The European Corn Borer EILEEN CULLEN and JOHN WEDBERG

A1220

The European corn borer EILEEN CULLEN and JOHN WEDBERG

Common name Life cycle — laying. High temperatures and low ScientificEuropean name corn borer European corn borers pass the humidity will increase moth mor- —Ostrinia nubilalis winter as full-grown larvae, usually tality. The number of viable eggs laid Appearance living inside old corn stalks, in the depends upon the availability of stems of weed hosts, or in vegetable drinking water. (Dew is considered a Full-grown larvae of the stems left in the field. Spring devel- source of drinking water.) Heavy European corn borer range in length 3 opment begins when temperatures rains interfere with moth activity, but from ⁄4 to 1 inch and vary in color exceed 50°F. The larvae pupate showers do not. from gray to creamy white. The body during May, and moths appear in The first moths of the year are is covered with numerous dark spots June. Cool weather or drought may attracted to the tallest corn for egg and the head is black. Adults are delay spring development of borers laying. Female moths deposit egg straw-colored moths with a 1-inch while warm weather and adequate masses on the underside along the wingspread. Male moths are slightly moisture may accelerate it. Due to the midrib of lower leaves of young corn smaller and distinctly darker than lake influence and other moderating plants. It takes approximately 6 days females. Adult females lay eggs on factors, spring moth appearance for eggs to hatch at typical late-June the underside of leaves near the 1 3 lingers into July in the eastern and temperatures in central Wisconsin. midvein. These egg masses are ⁄8– ⁄16 northern portions of Wisconsin. Hot, Upon hatching, these first-gener- inches long and contain approxi- dry spring weather may kill pupae. ation larvae move almost immedi- mately 20–30 eggs which overlap like After emergence, moths spend fish scales. As they develop, the eggs the daylight hours in sheltered areas change from white to a creamy color. (weeds and grasses) in or bordering Just before hatching, the black heads corn fields. On warm, calm evenings, of the larvae become visible. This they fly about in corn fields and final egg stage is referred to as the deposit eggs. Below-normal tempera- “black-head” stage. tures will reduce flight activity thereby reducing mating and egg

European corn borer egg mass shortly before hatching.

European corn borer larva. “Pin-hole” damage caused by first- generation European corn borers.

THE EUROPEAN CORN BORER

Damage Processing sweet corn ately to the plant whorl where they First-generation borers begin to feed. As the leaves lengthen, The major injury to field corn is require this damage begins to look like pin- stalk tunneling which impairs plant less monitoring on processing sweet holes. Larvae from one egg mass will growth and reduces potential yield. corn than fresh market corn. Since the scatter over several adjacent plants. Second-generation larvae cause addi- corn matures later, leaf whorl feeding

By the third stage (instar) of develop- tional yield losses from broken stalks Ican be used as a sign of infestation. ment, larvae tunnel into the midrib of and dropped ears. Although canning Sample 10 plants in each of five the leaves, eventually burrowing into and fresh market sweet corn also I different areas in a field. the stalk of the plant. It takes approxi- suffer from stalk tunneling, the major During the whorl stage, treat if mately 22 days for borers to reach the damage in sweet corn is to the ears. 25% of the plants are infested full-grown (fifth instar) stage. Because borers damage the kernels, with live larvae or eggs, or show

Larvae that mature early in the corn intended for canning or freezing I signs of leaf feeding. season pupate inside the green corn must be handled and trimmed prior During the late-tassel to silk stalk. Approximately 12 days later to processing. This type of damage in stages, treat if you find live larvae moths emerge to mate and lay eggs fresh market sweet corn results in loss I or eggs on 4–5% of the plants. for the second generation. First-gener- of the entire ear because of consumer Repeat treatments every 3–5 days ation larvae that mature later in the rejection. if you continue to find live larvae season do not pupate. Instead, they Most of the ear damage occurs or eggs on 4–5% of the plants. enter a diapause condition (physio- from infestations arising during the Two to three applications may be logical arrest) and overwinter as full- row tassel (RT) stage†. This is when necessary during years with grown larvae. Most corn borers in tassels have emerged on most of the Second-generationheavy infestations. borers northern Wisconsin follow this latter plants and pollen or silks are present pattern. on 10% of the crop. attack late-planted processing corn. The summer moth flight, which Scouting suggestions Blacklight insect traps provide useful leads to second-generation larvae, Fresh market sweet corn records in June for first flight, and are peaks around mid-August in central even more worthwhile in August for Wisconsin. In tasseling corn, approxi- Fresh market sweet corn in monitoring the second flight. Scout mately 90% of the eggs are laid on the southern Wisconsin may be tasseled areas that have different planting two nodes directly above and below in late June when egg masses for the dates or varieties separately because the ear (four nodes total). Young first-generation borers start to hatch. these differences can affect the extent Monitoring for early evidence of borer larvae hide behind leaf sheaths and Iof egg laying. under ear husks. They enter the silk attack is crucial as treatments must be Scout every 5–7 days. Sample at channel at the tip of the ear. Larvae properly timed to keep the devel- least 10 plants in each of five oping ears essentially free of borers. that hatch after tasseling enter the I areas of a field. stalk or ear. The larvae that reach the Blacklight traps attract moths and Examine leaves for egg masses fifth instar during September and can help you monitor population and larvae; also look on husk intensity and activity. Contact your October will overwinter along with I leaves arising from the ear tip. those larvae that diapaused in August. countyI Extension office for trap data. Check emerging tassels and in Heavy rains occurring before the Begin checking plants for egg leaf collars for larvae; treat if you young borers tunnel into leaves and masses by June 15 in southern find live larvae or eggs on 4–5% Wisconsin. Examine 10 plants in stalks will kill many larvae either by I of the plants. washing them from the plant or by I at least five different areas. Repeat treatments every 3–5 days drowning them in the whorl or leaf Through early silk stage, treat if if you continue to find live larvae collar area. 5% of the plants have egg masses, or eggs on 4–5% of the plants.

