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. Icounty 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.