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ISSUE 12    July 21, 2005

SCOUT FIELD CORN FOR EUROPEAN CORN BORER INFESTATIONS

The degree day model for the univoltine-type of European corn borer indicates that flight is 75-90% emerged in the southern counties to 25-50% emerged in the more northern counties of North Dakota. The accumulated degree days (DD) using a base of 50 F for moth emergence include: 911 DD = 10% emerged; 986 DD = 25% emerged; 1078 DD = 50% emerged; 1177 DD = 75% emerged; and 1274 DD = 90% emerged. This is the major type of corn borer that attacks our corn in North Dakota.

In North Dakota, chemical treatments are not always warranted for controlling corn borer damage. If you didnít plant Bt corn for protection against corn borers, scout fields to determine the level of corn borers present. Look for egg masses on the underside of leaves and/or active larvae feeding in whorls or tassels. Egg masses contain 20-30 eggs and are white to cream colored and look like fish scales.

As the eggs mature, they become dark from the black head capsules of the young larvae ready to emerge, called the "black-head" stage. Larvae vary in color from gray to creamy white with dark spots and are 1 inch when mature. Larvae feed on the leaves and tassels creating a shot-hole appearance. Insecticides for control of corn borer must be properly timed. Most insecticide treatments for corn borer are applied too late with the assumption that the treatment was a failure. There is usually only 7-10 days window when the insecticide can reach the larvae before they tunnel into the stalk. When larvae are about the length of a dime, they tunnel into the stalk. Once larvae tunnel into the stalk, it is too late to expect effective control with any insecticide. Economic thresholds are: 40-50% of the plants with shot-holing or egg masses for dryland field corn and 25-35% of the plants in irrigated corn. Randomly check 10 plants at 5 locations across the field for larvae / egg masses.

Detailed management worksheets to assess the economic necessity of treating with an insecticide are available in NDSU Extension Publication 327, "European Corn Borer Management in North Dakota."

http://www.ag.ndsu.nodak.edu/aginfo/entomology/entupdates/ecb/mngborer.htm

Janet Knodel
Area Extension Specialist
North Central Region Extension Center
jknodel@ndsuext.nodak.edu

 

SAMPLING FOR EGGS OF THE BANDED SUNFLOWER MOTH

Last week sampling procedures for sampling adult banded sunflower moths were discussed. However, sampling for moths is difficult. A new technique involves sampling for eggs rather than the adult moths and offers the advantage of being quicker and more accurate.

Survey Locations:

Sample five, randomly selected buds in row 5 or about 18 feet in from the field margin at the sites marked in the diagram. Make sure you sample without regard to floral bud size but begin sampling when most plants are in stage R3 (buds are elongated to about ľ to ĺ inches above the nearest leaf and the bracts are covering the face of the bud; the yellow ray flowers are not yet visible). Avoid sampling the field corners and for very large fields additional samples along the field margin are advised.

x

x

x

x

x

x

x

x

Counting BSM Eggs in the Field

  1. Randomly choose a bud.
  2. Select a bract at random from the outer whorl; count the eggs on that bract and on the adjacent bracts to each side. Use a 3.5 X magnifier to best see the eggs. Head mounted magnifiers are advised as they leave your hands free to manipulate the plant.
  3. Locate a bract on the outer whorl directly across the bud from the original bract and repeat step two. Total the number of eggs on the six bracts.
  4. Determine an average egg count for the 5 plants at each site.

Economic Injury Level: The economic injury level for the banded sunflower moth in eggs per plant at different market values and at different plant population follows.

Market Value

Plant Population

($) per lb

16,000

18,000

20,000

0.10
0.14
0.18

4.8
3.5
2.7

4.3
3.1
2.4

3.9
2.8
2.2

Economic Distance: Economic distance is the distance from the field margin that an economically damaging population is expected. Economic distance works for the banded sunflower moth since it is an edge insect and populations decline from the field margins to the field interior.

example: If there is an average of 5 eggs at row 5, the economic distance will extend 34 yards into the field. To estimate economic distance, use this formula or the chart below.

Economic Distance = 18.52(EP) - 6.19; (EP = Average egg density at row 5 or 18 feet)

Economic Yield Loss at Distance into the Field

Average Egg Density
at Row 5 or 18'

 

Economic Distance (yards)

5
10
20
25

34
135
336
439

Based on 16,000 plants per acre and a market value of $0.14/lb.

Gary Brewer and Kirk Mundal
Department of Entomology
North Dakota State University


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