I larvae, or leaf-feeding damage. Two to three applications may be Repeat treatment every 3–5 days necessary during years with if more than one unhatched egg heavy infestations. mass remains per 20 plants. †From Vegetable Insect Management—With Emphasis on the Midwest edited by Rick Foster and Brian Flood. Meister Publishing Company, Willoughby, OH. 1995

Field corn Cultural control Spores produced by the bac- Field corn that’s been planted European corn borers overwinter terium contain a protein which, when early attracts egg-laying moths in in corn stalk residue. Therefore, ingested by a susceptible insect, rup- June. The guideline for determining removing corn by harvesting it for tures the gut lining, ultimately killing treatment needs for first-generation silage will kill a high percentage of the insect. There are many different borers depends on the number of the borers, perhaps as much as 80%. strains of Bt, each with specificity plants with fresh whorl feeding. Do Deep moldboard plowing (where soil toward a particular group of insects. not wait until plants show signs of erosion is not a major concern) and Bt has been commercially avail- severe leaf feeding before applying an stalk shredding will also substantially able as a foliar insecticide for lepi- insecticide. By this time the borers are reduce within-field populations of doptera larvae such as European corn larger and more difficult to kill and, borers. However, these procedures borer for decades. Transgenic Bt corn more importantly, they have already will not eradicate European corn hybrids with activity against tunneled inside the stalk and are pro- borer because it also infests numerous European corn borer larvae have been tected from the insecticide. non-cultivated plants. Also, the poten- commercially available since 1996. Both first- and second-generation tial for soil erosion and the costs Incorporation of the Bt toxin into the borers reduce potential yield by tun- involved with extra equipment and corn plant overcomes some of the dif- neling in the stalk. However, most of trips over the field limit the useful- ficulties associated with conventional the damage—broken stalks and ness of plowing and stalk shredding. Borer-resistant varieties insecticides (human exposure to toxic dropped ears—found at harvest time chemicals associated with mixing and is caused by second-generation Two types of borer-resistant corn loading procedures, death of benefi- borers. Due to the extended egg- plants are commercially available: cial insects, etc.). laying period in August, you’ll need plants with native resistance and Unlike Bt microbial sprays, which to sample fields weekly for an accu- plants that have been genetically have a field residual measured in days, rate picture of the potential damage. alterNativeed (transgenic). resistance the Bt toxin in transgenic Bt corn is Control refers to nat- expressed throughout the life of the Natural control urally occurring plant chemical com- corn plant. This gives more consistent pounds that can either repel or kill an and economic control when European Natural control plays a key role in insect. It is important to remember corn borer populations reach moderating European corn borer pop- that resistance does not necessarily threshold levels. However, there is a ulations. Weather conditions, preda- mean that the plant is immune to significant risk that with constant tors, parasitoids, and diseases take insect damage. Some seedlings, for exposure of target insects to Bt toxins, their toll on borer populations. Given example, produce DIMBOA, a chem- resistant pest populations may ideal conditions, natural controls can ical that is lethal to first-generation develop. kill off approximately 80–90% of the borers. However, by the time the plant To help protect against rapid eggs and larvae. Although augmenta- reaches an extended leaf height of development of insect resistance, the tive releases of predators and para- 17–24 inches, the chemical is suffi- U.S. Environmental Protection sitoids cannot be solely relied upon to ciently diluted for borers to feed Agency has mandated that growers keep populations at acceptably low safely. Check with your seed supplier follow insect resistance management levels, conservation of these natural for information on the relative native (IRM) protocols when planting trans- enemies is an important part of pest resistance or susceptibility of the vari- genic Bt corn hybrids. A minimum of management programs. We typically etiesT youransgenic are considering. Bt corn hybrids 20% of each field planted to Bt corn have 5–7 years of relatively low borer are must include a refuge area of non-Bt populations separating outbreak an example of genetically modified corn borer corn. This creates a refuge years, and this is primarily a result of organisms (GMOs) in which corn has of susceptible European corn borers natural control. Using insecticides had a gene inserted from an unrelated that have not been exposed to the Bt only when absolutely necessary is an organism, in this case Bt. Bt is an toxin. Susceptible moths from the important step in conserving natural abbreviation for Bacillus thuringiensis, refuge area mate with resistant indi- enemies. a naturally occurring soil bacterium. viduals that potentially could emerge

