LIFE CYCLEThe European corn borer usually goes through two generations a year in major corn growing areas in Nebraska. In some years, there may also be a partial third generation. In far northwest Nebraska, there may be a mixture of one and two generation borers. There are four stages in each generation-- egg, larva (borer), pupa, and adult (moth)). The larva goes through five "steps" of growth (each step is called an instar, (Figure 1)). As the larva feeds it becomes larger and must shed its skin to continue growing. Temperature is a major regulator of corn borer development, and accumulated temperature units (degree-days, base 50) can be used to predict the seasonal occurrence of the subsequent life stages following captures of the first moth of the season in pheromone or light traps. (Figure 2) The borer overwinters as a full grown fifth instar larva in corn stalks, cobs, and plant debris.
First GenerationOverwintered larvae change into pupae in the spring and emerge as first brood moths approximately ten to fourteen days later during May and June. Moths aggregate or gather in weedy or grassy areas, normally field margins, to mate and drink water, usually in the form of dew. On warm, calm, humid evenings in June, female moths fly from these protected areas into corn to lay masses of 15 to 25 eggs near the midrib on the underside of the leaves (< 1% of the eggs may be laid elsewhere on the plant). The egg masses are flat, about 1/4 " in diameter and overlap like the scales of a fish. Freshly laid eggs are white, then turn pale yellow and darken just before hatching. Eggs hatch within five to seven days depending on temperature. Eggs that are about to hatch have distinct black centers and are referred to as "blackheaded". This is due to the black head of the larva showing through the translucent egg shell. Blackheaded eggs will hatch within twenty-four to forty-eight hours. Newly hatched larvae disperse and soon establish themselves deep in the whorl, feeding on developing leaves during the first two larval instars. As the leaves grow and unroll from the whorl, the "shot-hole" feeding signs (small round holes scattered in the leaf tissue) can be seen. Following a feeding period in the plant whorl (approximately 2 weeks for each larva), third instar larvae leave the whorl, bore into stalks and excavate tunnels (cavities), in which they complete development. Fifth instar larvae of this first generation change into pupae within the plant cavity, from which the summer, or second brood moths emerge in July and August. Occasionally, larvae will pupate outside the stalk on the corn leaf.
Second GenerationSecond brood summer moths generally emerge in late July and August, behaving like first brood moths by moving to grassy or other dense low vegetation near or inside corn fields to mate and drink water. In weed-free corn fields these areas may be fence line bromegrass or adjacent soybean or alfalfa fields. If grassy weeds are present in the cornfield, moths may remain in the cornfield and aggregate in weeds or patches of volunteer corn. Summer moths lay over 85 percent of their eggs on the undersides of the ear leaf and the three leaves above and three leaves below. After hatching, second generation larvae feed on the leaves and in leaf axils for a few days (particularly if pollen is available), then move behind leaf sheaths and the leaf collar area, or into ear tips. Third instar larvae bore into the stalk, ear shank, or ear. These larvae usually overwinter and do not pupate until the following spring. In years of extended growing seasons with greater than average degree-day accumulations, a small proportion of the larvae pupate, and produce moths, giving rise to a third brood of moths. Eggs laid by these moths are not of economic significance, since the corn plant at this late date is normally well beyond the period of susceptibility.
DAMAGEEuropean corn borer larvae feed on all above ground tissues of the corn plant. They also bore into, feed, and tunnel within the tassel, ear, ear shank and stalk, forming cavities. Cavities produced by borers interfere with the translocation of water and nutrients. Cavities also reduce the strength of the stalk and ear shank, thereby predisposing the corn plants to stalk breakage and ear drop, which is aggravated by high winds or other adverse environmental conditions.
YIELD LOSSESYield losses due to damage by the larvae are primarily due to reduced ear and kernel size (physiological losses) as well as broken plants and dropped ears (potential harvest loss). Larvae feeding in the ear may cause seed yield loss and/or reduce quality in seedcorn, popcorn, and fresh market sweet corn. Within a range of 1 to 6 borers per plant, the relationship between the average number of larvae and yield appears to be linear and likely ranges from about 2 to 10% yield loss per borer per plant. Yield losses from ECB are highly correlated with 1) plant stage 2) water stress and 3) the hybrid.
