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ISSUE 10  JULY 8, 1999

 

WHEAT MIDGE UPDATE

    Scattered reports from around the central and northeast counties indicate that midge are now flying and
can be observed in fields at night. The only report of treatable levels, at the time of this writing, were
from northeast Stutsman county. Keep track of local reports for the next two weeks for updates on
your situation. Scout regularly in the evening when weather conditions favor flight.

    Currently we are at the time for peak emergence in much of the central counties. We expect midge flights
to last until we accumulate 1800 DD.

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Wheat Midge Degree Day Accumulations - July 6, 1999

 

EUROPEAN CORN BORER TRAP CATCHES INCREASE

    Moth captures in black light traps located around the region are all catching low levels of moths at
this time. With these captures, we need to begin monitoring corn for moths in the field and grassy
margins, eggs on the undersides of leaves, and larvae feeding in the whorl or higher on older plants.

Damage

    Yield losses due to European corn borer infestations are primarily due to stalk tunneling that
results in physiological stress. With persistent autumn winds and dry conditions, tunneling in stalks
and ear shanks increases the risk of stalk breakage and dropped ears.

Management - Natural Control

    Heavy rains that occur before borers are able to burrow into the plant may kill the worms by drowning
them or by physically removing them from the plant. The high humidity will promote disease outbreaks
affecting the larvae.

Management - Chemical

    The challenge of the crop manager is to distinguish when egg laying and larval populations can be
tolerated or they need to be controlled. Corn should be monitored weekly for at least five weeks
once plants exceed an extended leaf height of 17 inches. At this point, corn borer larvae will be able
to survive on the plant. Inspect plants for the presence of egg masses, whorl feeding, and active larvae.
Observing moth activity around field margins or within the field may alert you to developing infestations.

Field scouting for corn borers:

    Whorl stage corn . . . . Pull the whorls from 10 plants at 5 locations across the field. Select whorls
at random, avoiding damaged plants. Unwrap the whorl leaves; count and record the number of live
larvae found.

Use the corn borer worksheet to help make decisions about the profitability of treating an individual field.

Worksheet for Corn borer in whorl stage corn...  You fill in the blanks
1. ___ % of plants infested x ___ Avg. no. borers/plant = ___ Borers per plant
2. ___ borers per plant x ___ % yield loss per borer* = ___ percent yield loss
3. ___ percent yield loss x ___ expected yield (bu/acre) = ___ bushels/a loss
4. ___ bushel loss per acre x ___ price per bushel = $ ___ loss per acre
5. ___ loss per acre x ___ percent control** = $ ___ preventable loss/a
6. ___ preventable loss/acre - ___ cost of control per acre = $ ___ profit (loss)/acre

* 5 % for corn in the early whorl stage; 4 % for late whorl; 6 % for pretassel
** 80 % for granules; 70 % for sprays.

 

Economic Threshold (Corn borer/plant) when factoring Crop Value and Control Costs

Control Costs2 ($/acre)

Value of Corn Crop1 ($/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

1 Crop value = expected yield (bu/acre) X projected price ($/bu)
2 Control costs = insecticide price ($/acre) + application costs ($/acre)

 

Phillip Glogoza
Extension Entomologist


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