Watch for European Corn Borer Moths

Watch for European Corn Borer Moths

July 25, 2008

We will soon be seeing the second flight of European corn borers in Nebraska. Check the current information on light trap catches to get an idea of when to start scouting. Light traps near your area can be found through the UNL Entomology Department Web site at under Extension Programs, Field Crops Entomology.

Timely and accurate scouting is the key to managing European corn borer in (non-Bt) corn hybrids. Remember that conditions are localized and each field must be scouted to make accurate decisions. We have had it easy the last few years - flights have been relatively low; however, this cannot last forever. In fact, we had several reports of significant first generation corn borer activity this year in those cornfields that got planted relatively early, so stay vigilant.

Scouting Recommendations

Fields with green silks during the peak moth flight period are most susceptible to second-generation egg laying. The white, flat eggs overlap each other like fish scales and are laid in masses of five to 40 eggs. Eggs are most likely found on the underside of leaves, near the mid-rib, on the ear leaf and the three leaves above or below the ear leaf. Approximately 90% of the egg masses will be found on these middle seven leaves. A black spot is visible on the eggs for about 24 hours before they hatch. The spot is the head of the developing corn borer; this stage is often referred to as the black head stage.

To determine whether control would be profitable, examine 25 plants at four sites per field (100 plants total). Record the number of egg masses and the number of plants sampled. If you sampled only the middle seven leaves, multiply the number of egg masses by 1.1 to estimate the total present over the whole plant. Use this adjusted mean in the worksheet.

Infestations are most damaging when corn borers enter the stalk early in corn's reproductive cycle. There is a short time between first egg hatch and significant stalk tunneling when corn borers are best controlled. Concentrate scouting efforts in this early egg laying period and repeat every three to five days. Often, second generation egg laying may extend to 21 days or more. Although later hatching corn borers do not directly reduce grain yield as much, they may still cause stalk breakage or ear drop. Early harvest of fields damaged by corn borers and selecting varieties with good stalk strength and resistance to stalk rot can reduce this loss.

Calculating Control Costs and Benefits

Use the calculations outlined in the worksheet below to determine if an economic infestation is present. You will need to know the crop stage, expected yield, expected market price for corn, percent control with insecticide, and cost of control (insecticide plus application costs). An interactive version of this worksheet is available online at

Worksheet Adjustments

This worksheet will help you better evaluate the factors influencing the cost/benefit relationship for second generation European corn borer treatments. Average values are suggested in the worksheet but may be modified for local conditions. Consider the following factors:

  1. Borer survival is suggested to be 15%. Larval survival varies with weather conditions and irrigation. In irrigated corn, larval survival may be 20% or more, while in dryland corn with no significant rainfall, it may be 10% or less. Survival of eggs and small larvae decreases greatly in hot, dry weather or with extended periods of heavy rain.


  2. Yield loss will be about 4% per borer for infestations occurring before silks turn brown and 3% per borer after silks turn brown but before blister stage. These averages are based on published research but only account for physiological yield losses (reduced grain production) and do not consider yield loss from stalk breakage or ear drop. These factors are difficult to predict and vary with hybrid, cultural practices and weather.


  3. Percent control with insecticides is suggested to be 70%; change this value if you think that control will be different in your situation.


If treatment is needed, time insecticide applications to coincide with the beginning of egg hatch to achieve acceptable control. Generally, liquid and granular formulations of the same insecticide are equally effective against corn borer larvae. However, in considering other pests that may need to be controlled at this time of year (western bean cutworms, rootworm beetles, grasshoppers, spider mites), liquids may be preferred. Rates and restrictions of registered insecticides for European corn borer control can be found on the label or at the UNL Department of Entomology Web site.

Tom Hunt
Extension Entomologist
NEREC Haskell Ag Lab
Keith Jarvi
IPM Assistant, NEREC


Management Worksheet for Second Generation European Corn Borers
An interactive version of this worksheet is available online at
_______________ Number of egg masses per plant x 3 borers per egg mass* = _______________ borers per plant
_______________ Borers per plant x 4% yield loss per borer** = _______________ percent yield loss
_______________ Percent yield loss x _______________ expected yield (bu per acre) = _______________ bushels per acre loss
_______________ Bushels per acre loss x $_______________ sale price per bu = $_______________ loss per acre
$_______________ loss per acre x 70% control*** = $_______________ preventable loss per acre
$_______________ preventable loss per acre
-$_______________ cost of control (product + application costs)
=$_______________ profit (+) or loss (-) per acre if treatment is applied
If preventable loss exceeds cost of control, insecticide treatment is likely to result in economic benefit.
* Assumes survival rate of three borers per egg mass; may vary with weather and egg mass size.
** Use 3% loss per borer per plant if infestation occurs after silks are brown. The potential economic benefits of treatments decline rapidly if infestations occur after the corn reaches the blister stage.
*** 70% is an average, you may use another value if desired.


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