University of Illinois

No. 19/August 8, 1997

How Will You Manage Second-Generation European Corn Borers?

In last week's issue of this Bulletin (no. 18, July 25), I indicated that we were waiting for second-generation European corn borers to show their hand. Adults laying eggs for the second generation have been flying in southern Illinois for at least a couple of weeks, and moth flights have become more noticeable in western Illinois. Dave Mowers with Mowers Soil Testing Plus in Toulon (Stark County) told me that their captures of adult corn borers increased steadily (although the numbers were not large) during the week of July 21. Bill Craig, a private consultant in the area of Macoupin and Montgomery counties, reported that he found his first egg mass on July 28. Rick Weinzierl, Extension entomologist here on campus, told me that counts of corn borer moths in light traps near Manito (Tazewell County in central Illinois) have increased steadily to 200 to 300 per night within the last week.

Figure 1. European corn borer moths laying eggs for the second generation.

Figure 2. European corn borer egg mass.

Second-generation European corn borers always are more challenging to manage than first-generation borers. The second flight of adults typically occurs over a longer period of time than the flight of moths that lay eggs for the first generation. Add to this the potential for mixed ecotypes discussed in last week's Bulletin, and management of second-generation corn borers becomes frustrating at best and sometimes downright exasperating. However, frequent and careful monitoring pays dividends in yield protection if densities of second-generation corn borers are large enough to cause economic losses.

As is true for first-generation corn borers, the potential for yield loss caused by second-generation corn borers depends upon the time of infestation. In general, the percent of physiological yield loss caused per borer per plant is less for the second generation than for the first generation. If second-generation borers infest plants during pollen shed or when kernels are initiated, the percent yield losses per borer per plant are 4 percent and 3 percent, respectively. However, these figures do not include any yield loss attributable to broken stalks and dropped ears that may occur closer to harvest time.

Let's review some of the management questions and decisions that need to be made to manage second-generation European corn borers. If luck is with us, we won't have to face economic infestations of second-generation borers. However, it doesn't hurt to be prepared.

What types of fields are most attractive to moths laying eggs for the second generation? In general, European corn borer moths are attracted to cornfields that are pollinating and have fresh silks. However, if pollinating fields are not readily available (and they may not be in many areas because development of corn borers has been delayed), the moths lay eggs in any available cornfield, or possibly on other hosts. Keep a close watch on late-planted or late-maturing fields, but don't ignore early planted fields.

When scouting for second-generation European corn borers, what should I look for? Look for egg masses laid on the undersides of leaves near the midribs, and usually on the leaves in the ear zone (the ear leaf and three leaves above and below the ear leaf). However, during some years corn borer adults lay eggs on may parts of the plants, so examine entire plants when you initiate your scouting efforts. If you detect that most egg masses have been deposited in the ear zone, you can concentrate your efforts there and reduce the amount of time you spend in the field.

After I find egg masses, how soon will larvae tunnel into the stalks? Because the oviposition period for moths laying eggs for the second generation is much longer than for the first generation, you may encounter tunneling larvae and freshly deposited egg masses at the same time. After the larvae hatch from egg masses (3 to 7 days, depending upon the temperature), they move to the leaf collars, where they feed upon the tender leaf tissue. Within 10 to 14 days, again depending upon temperatures, the larvae develop to fourth instars that tunnel into the stalks, shanks, or ears.

When do I have enough second-generation borers that control is justified? A management worksheet (Figure 3) is the best way to make a decision about controlling second-generation borers. In the worksheet we offer some average numbers based upon research data from many years and many states. However, if you have experience that suggests other numbers are more suitable for your area, use your own information. For example, we suggest an average larval survival rate of 20 percent (about four larvae per egg mass). However, if the weather is extremely dry, survival may decrease to 10 percent. In some area of Illinois where corn is suffering from a lack of moisture, densities of corn borers could be diminished.

Figure 3. Management worksheet for second-generation corn borer.

_____ number of egg masses/plant (cumulative count, taken a few days apart)x4 borers/egg mass*=_____ borers /plant
_____ borers /plantx_____ 3% yield loss /borer** (do not use a decimal)=_____ % yield loss
_____ % yield loss (use a decimal)x_____ expected yield loss (bu/A)=_____ bu/A loss
_____ bu/A lossx$ ____ price/bu=$ ____ loss/A
$ _____ loss/A x_____ % control (use a decimal; 0.75)=$ ____ preventable loss/A
$ _____ preventable loss/A-$ ____ cost of control/A=$ _____ gain (+) or loss (-) per acre if treatment is applied

*Assumes survival rate of 20% (4 borers/egg mass).
**5% for corn in the early whorl stage; 4% for late whorl; 6% for pretassel; 4% for pollen shedding; 3% for kernels initiated. Use 3% per borer per plant if infestation occurs after silks are brown. The potential economic benefits of treatment decline rapidly if infestations occur after corn reaches the blister stage.

Is it difficult to time an insecticide application for control of second-generation corn borers? Yes. However, 75 percent control is possible. Timely and frequent scouting are the keys to obtaining good results if control is necessary. Because the egg-laying period is so protracted, you will not control all second-generation borers with one insecticide application. However, if the application is made just after peak moth flight while most of the larvae are still feeding in the leaf-collar areas, results can be satisfactory. Because corn borer larvae cause more injury when they attack during the pollen-shedding stage than when they attack during kernel initiation, missing the later-attacking borers usually results in less yield loss. Data published from the University of Nebraska suggest that economics usually rule out the feasibility of a second insecticide application.

What insecticides are suggested for control of second-generation borers? Table 1 shows the insecticides we suggest for control of second-generation borers. Note the absence of microbial insecticides from this list.

Table 1. Insecticides suggested for control of second-generation European corn borers in field corn.

InsecticidesAmount of product broadcast per acre
*Ambush 2Ea6.4 to 12.8 oz
Dyfonate II 15G6.75 lb
Lorsban 4E1.5 to 2 pt
Lorsban 15G6.5 lb
*Penncap-M2 to 4 pt
*Pounce 1.5G6.7 to 13.3 lb
*Pounce 3.2ECa4 to 8 oz
*Warrior 1EC2.56 to 3.84 oz

aApply Ambush 2E prior to the brown-silk stage.
*Use restricted to certified applicators.

Although microbial insecticides are effective for controlling first-generation borers, their efficacy subsides considerably for second-generation borers. If you determine the need for an insecticide, please follow all label directions and pay attention to all precautions. Also, please avoid applying insecticides when wind conditions increase the potential for drift.

How does Bt-corn hold up during infestations of second-generation corn borers? Most of you are aware by now that differences among different Bt-corn hybrids exist. The "concentration" of Bt endotoxin begins to decline in Mycogen and Ciba (Novartis) Bt-corn hybrids after pollination. Consequently, corn borers can survive in these Bt-corn hybrids late in the season. Whether these surviving corn borers cause much yield loss is debatable. Bt-corn hybrids that contain Monsanto's YieldGard technology express the Bt endotoxin well past pollination. Therefore, you shouldn't find too many, if any, corn borer larvae surviving in these Bt-corn hybrids. Assessment of performance of different Bt-corn hybrids will occur in many growers' fields this year, and we will have some of our own data again this fall.

Kevin Steffey, Extension Entomology, (217)333-6652