European Corn Borer

European corn borer is a destructive pest of corn in the United States and was the first target of Bt corn in 1996. Since then, European corn borer populations significantly declined and farmers have not had to worry about this pest for several decades. Recent resistance development by European corn borer to Bt hybrids in Canada and Connecticut is concerning, and farmers should plan to scout for European corn borer and injury to corn and report any potential issues in Bt hybrids.

Identification

Adult: European corn borer adults are small moths, approximately ½ to 5/8 inches long, that appear triangular when at rest. Both sexes have irregular wavy lines across the front wings, but females are a light tan color while males are darker brown (Photo 1). The tip of the abdomen is usually visible behind the wings on males.

Egg: European corn borer eggs are approximately ¼ inches long and somewhat flattened but are laid in small masses, overlapping like fish scales (Photo 2). The eggs are creamy white until they are ready to hatch, then the black heads of larvae are visible.

Larva: Young European corn borer larvae are dirty white with distinctly black heads when they first emerge from eggs (Photo 3A). As they get larger, the head turns dark brown. Older larvae may be pinkish-gray or pale brown and have a dark gray stripe down the back and four small spots on the top of each segment (Photo 3B). Mature larvae are approximately 1 inch long.

Pupa: European corn borer pupae are dark red to brown with slightly rounded heads and a pointed end (Photo 4).

Biology

Distribution: European corn borer is native to western Asia and Europe and was introduced to the United States in the early 1900s in Massachusetts. Now, European corn borer can be found in almost all corn-growing regions east of the Rocky Mountains, and it has been present in Iowa since 1942.

Host Plants: European corn borer has a number of host plants, but the most common ones in Iowa are field corn, popcorn, seed corn, sweet corn, sorghum, wheat, and vegetables (peppers, apples, beans, potato). They can also infest a variety of weeds, like foxtails, pigweeds, ragweeds, and smartweeds.

Life Cycle: Mature larvae of European corn borer overwinter in crop residue (e.g., cornstalks, corn cobs) or weed stems. In the spring, they resume development once temperatures exceed 50°F. Moths emerge and fly to aggregation sites, particularly dense grassy areas, to get moisture and find mates. Adults do not feed but seek out areas to drink water. Females are ready to mate after approximately two days, and it takes an additional three days to begin egg-laying. Small egg masses are deposited on the undersides of corn leaves, often near the midrib and closer to the stalk. Females may lay up to 60 eggs per egg mass, but the averages are 15 and 30 eggs/mass for the first and second generation, respectively. One female will lay up to 400 eggs over 10 nights. Depending on temperature, eggs hatch after 3-7 days, and larvae begin feeding on the plant. Larvae develop through five instars, which takes approximately 22-24 days but may take over 30 days, depending on temperature. Adults emerge from pupal cases about 7-10 days later. There are at least two generations of European corn borer in Iowa; occasionally, a partial third generation may occur during longer summers.

European corn borer development and activity is closely tied to corn growth and development, largely because corn contains a natural feeding deterrent called DIMBOA. The highest concentration of DIMBOA occurs in young corn plants, and the chance of larval survival increases once plants reach V9. Usually, most of the first-generation egg-laying in Iowa occurs when corn is V6-V10.

Dispersion and Dispersal: The dispersal dynamics of European corn borer adults are well understood and somewhat complex. Aggregation sites serve an important role in the European corn borer life cycle, and they come and go from these areas frequently. Aggregation sites are areas of dense vegetation, usually grasses in fencerows, ditches, and conservation lanes. Dense soybean stands may also serve as aggregation sites. Dew accumulates and is retained in these areas better than in cornfields. Adults use these areas to rest, find mates, and take in water, and drinking water is required for females before emitting sex pheromones. Adults typically leave aggregation sites around dusk and return, but not necessarily to the same ones, before dawn. Adults primarily leave aggregation sites to lay eggs on host plants at night.

Plant Injury

European corn borer larvae are the damaging life stage, and they can feed on almost any above-ground part of the corn plant. This feeding causes severe economic injury, which earned European corn borer the title of “the billion-dollar bug” prior to the commercialization of Bt hybrids. Young larvae (1^st and 2^nd instars) may leave a windowpane effect after feeding on leaves (Photo 5A), because they consume only the mesophyll. Young larvae may also chew holes through leaves in the whorl, which creates a series of holes (Photo 5B) called “shotholes.” Larvae may also tunnel into the midrib of the leaf, causing the leaf to break, or they may tunnel into the stalk (Photo 5C). During the reproductive stages of corn development, larvae may feed on leaves or kernels or tunnel into the tassel (Photo 5D), but the main source of injury is from tunneling into the ear shank (Photo 5F) or the stalk (Photo 5E). Second generation larvae primarily survive on pollen until they begin tunneling. Frass is likely to be visible, especially near entrance holes or in the leaf axil (Photo 5G).

Feeding by European corn borer larvae results in poor ear development, broken stalks, or dropped ears. Dry weather or late-season storms may result in stalk or shank breakage and loss of ears before harvest. Larval feeding during the mid-vegetative stages of corn growth (V6-V16) has a greater impact on potential yield reductions than larvae that initiate feeding during reproductive stages. From V6-V10, grain yield reductions are approximately 6% for each larva per plant. From R2-R4, yield reductions are approximately 3% for each larva per plant. Drought stress after European corn borer tunneling can exacerbate these losses and result in up to 12% grain yield reduction per larva per plant.

