Stalk Borers in Colorado Field Corn Fact Sheet No. 5.537 Insect Series|Crops by F.B. Peairs* European Corn Borer Quick Facts Description • Two species of stalk borers European corn borer eggs are laid in attack field corn in Colorado: masses of 15 to 30. The egg masses have a European corn borer, Ostrinia scaly, glossy white appearance (Figure 1). As nubilalis; and southwestern the eggs mature, the black head capsules of corn borer, Diatraea the developing larvae become visible, which Figure 3: European corn borer female (left) and grandiosella. is known as the “blackhead stage.” European male (right) moths. (Photos by F.B. Peairs.) corn borer larvae are cream colored to • Yield losses from either pinkish caterpillars which are marked with Life History species vary with the number small, round brown spots (Figure 2). The European corn borer usually has of larvae per plant and plant head capsule is dark or reddish brown. two generations in Colorado, although growth stage. Fully developed larvae are about one inch a single generation may occur at higher (25 millimeters) in length. Male moths are • Some naturally occurring elevations and a partial third generation distinctly darker and slightly smaller than has been observed on occasion. The two biological controls, including the pale yellow female moth (Figure 3). The generations are quite different in damage and insect predators, parasitic average adult wing span is about one inch. management because they occur at different wasps, parasitic flies, and The forewings are buff colored with darker stages in crop growth and development. pathogens have been bands running in wavy lines. First generation. European corn borers observed to affect both overwinter as fully grown larvae in corn species in the field. stalks and other plant residues. In early spring they pupate and emerge as moths • Bt corn varieties are effective during late May or early June. Adults fly at against both species. night but do not cause damage. On warm, calm, humid evenings in June, female moths fly from weedy or grassy margins into corn fields and lay eggs. Eggs are laid in masses on the underside of corn leaves, usually Figure 1: European corn borer egg mass. (Photo near the midrib. Once an egg mass reaches by S.D. Pilcher.) the blackhead stage, hatching will generally occur within 36 hours. Survival is poor on small plants (less than 18 inches). Larvae require from four to six weeks to complete development. Mature larvae pupate within the corn stalk. The second flight of moths emerges from mid‑July to early August to lay the eggs of the second generation. Second generation. Female summer moths prefer to lay eggs in corn that is tasseling and in the green silk stage. ©Colorado State University Later‑maturing fields are more attractive to Figure 2: European corn borer larva. (Photo by Extension. 6/02. Revised 10/14. F.B. Peairs.) egg‑laying moths than fields approaching maturity. These larvae usually overwinter and www.ext.colostate.edu do not pupate until the following spring. *F.B. Peairs, Colorado State University Extension entomologist and professor of entomology, bioagricultural sciences and pest management. 10/2014 Table 1: How to tell the difference between the European and Southwestern corn borers. European corn borer Southwestern corn borer Distribution Eastern Colorado, mostly in Southeast Colorado, rarely oc- sprinkler-irrigated corn in the curs as far north as Burlington. northeast. Adults Small, tan-to-gray moths. White moths with unmarked Wings marked with wavy wings. Wings fold tentlike brown lines. Wings fold flat. around body. Eggs Found in masses of 15 - 30. Found in masses of 3 - 5. De- Cream-colored, acquiring velop three reddish stripes as a black center as they near they mature. Usually on upper hatch. Usually on undersides leaf surfaces. Figure 5: Ear damage caused by the European of leaves. corn borer. (Photo by F.B. Peairs.) Larvae Cream to pinkish colored White larvae with distinct black larvae marked with distinct spots. Overwintering larvae brown spots. lose spots. Damage Tunnel in all parts of plant Similar to European corn except roots. borer, except girdles lower stalk in fall.