THE EUROPEAN CORN BORER

from the Bt corn, passing along sus- FIELD Management worksheet for ceptibility to the Bt toxin to their off- CORN FIRST-GENERATION European corn borer spring. Planting options that may be ______% of 100 plants infested x ______ave. # borers/infested planta used to meet the IRM planting = ______borers/plant requirement: (1) plant the refuge as a large block within or next to the Bt ______borers/plant x 5% yield loss/borer = ______% yield loss corn field, (2) split the planter to alter- ______% yield loss x ______expected yield (bu/a) = ______bu/a loss nate four or more rows (six or more ______bu/a loss x $______price/bu = $______loss/a preferred) of non-Bt corn with Bt corn, $______loss/a x _____ % controlb = $______preventable loss/a (3) plant field perimeters or end rows to non-Bt corn equal to 20% of the $______preventable loss/a – $______cost of control/a entire field, or (4) plant the refuge = $______gain (+) or loss (–) per acre if treatment is applied 1 area within ⁄2 mile (preferably within a 1 Determined by checking whorls from 10 plants. ⁄4 mile) of Bt corn fields. Note: bAllow 80% for most insecticides or, preferably, consult Extension publication Pest Mixing non-Bt seed with Bt corn seed Management in Wisconsin Field Crops (A3646) for an insecticide performance update. for use in the refuge is not permitted. For more information and recom- FIELD Management worksheet for mendations, refer to UW Extension CORN SECOND-GENERATION European corn borer publication Pest Management in Wisconsin Field Crops (A3646). a b Chemical control ______number of egg masses/plant x 2 borers/egg mass

For economic treatment thresh- = ______borers/plant olds and insecticide recommendations c ______borers/plant x 4% loss/borer = ______% yield loss for sweet corn, potatoes, snap beans, and peppers see Extension publica- ______% yield loss x ______expected yield = ______bu/a loss tion Commercial Vegetable Production in ______bu/a loss x $______price/bu = $______loss/a Wisconsin (A3422). On field corn, insecticide treat- $______loss/a x 75% control = $______preventable loss/a ments are not always warranted for $______preventable loss/a – $______cost of control/a controlling borer damage. Use the fol- lowing management worksheets to = $______gain (+) or loss (–) per acre if treatment is applied determine whether treating an infes- aUse cumulative counts, taken 7 days apart. tation will save you money. For treat- bAssumes survival rate of 2 borers/egg mass. c ment recommendations, refer to Use 3% loss/borer if infestation occurs after silks are brown. The potential economic benefits of treatment decline rapidly if infestations occur after corn reaches the blister stage. Extension publication Pest Management in Wisconsin Field Crops (A3646).

Copyright © 2005 University of Wisconsin-System Board of Regents and University of Wisconsin-Extension, Cooperative Extension Authors: Eileen M. Cullen is assistant professor and John Wedberg is professor emeritus of entomology, College of Agricultural and Life Sciences, University of Wisconsin-Madison and University of Wisconsin-Extension, Cooperative Extension. Produced by Cooperative Extension Publications, University of Wisconsin-Extension. University of Wisconsin-Extension, Cooperative Extension, in cooperation with the U.S. Department of Agriculture and Wisconsin counties, publishes this information to further the purpose of the May 8 and June 30, 1914 Acts of Congress; and provides equal opportunities and affirmative action in employment and programming. If you need this material in an alternative format, contact the Office of Equal Opportunity and Diversity Programs or call Cooperative Extension Publications at 608-262-8076. This publication is available from your Wisconsin county Extension office or from Cooperative Extension Publications. To order, call our toll-free number: 877-WIS-PUBS (947-7827) or visit cecommerce.uwex.edu. A1220 The European Corn Borer R-04-2005