Corn borer next to Mn map image

Quick Guide to European Corn Borer Management: Scouting and Decision-Making for First Generation

Ken Ostlie

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Copyright ©  2002  Regents of the University of Minnesota. All rights reserved.


The European corn borer causes more crop loss than any other corn insect in Minnesota. Economic infestations of first-generation larvae can be quickly identified and profitably controlled using this quick guide to scouting and insecticide decisions.

Scouting Instructions

  1. Target earlier-planted, taller fields for scouting.
  2. Visit each field every three to five days after moth flight begins. Check for moths in grassy field borders or contact your county extension office about flight status.
  3. Sample five locations per 40 acres on each visit.
  4. At each location, examine 20 plants for leaf feeding (or shotholing). See Figure 1 for example of shotholing. Record data on form at right.
  5. Dissect two or more plants with leaf feeding at each location and count the number of borers. Record this number and larval stage(s). Count only borers not tunneling in stalk or midrib.
  6. After completing samples, estimate borers per plant using the following formula:
    Borers/plant = (proportion of plants shotholed)
    X (borers/shotholed plant).
  7. Using local control costs and projected crop value for field, look up the economic threshold (= break-even density of borers) in the table on the back of this page.
    Multiply by the desired economic return, e.g., 1.5 for 150% return on control costs, to calculate an action threshold.
  8. Compare estimated borer density with the action threshold you just calculated.
  9. If borer density exceeds threshold, quickly make arrangements to treat. Sketch diagram of field for applicator and indicate surrounding crops, homesteads, field entrances, power/telephone lines, and bee yards. Select insecticides from BU-0500, Insecticide Suggestions for Insect Pests of Minnesota Field Crops.
  10. If not, resample in three to five days provided:
    • moth flight is still under way;
    • larvae are young (less than 3rd stage); and
    • tunneling is not prevalent.
FIELD DATA FORM
Date _______ Field ID_______ Acres_______
Co.________ Twnshp._______ Sec._________
Crop Value ________ Control Costs_______
 Sample  # Shotholed
  /20 plants 		  Borers per
   shotholed
       plant		   Borer
 Stage(s)
1   _______   _______   _______
2   _______   _______   _______
3   _______   _______   _______
4   _______   _______   _______
5   _______   _______   _______
6   _______   _______   _______
7   _______   _______   _______
8   _______   _______   _______
9   _______   _______   _______
10   _______   _______   _______
    =======   =======   =======
Total   _______   _______  
No. plants   _______   _______  
Avg   _______   _______  

Estimated larvae/plant =
(proportion plants shotholed) X (borers/shotholed plant)
   = _______ X _______ = _______

Action threshold
   = economic threshold X return
   =_______ X _______ = _______

Decision: ___ Treat ___ Donāt Treat ___ Resample

For a more complete discussion of European corn borer, see BU-2322 European Corn Borer Development and Management, and FO-5970 Managing First-Generation European Corn Borer.

Figure 1. Leaf injury (shotholing) caused by young corn borer

Corn borer image

Figure 2. Where to look for young corn borers and leaf injury

Corn image with whorl and exposed collar

Economic Thresholds 1 (Borers/plant) for
First-Generation European Corn Borer
Crop Value 2 ($/Acre)

Control
Costs3
($/Acre)
  200  250  300  350  400   450  500   550  600
6 0.75 0.60 0.50 0.43 0.38 0.34 0.30 0.27 0.25
7 0.88 0.70 0.58 0.50 0.44 0.39 0.35 0.32 0.29
8 1.00 0.80 0.67 0.57 0.50 0.45 0.40 0.37 0.34
9 1.12 0.90 0.75 0.64 0.56 0.50 0.45 0.41 0.38
10 1.25 1.00 0.83 0.71 0.63 0.56 0.50 0.46 0.42
11 1.38 1.10 0.92 0.79 0.69 0.61 0.55 0.50 0.46
12 1.50 1.20 1.00 0.86 0.75 0.67 0.60 0.55 0.50
13 1.63 1.30 1.08 0.93 0.81 0.72 0.65 0.59 0.54
14 1.75 1.40 1.17 1.00 0.88 0.78 0.70 0.64 0.59
15 1.88 1.50 1.25 1.07 0.94 0.84 0.75 0.68 0.63
16 2.00 1.60 1.33 1.14 1.00 0.89 0.80 0.73 0.68
17 2.13 1.70 1.42 1.21 1.06 0.95 0.85 0.78 0.71
18 2.25 1.80 1.50 1.29 1.13 1.00 0.90 0.82 0.75

1 Calculation makes four assumptions:

  1. scouting is 100% efficient,
  2. each scouted larva will produce one tunnel (=100% survival),
  3. each tunnel will cause 5% yield loss, and
  4. an insecticide application will control 80% of the larvae.

2Crop value = expected yield (bu/acre) X projected price ($/bu)

3Control costs = insecticide price ($/acre) + application costs ($/acre)



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