Monitoring European Corn Borer

Both first- and second-generation corn borers cause yield losses in many areas of Illinois. The trend toward planting long-season corn varieties very early has contributed to larger populations of corn borers.  When planted early, long-season hybrids can be attacked by the first generation and may still be attractive to the second generation. In addition, most reduced-tillage systems, especially those that do not include plowing, permit increased survival of the overwintering population of borers.

As a first step in managing European corn borers, growers should consider selecting a hybrid that is resistant or tolerant. Some "conventional" hybrids are resistant to first-generation corn borers, and others have some degree of tolerance to corn borer injury. Genetically transformed hybrids expressing the Bt gene that produces the toxic protein should provide season-long control of European corn borers. However, the decision to plant Bt-corn hybrids should be accompanied by considerations for managing the potential for the development of corn borer resistance to the Bt gene. Refer to Bt-Corn for Control of European Corn Borer for more details about Bt-corn and resistance management.

Scouting and Decision Making for First-Generation European Corn Borer
Scouting Procedure. Corn that is planted early (the fields with the tallest corn) should be monitored closely during June and early July for signs of whorl feeding by corn borer larvae. The fields with the tallest corn are the most attractive to moths that are lay-ing eggs for the first generation. Plan to scout cornfields for injury at least once a week for 2 to 4 weeks after the initial corn borer moth flight, generally from early June to early July. Grassy areas and soybean fields that border cornfields may harbor large numbers of corn borer moths because female moths rest in these areas during the day. Check these areas for moths before you enter the field to determine if they are present.

To determine whether or not an insecticide treatment is economically justified, examine 100 plants (10 consecutive plants at 10 different locations in a field, always avoiding field edges) for shot-hole feeding in the whorl leaves. At each location, unroll the whorl leaves of an infested plant (one with shot-hole feeding) and count the live corn borers. Calculate the percentage of plants infested and the average number of live borers per infested plant. Also note the location of the corn borer larvae. Those that are still in the whorl leaves can be controlled; those that have bored into the stalk are protected from the insecticide. If all larvae have left the whorl leaves and bored into the stalk, treatments will not be effective. For best results, treatment should be applied soon after egg hatch to kill the young larvae before they bore into the plant. The larvae begin tunneling into the stalks about 10 days after hatching.

Decision Making. To decide whether or not it is profitable to treat a field infested with first-generation corn borers, you need the following information.

• average percentage of plants with whorl feeding
• average number of larvae per infested plant
• crop growth stage
• expected yield per acre
• anticipated value of grain per bushel
• cost per acre for insecticide treatment

Enter these data into the management worksheet for first-generation corn borer to calculate the gain or loss if an insecticide is applied. Following is an example (completed form is found in Insecticides Specified in this Chapter).

Assume a 20 percent infestation (20 of 100 plants with whorl-feeding injury caused by first-generation borers) of early whorl-stage corn, with an average of 1.5 corn borer larvae per plant. Expected yield is 160 bushels per acre, and the corn price is $3.00 per bushel. Also assume 80 percent control with granules, and cost of control is $12 per acre. Enter this information into the worksheet for first-generation corn borers, as indicated on the example worksheet. Obviously, 20 percent infestation in this example does not warrant a treatment ($12-per-acre control cost – $5.76- per-acre preventable yield loss = – $6.24 per acre loss if the field is treated). However, if infestation were 50 percent, control would be economically justified ($14.40-per-acre preventable yield loss – $12 per-acre control cost = $2.40 gain). Typically, if expected yield, price per bushel of corn, or anticipated percentage control increases, economic justification for control is more likely. Conversely, if expected yield or price per bushel of corn decreases, or if cost of control increases, economic justification for control is less likely.

Much of the information and suggested guidelines on the worksheets were derived from research trials conducted over many years in numerous locations throughout the Corn Belt. However, if your experience or environmental conditions in your area suggest that other figures might be more accurate, use them instead. For example, if you believe you can achieve 90 percent control with a certain insecticide, use 90 percent instead of 80 percent (our average guideline). If you estimate that survival is less or more than 20 percent (for whatever reason), multiply the percentage survival (decimal point) by 23 (average number of eggs in a mass) to obtain an estimated average number of borers per plant.

Scouting and Decision Making for Second-Generation European Corn Borer
Scouting Procedure. Start checking for egg masses when moth flight is under way, usually from July through mid-August. Concentrate initial scouting efforts in late-planted fields, where the probability of an economic corn borer infestation is greatest. Scout fields at least once a week and continue scouting throughout the egg-laying period. Examine a minimum of 50 plants, selected at random throughout the field, and count the number of corn borer egg masses found on each plant. Although 90 percent of the egg masses are laid on the ear leaf and the three leaves above and below the ear leaf, you should plan to examine all the leaves on each plant. Decision Making. To determine if it is profitable to treat a field infested with second-generation corn borers, you need the following information:

• average number of European corn borer eggmasses per plant
• crop growth stage
• expected yield per acre
• anticipated value of grain per bushel
• cost per acre for insecticide treatment.

Because the egg-laying period of the second flight of moths is extended, you should accumulate egg counts for consecutive scouting trips. For example, if you find an average of 0.2 egg mass per plant on your first scouting trip and 0.4 egg mass per plant on your second scouting trip 5 days later, add the two counts to determine the cumulative count. However, do not accumulate the number of plants sampled; this number should remain constant. In this example, the cumulative count is 0.6 egg mass per plant.

Enter these data into the management worksheet for second-generation corn borer to calculate the gain or loss if an insecticide is applied.

For best results, treatment should be applied soon after egg hatch to kill the young larvae before they bore into the plant. The larvae begin tunneling into the stalks about 10 days after hatching.

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