Table 2. Losses per borer. When leaves emerge, the “shot hole” % Grain Yield Loss feeding signs (Figure 4) in the leaves can be Plant Growth Stage per Borer per Plant seen. Most of the mature larvae will bore Early Whorl 5.5 into the stalks, feed, and finish development Late Whorl 4.4 there. First generation damage includes leaf Pretassel 6.6 feeding and stalk boring. Most second generation eggs are laid in Pollen Shed 4.4 the middle of the plant, on the undersides Blister 3.0 of the ear leaf and the three leaves above Dough 2.0 and the three leaves below the ear leaf. Newly hatched second generation larvae first feed on leaves and pollen in leaf axils and then move to leaf sheaths and collars, and ear tips. Partially grown larvae bore into the tassels, ear shanks, ears, and stalks. Second generation larvae cause ear damage (Figure 5); they tunnel in the shank (Figure Figure 6: European corn borer tunneling in corn 6); and feed on silks, kernels, and cobs. shank. (Photo by S.D. Pilcher.) Signs of infestation include: dropped ears, broken shanks, stalk breakage, sawdust‑like Planting and harvest dates may affect castings on leaves, and holes in the stalks. infestations and damage. Early planted Yield losses depend on the number crops and shorter season hybrids are more of larvae per plant and the stage of crop attractive to females in the first flight, growth. Losses per borer, for infestation which prefer taller corn. Later planted of one to six borers per plant, are given corn and longer season hybrids will be in Table 2 (taken from Mason, C. E. et al. less mature and therefore more attractive 1996. European Corn Borer Ecology and to females during the second flight, which Management. North Central Regional are attracted to flowering corn plants. The Extension Publication 327, Iowa State longer an infested crop remains in the University, Ames, IA). field, the more losses will be incurred from lodging and ear drop. More heavily infested Management fields should be harvested first. Heavily Figure 4: Feeding signs of the European corn Pathogens such as the fungus, Beauveria infested fields should be harvested as soon borer, often called “shot-holes”. as potential losses outweigh additional early (Photo by S.D. Pilcher.) bassiana, and the protozoan, Nosema pyraustae, may weaken or kill European harvest and drying expenses. corn borers. Lady beetle larvae and adults Some hybrids have useful resistance to Damage and lacewing larvae feed on eggs and newly the first generation European corn borers, Newly hatched, first generation larvae hatched larvae. A fly,Lydella thompsoni; which feed in the whorls and later enter the feed first on the leaf near where they and wasps including Eriborus terebrans, and stalk. Some hybrids tolerate stalk and shank hatched. As the larvae grow they move Macrocentrus grandii, parasitize European boring and thus are less likely to lodge to the whorl or leaf sheath area and feed. corn borer larvae. or lose ears. Seed companies can provide it will be important to consider resistance at each sample site). Once larvae have information on these traits in their hybrids. management requirements when selecting entered the stalk, control is impossible. There also are many hybrids, known hybrids and their acreage allocation. Survival of second generation larvae is as Bt corn hybrids, on the market that Zone 3 (Eckley, Wray, Wauneta) highest during pollination, so treatments are resistant to corn borers through the is characterized by light soils, relatively should be considered when weekly scouting insertion of a bacterial gene that causes the uniform crop maturity, and consistently has an accumulated total of 25 percent of plant to produce an insecticidal protein. large first and second generation European plants with egg masses. This percentage The level of European corn borer control corn borer flights. Also, second generation should be raised to at least 50 percent after achieved with Bt corn hybrids labeled for flights typically extend over long periods pollination. If egg laying continues after corn borers generally has been equal to or of time. Economic infestations from the treatment, a second application may be better than that obtained with insecticides. either generation are likely and often justified. The complex method is described Bt corn should be used only where the both generations have to be treated in the in the online High Plains Integrated Pest risk of European corn borer infestation same field. In addition, it frequently has Management Guide at highplainsipm.org. is high. Colorado State University been difficult to obtain adequate second Cooperative Extension entomologists generation control with a single insecticide recognize the following European corn application. The use of Bt corn hybrids are Southwestern Corn Borer borer risk areas within the Golden Plains recommended for this area, regardless of Description area (Kit Carson, Phillips, Washington planting date or maturity. Southwestern corn borer eggs (Figure 7) and Yuma counties) based on more than If you are not in one of the zones are laid in groups of three to five, and a decade of pest survey information. Pest described above, take a look at your average overlap each other slightly. They initially survey data are insufficient to characterize expenditures for corn borer management are creamy white but develop three reddish European corn borer risk for other parts of (scouting, insecticides, application) and stripes as they near hatch. Larvae are dull the state. your average losses to corn borers over white with dark brown to black spots Zone 1 (Burlington, Bonny Dam, the last five years. Your annual total of forming a regular pattern along the body Kirk) is characterized by heavy soils and management costs and crop value lost (Figure 8). These spots generally disappear consistently low light trap catches. Areas should be similar to the cost of switching. in overwintering larvae (Figure 9). Fully in this zone generally are not expected Granules applied by ground and air, as to have economic European corn borer well as some center‑pivot applied liquids, infestations, although growers in the Kirk have given