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European corn borer larvae overwinter as 5th instar larvae in corn stalks, stems of other plant hosts, and in plant debris on or in the soil. In the spring, when temperatures reach 50 °F, the larvae start pupating into adult moths, which occurs in about 7-10 days. The moths have alternating yellow and brown wavy lines across each wing and male and females are similar in appearance.
Female moths seek the tallest corn (usually the first planted) to lay eggs for the 1st generation. Eggs are laid in clusters (~23 eggs/cluster) that overlap with an appearance similar to fish scales (Figure 1). Each egg is about the size of a pencil tip. Maturing larvae in the egg have a visible black head capsule, often referred to as the black head stage. The larvae emerge from the eggs in about 5-7 days and start feeding in leaf collars and in the whorl prior to reaching the 3rd instar, after which they bore into the plant. Fully grown larvae are about 1-inch long with flesh colored bodies and two distinct brown spots on each segment, with brown to black heads.
Figure 1. European corn borer egg mass, larva, adult moth, and pupa.
Scouting for 1st generation ECB larvae and damage should start after eggs have hatched (Table 1). Larvae feed on plant leaves in the whorl, chewing small holes in the leaves which create a "buckshot” effect as leaves unfurl. As larvae mature to the 3rd or 4th instar growth stages, they begin to tunnel into the leaf midrib or stalk where larval development is completed and pupation begins for the next generation.
When ECB moths are found in light or pheromone traps, scouting for 2nd generation egg masses should commence. Fields most likely to be infested have green, succulent corn that is shedding pollen or has green silks in late July and early August, usually the latest planted fields in the area. Particular attention should be placed in silage production areas where longer maturity products are used which could be at higher risk of infestation. Egg masses, that are usually on the underside of the leaves located above and below the ear zone, should be counted for a 3 to 4-week period during egg laying. The 2nd generation larvae pass through the same larval stages as the 1st generation. A 3rd generation may occur in southern areas.
Figure 2. 2nd generation European corn borer feeding in ear shank and associated frass.
Stalk tunneling affects the plant's ability to transfer nutrients and water, causing a potential loss in yield. Stalk breakage, lodging, shank tunneling, ear drop, and secondary stalk rots can also contribute to the potential for yield loss. According to Purdue University, an infestation averaging1.0 ECB larva/plant, just prior to tasseling, may reduce yields by 6.6% (Table 2).3 Table 2, shows the potential percentage of yield loss caused by ECB at various growth stages of corn. Extensive use of B.t. corn technology has contributed to a considerable suppression of ECB populations and economic return.1,2 An analysis of historical ECB damage in Minnesota estimated that B.t. corn for ECB provided an average benefit of $17.24/acre.
B.t. corn products for ECB, corn rootworm (Diabrotia virgifera), and corn earworm (Helicoverpa zea) management were estimated to be planted on 76% of the United States corn acres in 2013.3 Even growers of non-B.t. corn have realized economic benefits from the suppression of ECB. A study encompassing a five state region including Iowa, Nebraska, Illinois, Minnesota, and Wisconsin estimated the combined cumulative benefit value to be 16.9 billion dollars in ECB protection to B.t. and non-B.t. corn growers during the period from 1996-2009.2 However, ECB populations can still threaten non-B.t. corn products and growers using non-B.t. corn products may need to rely on precisely timed insecticide applications for yield and grain quality protection.
Economic thresholds for ECB are dynamic as they change based on plant growth stage at time of infestation (Table 2), expected yield potential, estimated price/bushel, and insecticide treatment cost/acre. Additional field information such as percentage of plants with whorl feeding and average number of larvae/infested plant should be determined to assess 1st generation infestation levels. For 2nd and later generations, determine average number of egg masses/plant. Granular or liquid insecticide applications directed over the whorl provide about 80% control of 1st generation larvae.1,2,3,4 Insecticide applications for 2nd generation larval control is estimated to be 60 to 75%.1,2,. Field scouting and timing of insecticide application are critical for ECB control. Once larvae begin boring into the stalk, insecticide applications are ineffective. Multiple applications may be necessary to provide adequate control for both 1st and 2nd generations of ECB. It’s important to remember that some insecticides can flare mite populations, primarily due to the loss of natural enemies killed by the insecticides.6 Scout for mites within 5 days after an insecticide application. Management worksheets are available from state Extension Services that outline scouting procedures and help calculate thresholds.1
Corn producers may want to consider protecting yield potential by selecting B.t. corn products that provide protection against ECB yield loss. VT Double PRO® corn technology has dual modes of action against ECB and other above ground insects. Genuity® VT Triple PRO® corn and SmartStax® corn technologies provide protection against ECB, other above ground insects, and corn rootworms.
Sources: 1Gray, M.E. 2014. Remember the European corn borer? The Bulletin, June 30, 2014. University of Illinois. 2Hutchison, W.D., Burkness, E.C., Mitchell, P.D. and Moon, R.D. 2010. Areawide suppression of European corn borer with Bt maize reaps savings to non-Bt maize growers. Science 330:222–225. 3Fernandez-Cornejo, J., Wechsler, S., Livingston, M., and Mitchell, L. 2014. Genetically engineered crops in the United States. Economic research report number 162. Economic Research Service. USDA. 4Ostlie, K.R., Hutchison, W.D., and Hellmich, R.L. Bt corn and European corn borer, long-term success through resistance management. University of Minnesota. 5Cook, K.A., Ratcliffe, S.T., Gray, M.E., and Steffey, K.L. 2003. European corn borer. Insect Fact Sheet. Integrated pest management. University of Illinois. https://ipm.illinois.edu/fieldcrops/insects/european_corn_borer.pdf 6Ostlie, K.L. 2012. Managing two-spotted spider mite on corn. University of Minnesota. 7Krupke, C.H., Bledsoe, L.W., and Obermeyer, J.L. 2010. European corn borer in field corn. Bulletin E-17-W. Purdue University. Web sites verified 4/30/18. 141003080313