Corn borers reduce yields before harvest

Second generation European corn borers are still out in the fields causing injury. I have seen early planted fields (late April) with an average of 1.9 larvae per stalk and late planted fields (early June) with an average of 3.5 per plant. These second generation borers inflict yield losses in two ways. First, internal stalk tunneling causes physiological injury to the plant by disrupting the flow of water, nutrients, and sugars. This injury can translate into grain yield losses of 2-3 percent per plant. Second, larvae tunnel into the ear shank or stalk, causing the ears to drop onto the ground before harvest. This is called harvest loss and is in addition to the physiological losses. If harvest losses are observed in a field from dropped ears, it typically represents the "tip of the iceberg" with the physiological losses resulting in even greater yield reductions.

While nothing can be done now to curb physiological losses, potential harvest losses are just around the corner and can be reduced by early harvesting. Several factors strongly influence corn borer harvest loss. These are the corn borer infestation level, corn hybrid stalk or ear shank strength, the number of days corn stands in the field after maturity, and weather. In Minnesota, a mild, dry fall in 1990 resulted in less than 1 percent harvest loss while a windier and wetter fall in 1991 caused 2-7 percent harvest loss despite similar shank infestation levels. Potential harvest losses will vary widely from field to field and hybrid to hybrid.

European corn borer in ear shank can cause dropped ears.

Stalk rot can complicate the picture. Tunneling by corn borers provides stalk rots with a convenient entry point. More stalk rots usually accompany a corn borer infestation. Stalk rots attack the pith of lower internodes, contribute to premature plant death, weaken the stalks and thereby cause additional lodging.

Should you harvest early? Farmers typically delay harvest to reduce grain moisture and save on drying costs. But a corn borer infestation increases the risk of harvest losses. As ear shanks dry, they become more brittle and more likely to drop when buffeted by strong winds. Likewise, lodging from stalk rots or breakage in tunneled stalks will increase. The dilemma for most farmers is a simple one. Do I harvest early to reduce harvest losses at the expense of increased drying costs? Regrettably, there's no easy answer, but we can make an educated guess.

The cost to dry down wetter corn can be estimated by considering corn market value, LP gas and electricity prices, and potential yield. Drying efficiency will vary from dryer to dryer, but the following formula may help in calculating energy costs:

*(cents per point per bushel) **(bu/acre)

Suppose harvesting was initiated at 30 percent moisture rather than 25 percent to reduce harvest losses, the field was expected to yield 150 bushels per acre with LP gas at $0.65 per gallon, electricity price of $0.10 per kwh and a corn price of $3.00. In this example, it would cost $10.50 or the equivalent of 3.5 bushels per acre to pay for the extra drying costs. Therefore, early harvest would be advised if the farmer expects more than 3.5 bushels per acre in ear drop.

How can you identify which fields to harvest early? Begin by examining each field for its potential ear droppage and stalk breakage. At four locations in each field, examine 25 consecutive plants using a push-pull test. The "push" part of the test checks the potential for additional stalk breakage or lodging. Count the number of stalks that have already broken or lodged below the ear. For all other plants, simply push the stalk several internodes above the ear, displacing each plant at least one foot from vertical. Add the number that break below the ear to those already broken. This is your estimate of the percentage of plants that could lodge.

The more critical test is the "pull" part since it estimates ear droppage. Jerk each ear downward. Tunneled ear shanks will usually snap off or collapse when the ear is pulled. Split the ear shanks and confirm that corn borer tunneling has occurred. Count the number of tunneled ears at each location. Add the counts for the four locations to determine potential ear drop. A ballpark estimate for potential loss in bushels per acre can be obtained by multiplying the percent ears affected by the estimated yield (percent of ears tunneled or broken X bushels per acre). Rank your fields from highest potential ear drop to lowest and harvest the higher risk fields first.

Information in this article was originally developed by Ken Ostlie, extension entomologist, and Dale Hicks, extension agronomist, at the University of Minnesota.

This article originally appeared on page 174 of the IC-476(24) -- October 7, 1996 issue.

Updated 10/06/1996 - 1:00pm