the best first generation control area experience occasional problems. results in university tests. Aerially‑applied Growers in Zone 1 should select well- and center‑pivot applied liquids have adapted non-Bt corn hybrids, scout for performed better than granules for control insect pest problems, and apply appropriate of second generation larvae in university insecticides if justified. The exception might tests. Monitor treated fields for spider be the Kirk area because it has a long (four mites as applications for control of second to five week) second generation flight in generation are often associated with mite Figure 7: Southwestern corn borer eggs. (Photo some years, increasing the probability that outbreaks. See highplainsipm.org for by S.D. Pilcher.) late planted or late maturing varieties will available insecticide products. experience economic infestation. Bt corn hybrids might be an appropriate choice for Treatment Guidelines these situations. The need to treat European corn borer Zone 2 (Yuma, Clarkville, Holyoke) can be determined by a simple method has a high probability of late planting or based on average Colorado conditions, or late maturing varieties due to heavy soils. by a more complex method which takes These areas also have a consistent history into account treatment costs, individual Figure 8: Southwestern corn borer larvae. of a prolonged second generation flight field yields, and current market conditions. (Photo by F.B. Peairs.) that result in economic infestations. The Incorrect treatment decisions, by either Bt trait would be an appropriate choice method, are more likely with second for late planted or late maturing hybrids generation infestations because the second in these areas. The prolonged flight makes generation egg laying period, which can last treatment decisions very difficult. The up to four weeks, makes proper treatment infestations accumulated over the season timing very difficult. Try examining five are economically significant, but not sets of 20 plants per 50 acres of corn – enough of the infestation occurs in any fewer samples will increase the chance for two week period to justify the use of an incorrect treatment decisions. insecticide. The first generation flight is By the simple method, chemical control consistent enough in these areas that Bt of the first generation is economical when hybrids might also be considered for early 25 percent of the plants have feeding Figure 9: Southwestern corn borer planting situations. Although there is not a damage and live larvae are present in the overwintering larvae. (Photo by S.D. Pilcher.) consistent need for Bt hybrids in this zone, whorls (check five whorls for live larvae in the whorls and stalks of the plant. After Management pupation, a second moth flight occurs. Eggs Management options are similar to generally are laid on the upper leaf surfaces those for European corn borer. Some in the ear zone of tasseled plants. When general predators, such as lady beetles second generation larvae have completed and lacewings, feed on southwestern corn their development, they tunnel down borer eggs and larvae. Early planted corn to the base of the plant to prepare their may escape second generation damage. overwintering site. Here they chew a groove Crop rotation may help reduce yield losses. around the inside of the stalk a few inches Cultural practices that promote strong above the soil surface. Then they tunnel out healthy stalks help mitigate losses due to FIgure 10: Southwestern corn borer moth. a hibernation cell in the underground part (Photo by J.L. Capinera.) lodging. Early harvest can help minimize of the crown of the corn plant, go through lodging and girdling losses. Fall and winter a molt in which they lose their spots, and stalk destruction, on an area wide basis, can finally, plug up the entrance to the cell. reduce overwintering populations. Short season varieties often escape Damage second generation damage. Bt corn hybrids First generation larvae feed in the labeled for corn borers are as effective whorls of young corn plants and, unlike against southwestern corn borer as they are European corn borer, do well on whorl against European corn borer. stage plants of any age. This feeding may result in “dead heart,” due to the death of Treatment Guidelines the growing tip. About half way through Check corn for first generation egg their development they bore into the stalk. masses and larvae from mid-May through FIgure 11: Southwestern corn borer lodging. Young second generation larvae (Photo by S.D. Pilcher.) June. Second generation should be scouted appear near tasseling and feed in the ears from mid-July through August. Consider or leaf sheaths. Half‑grown larvae of the grown larvae are from 1 to 1.25 inches long. chemical control if 25 percent of the second generation bore into the stalk. plants have egg masses or newly‑hatched Moths are white to pale yellow in color and The stem girdling during preparation for about 3/4 inch long (Figure 10). The wings larvae of either generation. The comments overwintering weakens plants and makes on chemical control of European corn have no distinct markings and are held them susceptible to lodging (Figure 11). tentlike over the body. borer, regarding application methods and Grain yield losses, exclusive of lodged or spider mite concerns, also are valid for girdled plants, of about 30 percent have Life History southwestern corn borer. been recorded for plants infested with both The southwestern corn borer and the first and second generation larvae. European corn borer have similar life histories. Overwintering larvae pupate in the spring and emerge as adults about Interesting Web sites: 10 days later. Adults are active primarily at night but can be found during the Bt Corn Hybrid Information: day under leaves. These moths lay eggs http://cls.casa.colostate.edu/transgeniccrops/what.html that produce the first generation of High Plains Integrated Pest Management: www.highplainsipm.org southwestern corn borers. Generally, eggs are laid on the upper surfaces of leaves. Larvae hatch and feed for four to six weeks

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