the threshold (Table 2 or 3), stop sampling. If the sample is below the threshold, sample the following week. If the sample remains below the threshold, sample a third and final week. If the threshold is not reached, an economic infestation of bean leaf beetles should not occur in your pod-stage soybeans.
  6. If the first-generation population is above the threshold, do not spray now, but scout the fields again in late August to monitor for the first-emerging beetles of the second generation. When the first beetles appear, spray the field with an insecticide (45-day preharvest interval or less). Based upon the population size of the first generation, it is expected that the second generation will exceed the economic threshold. Fields can be sampled for first-generation beetles by using either a drop cloth or a sweep net.

Drop cloth

Sweep net

Table 1. Predicted dates for peak emergence of first-generation bean leaf beetle adults. Top number = degree-day accumulations (base 46°F) as of June 30, 2004; bottom number = projected date for peak emergence of first-generation bean leaf beetle at 1212 DD.

Date of

Soybean Emergence
Decorah

(Northeast)
Burlington

(Southeast)
Des Moines

(Central)
Omaha

Southwest
Spencer

(Northwest)
May 1-7 1085

July2
1382

June 27
1222

June 30
1232

June 29
988

July 8
May 8-14 989

July 9
1225

June 30
1063

July 5
1053

July 6
850

July 13
May 15-21 831

July 15
1098

July 4
961

July 9
960

July 9
784

July 16
May 22-28 708

July 20
918

July 10
810

July 15
809

July 14
696

July 19

Table 2. Economic thresholds for first-generation bean leaf beetles (average number of beetles per 3 foot of row).

Management cost ($/acre)
$/bu 7 8 9 10 11 12 13 14 15
15 2.0 2.2 2.5 2.8 3.0 3.3 3.5 3.8 4.1
14 2.1 2.4 2.7 2.9 3.2 3.5 3.8 4.1 4.3
13 2.3 2.6 2.9 3.2 3.5 3.8 4.1 4.3 4.6
12 2.4 2.8 3.1 3.4 3.7 4.1 4.4 4.7 5.0
11 2.6 3.0 3.3 3.7 4.1 4.4 4.8 5.1 5.5
10 2.9 3.3 3.7 4.1 4.4 4.8 5.2 5.6 6.0
9 3.2 3.6 4.1 4.5 4.9 5.3 5.8 6.2 6.6
8 3.6 4.1 4.5 5.0 5.5 6.0 6.5 7.0 7.5
7 4.1 4.6 5.2 5.7 6.3 6.8 7.4 7.9 8.5
6 4.7 5.3 6.0 6.6 7.3 7.9 8.6 9.2 9.9
5 5.6 6.4 7.2 7.9 8.7 9.5 10.3 11.1 11.8
4 7.0 7.9 8.9 9.9 10.9 11.8 12.8 13.8 14.8

Table 3. Economic thresholds for first-generation bean leaf beetles (average number of beetles per 20 sweeps).

Management cost ($/acre)
$/bu 7 8 9 10 11 12 13 14 15
15 8.1 9.2 10.2 11.3 12.4 13.4 14.5 15.6 16.6
14 8.6 9.8 10.9 12.1 13.2 14.3 15.5 16.6 17.8
13 9.2 10.5 11.7 12.9 14.2 15.4 16.6 17.9 19.1
12 10.0 11.3 12.6 14.0 15.3 16.6 18.0 19.3 20.6
11 10.8 12.3 13.7 15.2 16.6 18.1 19.5 21.0 22.4
10 11.8 13.4 15.0 16.6 18.2 19.8 21.4 23.0 24.6
9 13.1 14.8 16.6 18.4 20.2 22.0 23.7 25.5 27.3
8 14.6 16.6 18.6 20.6 22.6 24.6 26.6 28.6 30.6
7 16.6 18.9 21.2 23.5 25.8 28.1 30.3 32.6 34.9
6 19.3 22.0 24.6 27.3 30.0 32.6 35.3 38.0 40.6
5 23.0 26.2 29.4 32.6 35.8 39.0 42.2 45.4 48.6
4 28.6 32.6 36.6 40.6 44.6 48.6 52.6 56.6 60.6

This article originally appeared on pages 90-91 of the IC-492(15) -- July 12, 2004 issue.


Source URL:
http://www.ipm.iastate.edu/ipm/icm//ipm/icm/2004/7-12-2004/predictblb.html