1) Plant stage. Percentage yield loss differs depending on the plant growth stage at the time the damage occurs. Cavity formation by the first generation borer usually occurs before tassel emergence resulting in approximately 5 percent yield loss per borer per plant. Yield losses (per borer) from second generation larvae vary widely because cavity formation may occur over several weeks, and rapid physiological changes occur as the ear is approaching maximum size and physiological maturity. The average value of about 5 percent yield loss per borer per plant appears appropriate until shortly after pollen shed. As the ear advances from the blister stage to physiological maturity, the yield reduction per borer rapidly decreases.
Percentage yield loss caused by European corn borer for various corn growth stages.North Central Regional Extension Publication No. 327)
2) Moisture stress. Moisture stress appears to play a significant role in the plant's response to the borer. Percentage yield losses/borer generally increase as soil moisture decreases. Conversely, adequate moisture conditions for the plant will reduce ECB losses. The capacity to optimally irrigate therefore, is an important factor in reducing yield losses.
3) Hybrid. Hybrids vary in their response to feeding by the European corn borer. Resistance to whorl, and sheath and collar feeding, is available in some commercial hybrids. Some varieties have the genetic capacity to yield better than others in the presence of borer damage. Relative stalk strength, firmness of the plant rind, and shank strength influence borer establishment on the plant, the amount of stalk breakage and the capacity for ear retention in the event of a corn borer infestation. Several seed corn companies are currently marketing Bt transgenic corn. Bt-corn hybrids produce an insecticidal protein derived from the bacterium Bacillus thuringiensis, commonly called Bt. These Bt hybrids provide a very high degree of control against damage by the first generation borer. Control of second generation European corn borer larvae varies from moderate to excellent depending on the hybrid and the degree and location of expression of the insecticidal trait. In all cases however, control should either compare favorably or significantly exceed that which could be achieved be a conventional insecticide application. Follow all resistance management recommendations when planting Bt corn.
METHODS FOR TRACKING SEASONAL ACTIVITYThere are several methods to help determine the seasonal peak activity of the ECB moth, including use of light traps and pheromone traps, degree-day accumulations, a computer software program and fieldscouting. Except for field scouting, none of other methods are field specific and cannot be used to predict the magnitude of an infestation of ECB.. So, regardless of the method used to monitor seasonal activity, site specific scouting of individual fields must be used to determine the need for and optimum timing of insecticide applications.
Light traps attract insects to a light source, and then trap them in a receptacle containing a killing agent. Trapped insects should be counted on a regular basis (daily, or several times a week). A fluorescent 15-watt ultraviolet (black light) lamp commonly serves as the light source. The disadvantage of light traps is that several species of flying insects are attracted to the light source and therefore the nightly catch needs significant sorting to assess ECB moth numbers. (See the end of this document for sources of light traps).
European corn borer pheromone traps contain a substance that mimics a part of the chemical communication system between male and female corn borer moths. Specifically, it mimics the chemical emitted by those females that are receptive to mating. Only male ECB moths are attracted and captured and therefore no sorting is required and counting is relatively quick. This lure (Iowa type) is commercially available and is used within a rigid paper trap coated with a sticky substance that retains the attracted insects. Trap catches are monitored on a regular basis. At present, little information is available regarding the interpretation and use of pheromone traps for surveying ECB. They have been used, however, to detect the initiation of moth flights. (See the end of this document for sources).
A third method uses temperature accumulations called degree days. Since temperature is a major regulator of borer development, accumulated temperature units (degree days, 50o F base temp.) can be used to predict the seasonal occurrence of the subsequent life stages following capture of the first moth of the season in pheromone or light traps (Figure 2). Degree-day accumulations not tied to the first moth capture are much less accurate in predicting corn borer development.
Computer software is available that runs on IBM and compatible computers equipped with at least one 5 1/4 inch floppy drive. The software contains a phenology model that predicts the egg-laying time of second brood (summer) moths based on the age distribution from a collection of the earlier first generation larvae (the management portion of the model will be discussed later). Degree-days for the phenology model may be based on either 30-year county averages (program default values) or local temperatures for the current year. This is the most accurate method to determine scouting times for second generation egg laying. Correctly identifying the larval stages or instars of the first generation larvae is important to the success of running the model.
