Integrated Crop Management

Scout first-generation bean leaf beetles now

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. Larry Pedigo and his students at Iowa State University have 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. This article explains this new management concept, which we first presented last year.

The beetle has two generations a year in Iowa. The overwintered population (not generation), which fed on soybean during May through June, is actually the second-generation beetles from last year that hibernated through the winter. Females from the overwintered population lay eggs that develop into first-generation beetles that emerge in July. First-generation adult populations usually peak in the late vegetative or the early reproductive soybean stages, whereas the second-generation adults peak during the pod-fill stage. 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 in the field. The degree days for the first-generation adults were estimated to be 1212 degree days with a developmental 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 for the first-generation adults in five different areas of Iowa. Table 2 shows the dates predicted for the peak emergence of first-generation adults at these locations.

The first generation can be sampled to predict the size of the second generation. Tables 3 and 4 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 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 Table 2 (left-hand column) 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 1 week after the predicted peak emergence. If the number of beetles reaches or exceeds the threshold (Table 3 or 4), 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 soybean.
  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. Degree-day accumulations for first generation bean leaf beetle adults (1212 degree-days with developmental threshold of 46°F) from the date of soybean emergence through July 9, 2001.

Degree-Day Accumulations
Date of

Soybean

Emergence
Decorah

(Northeast)
Burlington

(Southeast)
Des Moines

(Central)
Omaha

(Southwest)
Spencer

(Northwest)
May 1-7 1280 1487 1489 1594 1302
May 8-14 1177 1329 1379 1484 1231
May 15-21 1068 1209 1251 1341 1119
May 22-28 908 1047 1091 1185 989

Table 2. Predicted dates for peak emergence of first-generation bean leaf beetle adults.

Date of

Soybean

Emergence
Decorah

(Northeast)
Burlington

(Southeast)
Des Moines

(Central)
Omaha

(Southwest)
Spencer

(Northwest)
May 1-7 July 7 June 30 June 30 June 27 July 7
May 8-14 July 11 July 7 July 5 July 1 July 10
May 15-21 July 14 July 10 July 9 July 6 July 13
May 22-28 July 20 July 16 July 14 July 10 July 20

Table 3. 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 4. 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 144-145 of the IC-486(18) -- July 16, 2001 issue.


Source URL:
http://www.ipm.iastate.edu/ipm/icm//ipm/icm/2001/7-16-2001/scoutblb.html