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No. 13/June 20, 1997

European Corn Borer Moth Captures Impressive in Some Sites

The 1997 European corn borer season lurched forward during the past week! Steve Wendzel with Crop Production Services, Galesburg, Illinois, is monitoring the corn borer flight with 19 pheromone traps located in western counties. Beginning in early June, moth captures were common in many of Steve's traps; and, by June 5 through June 13, numbers were very respectable in some traps. Wayne Buhler, an entomologist with Purdue University, is using numerous pheromone traps in an experiment located in east-central Illinois and, during the second week of June, collected 195 moths in one of his traps. Wayne indicated that he caught moths for the first time this season in Benton County, Indiana, on June 4. Doug Gucker, Piatt County Extension Service, reports that he has been observing European corn borer moths since June 5 in action sites. Dave Feltes and Jim Morrison, Extension Service educators, also have observed moths in northwestern Illinois counties since approximately June 4. On June 17, Howard Brown, an agronomist with Pioneer Hi-Bred International, Inc. reported that a cornfield near Niantic, Macon County, had whorl-feeding injury and active early instar European corn borers in the whorls. Howard also found egg masses on several plants. Corn borer season is under way!

European corn borer moths, male left, female right.

European corn borer egg mass.

First-generation European corn borer whorl-feeding injury.

In an earlier issue of this Bulletin (no. 11, June 6), we provided a table that offered some information on European corn borer lifecycle events based upon accumulated degree-days. Bob Scott, Illinois State Water Survey, has provided (Table 1) some projections for these corn borer development events based upon first captures of moths in northern, central, and southern Illinois. These projections are based upon average historical temperatures for each of these regions of the state. The information that Scott has provided appears to be right on track. For instance, we should expect to find first-instar larvae and egg hatch in central Illinois by June 15. This is based upon an initial capture of moths on June 1 and a projected accumulation of 212 heat units by June 15. This projection matches very nicely with Howard Brown's observation in Macon County.

Table 1. Projected degree-day accumulations (developmental threshold of 50 degrees F) from initial capture of moths in the spring to first occurrence of life stage or activity of first generation European corn borers (from European Corn Borer: Ecology and Management, NCR Publication no. 327, Iowa State University, Ames).

Accumulated degree- daysFirst occurrence of eventGeneral activityProjected date for Northern Illinois (first moth capture, 6/7)Projected date for Central Illinois (first moth capture, 6/1)Projected date for Southern Illinois (first moth capture, 5/24)
212Egg hatch (first instar)Pinhole leaf feedingJune 19June 15June 7
318Second instarShot hole leaf feedingJune 24June 20June 14
435Third instarMidrib and stalk boringJune 29June 25June 19
567Fourth instarStalk boringJuly 5June 30June 24
792Fifth instarStalk boringJuly 15July 9July 2
1,002PupaChanging to adultJuly 23July 17July 10



How do I scout for the first generation of European corn borers? When corn plants have an extended leaf height of 15 inches, scouting should begin in earnest. If possible, examine at least 20 consecutive plants in each of five random areas for every 40 to 50 acres within a field. In very large fields, it will be more practical (although less precise) to examine 25 consecutive plants in each of five random areas for every 80 acres. It's also a good idea to walk at least 100 feet into the field before sampling. If more than one variety of corn is being grown or if different planting dates occurred within the same field, it is important to consider each section as a separate field when scouting. Plants should be checked for fresh whorl-feeding damage, and the percent of infested plants calculated. For every 20 to 25 plants examined, remove the whorl leaves from two plants and check for the presence of live borers (Figure 1). This allows you to estimate the average number of borers per infested plant. After the field has been scouted, you should fill out a management worksheet and make the appropriate decision (Figure 2).


Figure 1. Management worksheet for first-generation corn borer.

_____ % of 100 plants infested (use a decimal)x_____ average no. borers /infested plant=_____ borers /plant
_____ borers /plantx_____ % 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 loss (use a decimal)x$ ____ price/bu=$ ____ loss/A
$ _____ loss/A x_____ % control (use a decimal; 0.8 for granules, 0.5 for sprays)=$ ____ preventable loss/A
$ _____ preventable loss/A-$ ____ cost of control/A=$ _____ gain (+) or loss (-) per acre if treatment is applied

*5% for corn in the early whorl stage; 4% for late whorl; 6% for pretassel.

Figure 2. Body lengths and prothoracic shield widths for instars of the European corn borer.

In using Figure 1, keep these points in mind. Prothoracic width is preferable for determining instars because less variation is associated with this measurement than body length. Note that the unshaded portion of the body-length scale represents the upper range in length of each instar. Also, note the brackets around stages 4 and 5; these two instars usually are found tunneling in the stalks. Consequently, they cannot be controlled with insecticides.

If control of an infestation of European corn borers is justified, you can select among several insecticides (Table 2) that provide essentially equivalent control. Remember, granulars are usually more effective than sprays for controlling first-generation corn borer larvae in the whorls, unless the sprays can be directed right over the rows.

Table 2. Suggested insecticides for first-generation European corn borer in field corn.

InsecticideAmount of product per acrePlacement
*Ambush 2E6.4 to 12. 8 ozbroadcast
Dyfonate II 15G4 to 8 oz per 1,000 ft rowover whorls
Dyfonate II 15G5 to 6.75 lbbroadcast
Lorsban 4E1-1/2 to 2 ptbroadcast
Lorsban 15G5 to 6.5 lbbroadcast
Lorsban 15G3.5 to 8 oz per 1,000 ft rowover whorls
*Penncap-M2 ptover whorls
*Penncap-M3 to 4 ptbroadcast
*Pounce 1.5G6.7 to 13.3 lbbroadcast
*Pounce 3.2EC4 to 8 ozbroadcast
*Warrior 1EC2.56 to 3.84 ozbroadcast

* Use restricted to certified applicators only.

Mike Gray and Kevin Steffey, Extension Entomologists, (217)333-6652