European Corn Borer Larval Instar Determination Table
A final method of tracking the seasonality of European corn borers relates to scouting efforts in the field for the life stages of the borer. Scouting individual fields is still the BEST way to assess ECB population progress and magnitude of infestation. A video tape on scouting for European corn borer is available for additional specific information in addition to the information stated elsewhere in this document.
CULTURAL CONTROLSStalk shredding, plowing, grazing, or burning stalks. These activities may reduce overwintering corn borer populations in individual fields, but do not result in reduced damage the next year since eggs are laid by the winged adult and fields may be repopulated by moths moving in from other fields.
Variety selection. As previously noted, percentage survival of borers feeding on the corn plant varies with the corn hybrid. Some varieties of corn are resistant to first generation ECB larvae whorl feeding. In resistant hybrids, borers die shortly after feeding on plant whorl tissue although some "shot holing" may be noticed. Because the resistance factor causes death of the insect, it is called antibiosis and is largely attributed to a substance in the plant called DIMBOA. These hybrids maintain this resistance trait up to tassel emergence. Some varieties of corn may also express sheath and collar feeding resistance applicable against second generation borers. This resistance trait may be in addition to or apart from resistance to whorl feeding. As in whorl feeding resistance, borers die soon after feeding. There also is variability across hybrids in the plant's response in relation to stalk and ear shank strength, plant rind toughness and ear retention characteristics. These resistance traits are classified as tolerance.
Maturity group.The maturity group of the hybrid will dictate when the hybrid will flower and be most attractive to the egg-laying moth. An early maturity group hybrid flowering (tasseling and pollinating) before peak moth flight will have significantly fewer corn borers. Conversely, long season hybrids that pollinate during the summer moth flight, are particularly susceptible to borer establishment. Check with your seed corn representative for information on specific hybrids. Also see E.C. 105, "Nebraska Corn Performance Tests," published annually by the Nebraska Cooperative Extension Service and the Nebraska Agricultural Research Division. This publication includes information on the percentage of broken plants and percentage of dropped ears for the tested corn hybrids.
Planting date. Planting date is important because the height of the corn relative to surrounding corn during the first brood moth flight and the maturity of the plants relative to surrounding corn during second moth flight will helpdetermine the vulnerability of the hybrid to an ECB infestation. Moths of the first flight in June are most attracted to the taller corn, but will lay some eggs on shorter corn (2- to 4-leaf stage). Borer establishment will be poor in corn plants with less than six fully expanded leaves (whorl height approximately 10 inches). The second flight of moths emerging in late July and early August prefers to deposit eggs on corn at or near pollen shedding. If corn is planted late and/or a particularly long season variety is planted, the late maturation of that corn relative to surrounding corn may make it highly attractive to egg laying moths.
Early harvest. The longer corn remains in the field after maturity, the greater the amount of stalk lodging, breakage, and ear drop. This is especially true in fields heavily infested with ECB. Scout fields in the fall and make an assessment on potential harvest losses from stalk breakage and ear droppage. Then compare early harvest and drying expenses to potential losses if the corn is not harvested until an acceptable moisture level is reached.
NATURAL CONTROLSWeather. The environment plays a major role in the severity of ECB infestations. Cool nights (below 60oF) reduce egg laying, and hot, dry, windy weather will dry egg masses causing them to curl and drop from the leaf. Heavy, drenching rainfall just after egg hatch will drown small corn borers and cause a high mortality of moths. Even when plants are in an optimal growth stage, temperature-related climatic variables, such as moisture stress and evaporation, will kill significant numbers of newly hatched larvae. Weather conditions during adult moth mating, egg laying and egg hatch are very important in determining the size of the corn borer populations in a given year.
Disease organisms. Two disease organisms are sometimes effective in reducing corn borers. One is a widespread fungus, Beauveria bassiana, that attacks and eventually kills the larvae. More than 50% of overwintering populations of borers can be killed in some years. The other, a protozoan organism, Nosema pyraustae, infects eggs, larvae, pupae, and adults. This results in fewer eggs being deposited and reduced larval survival.
