University of Illinois

No. 20/August 7, 1998

European Corn Borers, Anyone?

Although I didn't provide scouting and management guidelines for European corn borers in last week's issue of the Bulletin as I promised, I'm not sure you missed much. Second-generation corn borers are most notable for their absence this year. Conventional wisdom suggests that this year's unusual weather created significant mortality among corn borers throughout most of the state. My graduate student, Maria Venditti, and her crew have found very few signs of life of second-generation European corn borers during their weekly scouting trips to four corn fields in Sangamon County. Nevertheless, a few folks have reported finding both early and mid-instar corn borers in some areas of the state. For example, Randy McElroy with DeKalb Genetics observed first-instar corn borers in the whorls of about 20 percent of the plants he examined in at least one field south of Route 50. That's right, I said whorls. Randy was looking at late-planted corn, so the symptoms of the infestation resembled the symptoms we usually associate with first-generation corn borers.

Moths laying eggs for the second generation are attracted to actively pollinating corn.

Once again, some folks have found unusual mixtures of corn borer instars in some areas of Illinois. Bill Craig, an independent consultant in Macoupin County, has observed both first- and fourth-instar corn borers in some fields in his area. Some of you may recall our speculative explanation during previous years when this has happened. Entomologists have determined that there are three "ecotypes" of European corn borers throughout North America. These ecotypes are determined by generations completed per season: one generation per season (univoltine), two generations per season (bivoltine), and three or more generations per season (multivoltine). Throughout Illinois during most years, the bivoltine ecotype predominates, although the multivoltine ecotype occurs in southern counties. The univoltine ecotype generally is distributed through the northern halves of South Dakota, Minnesota, Wisconsin, and Michigan and north into Canada.

European corn borer egg mass.

Some evidence suggests that we can experience both the bivoltine and univoltine ecotypes during some years in Illinois. In essence, the single peak of moth flight of the univoltine ecotype fits snugly between the two peaks of moth flight of the bivoltine ecotype. If such is the case, scouts may find mixtures of several sizes of corn borer larvae in some fields. Also remember that the flight of moths that lay eggs for the second generation extends over a relatively long period of time. Consequently, finding different sizes of corn borer larvae in a field could easily be due to the extended egg-laying period.

Looking for corn borer egg masses on the undersides of corn leaves.

Now, on to some scouting and management guidelines. Second-generation European corn borers always are more challenging to manage than first-generation borers. Extended moth flight and mixed ecotypes create frustrations for many. However, frequent and careful monitoring will pay dividends in yield protection if densities of second-generation corn borers are large enough to cause economic losses.

Mature corn borer larva inside a corn stalk.

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 percentage 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 percentage yield losses per borer per plant are 4 and 3 percent, respectively. However, these figures do not include any yield loss attributable to broken stalks and dropped ears.

In general, European corn borer moths laying eggs for the second generation are attracted to corn fields that are pollinating and have fresh silks. However, if pollinating fields are not readily available, the moths lay eggs in any available corn field, or possibly on other hosts. In fact, late-planted corn in the whorl stage would also be attractive to corn borer moths. Keep a close watch on late-planted or late-maturing fields, but don't ignore early planted fields.

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.

Because the egg-laying 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, as stated previously. 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.

A management worksheet (Figure 1) 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. As many have observed this year, survival may be well below 10 percent in some areas.

Figure 1. Management worksheet for second-generation European corn borers.

_____ number
of egg
count, taken a
few days
X4 borers/egg
_____borers/plantX_____ 3% yield
loss/borer**(do not use a
=_____ % yield
_____ % yield
loss (use a
X_____ expected
yield loss
=_____ bu/A
_____ bu/A
X$_____price/bu=$_____ loss/A
$_____ loss/AX_____ %control (use a
decimal; 0.75)
X$_____ cost of
=$_____ gain
(+) or loss (-)
per acre if
treatment is

* 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 .

Timely and frequent scouting is the key to obtaining good results if control of second-generation corn borers is necessary. Because the egg-laying period is so protracted, you usually cannot 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.

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

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

InsecticidesAmount of product
broadcast per acre
*Ambush 2E6.4 to 12.8 oz
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

* Use restricted to certified applicators.

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