Predators and parasites. Several birds feed on overwintering borers within the stalks, while lady beetle larvae and adults and lacewing larvae feed on eggs and newly hatched larvae. A native lady beetle Coleomegilla maculata , can be a significant predator of ECB egg masses. A tachinid fly, Lydella thompsoni, is a common and widespread parasite of ECB and has some success in biological control. Two wasps, Eriborus terebrans and Macrocentrus grandii parasitize 2nd to 4th-instar ECB larvae and are widespread in Nebraska. Another parasitic wasp, Sympiesis viridula, may also be present in Nebraska, but its impact is small during most years.
WHEN TO TREAT FOR ECB ON CORN
First Generation:Plan to scout all corn fields for a 2- to 4-week period following peak moth flight as determined by blacklight or pheromone trap collections in your area. (See the end of this document for sources of light traps and pheromone traps). This generally occurs from June to July from south to north across Nebraska. Because of the higher altitude, moth flights in southwestern Nebraska will be similar to northern Nebraska and the latest flights will be in northwestern Nebraska. To determine the need to treat for first generation borer, examine the corn whorls in each field, noting the percent of total plants damaged with "shot-holes", and determine the average number of larvae per damaged plant. Sample enough plants at enough locations in each field to ensure that sample estimates are representative of the entire field. Where possible samples should be taken from at least five representative sets of 20 plants for every 40 to 50 acres. The chances of makIng a wrong decision increase greatly if fewer samples are taken. Pull the whorls from five or more damaged plants at each site, unroll the whorl leaves and count the number of live corn borers per plant. Mortality of newly hatched larvae is very high. If possible, avoid making a treatment decision until larvae have aged to second instar or older but are still in the plant whorl and all infested plants show clear evidence of whorl feeding. Borers still in the whorl can be controlled chemically. If all larvae have left the whorl, it is too late to attempt control.
To make a decision on first generation borer treatment, the following information is needed:
Example:Assume that 50 percent of the corn plants in a field are damaged with an average of 4 larvae/damaged plant, that final yield expectation is 125 bu/acre, and that corn is worth $2.75/bu. Additionally, let us assume a 5 percent yield loss for each borer/plant, insecticide costs are $8.00/acre, and that application costs are another $4.00/acre (total = $12.00/acre). Estimated percent control is 75 percent.
ECB Treatment Decision Calculation Table
Preventable loss amount vs. total costs = $25.78 vs. $12.00(AN INSECTICIDE APPLICATION IS ECONOMICALLY JUSTIFIED IF PREVENTABLE LOSS EXCEEDS THE TOTAL COST OF APPLICATION).
To perform your calculations on our new electronic worksheet,
|Days after initiation of egg laying||Proportion of egg-laying completed||Days after initiation of egg laying||Proportion of egg-laying completed|
Example:A field of corn is at the blister stage and expected to yield about 175 bu/Acre with an anticipated market value of $2.50 per bu. Control costs are $13.00/acre and the expected percentage control with the single application is 75%. Further suppose that a scout estimates a population of egg masses in a field at 0.25 masses/plant by observing and counting an average total of 1 egg mass every fourth plant. This scouting period was 5 days after the first eggs were found in this field and estimates are that approximately 32% of the eggs have already been laid (Table 2, 5 days after finding first egg mass and back the date another 3 days = 8 days after initiation of egg laying)
1. Calculating the economic threshold or ET:
- CC = Estimated control costs (includes the cost of the insecticide and cost of application.
- YL = The percentage estimated loss in yield per borer per plant. (Consult table 3 below).
- MV = The estimated market value of the corn when you intend to sell it.
- EY = The expected yield in bushels/acre.
- PC = The percentage control expected if an insecticide is applied.
Therefore, based on this example, a potential pest density of 1.32 borers/plant is necessary to break even.
2. Calculating the potential population density or PPD:
- SV = Average proportion of individuals surviving through the damaging stage. Use the conservative value of 0.20.
- EM = Average number of eggs in each mass. Use the number 23.
- MP = Average number of egg masses per plant based on scouting.
- PO = Proportion of eggs already laid, based on a 20 day ovipositional period in each field (See Table 2).
Therefore since the potential pest density of 3.59 borers per plant exceeds the economic threshold of 1.32 borers per plant, it would be profitable to apply an insecticide.
The key to controlling second generation borers is to time applications when the first egg masses begin to hatch. Best control and economic return will be achieved when initial egg hatch coincides with reaching the economic threshold. As the plant matures beyond the blister stage, potential economicbenefits of a successful insecticide application rapidly decline.
There is uncertainty about the feasibility of multiple applications of insecticides to offset the extended ovipositional period of the moth during some years. Limited research indicates a well-timed single application is sufficient in most years. If other insect pests are present and or ECB moth population levels are high, liquid formulations are preferred over granules for second generation control because of their broader spectrum of activity, and the added advantage of obtaining some moth control.
BE ALERT FOR POSSIBLE BUILDUP OF SPIDER MITES AFTER INSECTICIDE APPLICATION.
METHODS OF INSECTICIDE APPLICATIONEuropean corn borer insecticides can be applied as granules or sprays. Granules can be applied for first generation control by tractor-mounted or drawn equipment, high clearance ground applicators, or byaircraft. Granules for second generation control can be applied with high clearance ground applicators or aircraft.
Liquid insecticides can be applied through center pivot irrigation systems (chemigation). In Nebraska, chemigation is regulated by the Nebraska Chemigation Act. Applicators must be certified chemigation applicators and all injection sites must be permitted. This method, while proven effective when compared with more conventional application techniques, requires proper equipment (including safety devices), an understanding of how to use it properly, and monitoring to ensure proper application, safety to others and to the environment. Proper equipment, procedures for calibration, and other instructions for application through center pivot systems is provided in two documents - "Applying Insecticides Through Center Pivots" (G84-703) and "Anti-pollution Devices for Applying Chemicals Through Irrigation Systems" (G73-43). Refer to product labels for specific instructions in handling and proper injection procedures, precautions, and restrictions.
Conventional ground sprayers, aircraft, or high clearance sprayers can be used for applying liquid insecticides. Excepting chemigation, liquids have not been effective against first generation borers because the water volume is not sufficient to carry the insecticide inside the plant whorl. High clearance sprayers or aircraft fitted with multiple nozzles to ensure thorough coverage of leaf axils and the ear zone can be used for control of second generation borers.
Recommended insecticides and rates of application can be found in the current edition of Extension Circular EC-1509, "Insect Management Guide for Nebraska Corn and Sorghum," available from your Nebraska Cooperative Extension office. Products based on the naturally occurring bacterium, Bacillus thuringiensis are effective against first generation corn borers and have advantages over other insecticides in terms of applicator safety, and reduced toxicity to insect predators and parasites, honeybees and wildlife. These products have not been as effective as synthetic insecticides when used for control of second generation corn borers.
PEST MANAGEMENT SAFETYAlways use integrated pest management programs when making plans to reduce losses due to pests. Integrated pest management (IPM) is a sustainable approach to managing pests by combining biological, cultural, physical, and chemical tools in a way that minimizes economic, health, and environmental risks. Economic risks can be minimized through the utilization of approved pest scouting techniques and by implementation of economic thresholds to make pest management decisions. Health risks can be minimized by following all safety directions provided on pesticide labels. Environmental risks can be minimized by avoiding pesticide usage when possible and by following all environmental or wildlife safety guidelines provided on the pesticide label when pesticides must be used. Insecticide Recommendations for Second Generation European Corn Borer Control in Field Corn
- Light Traps:
- O. B. Enterprises, Inc., 4585 Schneider Drive, Oregon WI 53575, 608-835-9416
- Pheromone Traps:
- (Scentry Wing traps with Iowa Strain ECB lure)
- Ecogen, Inc., Scentry Division, 2005 Cabot Blvd. West, Langhorne PA 19047
- Phone: 1-800-220-3326, or (215)-757-1590.
- Great Lakes IPM, 10220 Church Road NE, Vestaburg, MI 48891
- Phone: (517) 268-5693
- European Corn Borer Phenology and Management Model:
- Send orders with payment (payable to University of Nebraska) to Bob Wright,
- University of Nebraska South Central Research & Extension Center.
- Box 66, Clay Center, NE, 68933-0066. Or, contact Bob at (402) 762-4439.
- Specify 3 1/2 or 5 1/4" disk size. $71.23 (includes sales tax, shipping and handling)
- Insect Scouting in Corn, 26 min., VHS format; $29.95 + sales tax;
- Send orders with payment (payable to University of Nebraska) to
- Field Scouting, 104 ACB, UNL, Lincoln NE 68583-0816.