Although direct feeding on kernels is usually insignificant for European corn borer in field corn, feeding can allow entry of ear mold pathogens, which could pose serious health concerns for humans and animals.

Risk Factors

Planting of non-Bt corn hybrids is the biggest risk factor for European corn borer injury. First generation adults are most attracted to early-planted or the tallest cornfields for laying eggs. Second generation adults are more attracted to late-planted or late-maturing corn, sweet corn, or the shortest cornfields.

Scouting

It is necessary to scout for European corn borer to assess populations and determine whether economic losses are likely. The optimal timing of insecticide applications is critical to ensuring successful and economically sound applications are made. The initiation of scouting is based on accumulated degree days since the biofix, which is when the first adult moth is captured in the spring.

Moth trapping: Because accurate timing of scouting largely relies on tracking growing degree days (GDD) since first adult activity, it is important to plan to monitor moths in the spring each year. Pheromone traps are used to do this, which use synthetic lures to attract males to traps. Metal, mesh Hartstack traps are the most sensitive trap for capturing European corn borer males, followed by mesh Heliothis traps (Photo 6). The wing-style traps used for other moth species are not very effective at capturing European corn borer and should not be used for this purpose; funnel or bucket traps are not recommended either.

In early to mid-May, find a grassy area near a cornfield to place a Hartstack or Heliothis trap; these grassy areas are likely to be used as aggregation sites for European corn borer moths. Place the base of the trap at least six inches below the top of the vegetation. Check the traps often: each day would be preferred to ensure moths are identifiable, however, plan to check traps at least once per week. Swap out the pheromone lures every 2 weeks, and make sure to dispose of them properly. In Iowa, use the lures labeled as “IA” or “Z-race.” Once the first moth is captured (biofix), trapping is no longer required for European corn borer monitoring. However, traps could be placed in the field again in July to capture the second-generation moths.

First generation – larval scouting: Begin scouting for first generation European corn borer 170 growing degree days (GDD) after detecting the first moth in the spring or beginning at the V6 stage of corn growth. Look for larvae, feeding injury in the whorl, or feeding on young leaves (see Plant Injury section). To determine whether an insecticide application is necessary, sample 20 consecutive plants in five areas of the field (100 total plants) for every 50 acres. Look for leaf feeding, then for larvae. Plants with no leaf feeding are unlikely to have larvae. If plants have feeding injury, pull and unwrap leaves from the whorl to find larvae and frass. If multiple hybrids are planted in a field, consider each hybrid a separate field. Use the cost-benefit analysis table in Rice and Hodgson 2017 (Table 3) to determine if an insecticide is economical.

Second generation – egg mass scouting: Scouting for second generation European corn borer is entirely different from early-season scouting and the focus is on egg masses. First, determine when egg-laying began based on the biofix and GDD; egg-laying typically begins 1,192 GDD after the first moth capture in the spring, but more generally occurs over 20 days beginning around the VT/R1 growth stage of corn. Begin scouting 8-10 days after the first date of egg laying to determine egg density in the field. To determine whether an insecticide application is necessary, sample 20 consecutive plants in five areas of the field (100 total plants) for every 50 acres. Look for egg masses on the ear leaf, three leaves above the ear, and three leaves below the ear (7 total leaves; Figure 1) to determine the number of egg masses per plant. Then, use the cost-benefit analysis table in Rice and Hodgson 2017 (Table 5) to determine if an insecticide is economical. After the R3 growth stage, there is no need to scout for new egg masses if the threshold has not been reached because plants that no longer have green silks are not attractive to egg-laying females.

The University of Minnesota has an online spreadsheet that contains the cost-benefit analysis tables that can be used: https://docs.google.com/spreadsheets/d/122oK9YZ5NoTjgfYldi2I7tUHIlMHweHZgh3ORF1pk54/edit?gid=424622044#gid=424622044.

Scouting tips! If you are attempting to find European corn borer larvae that have tunneled into the stalk, do not split the stalk with a knife. Instead, peel it back until you find the larvae. Sometimes, you can find the larvae within the stalk by grabbing the stalk and feeling for where it is no longer “squishy” (Photo 7). There is also sometimes frass on the outside of the stalk or holes where larvae have entered/exited (Photo 7).

Management

European corn borer is usually effectively managed with Bt corn hybrids in the Corn Belt, but scouting and management is especially critical in non-Bt fields or high-value crop fields (food-grade corn). Additionally, with resistance development in other areas, understanding management of European corn borer is likely to increase in importance if field failures are detected in the Midwest.

Tracking development: As mentioned in the Scouting section, the development of European corn borer can be predicted using growing degree days, but the accuracy of this relies on detection of moths in the spring. Once the first moth is detected in your area in the spring, use Table 1 to estimate when scouting for first generation larvae or second-generation egg masses should take place. The Iowa Environmental Mesonet Pest Maps and Forecasting page can aid in tracking degree days for scouting European corn borer, but remember to change the start date to when moths are captured in your area!

Table 1. Key degree days, life stages, and associated activity of European corn borer development. Modified from Rice and Hodgson 2017.

Cultural: Extremely early planting of non-Bt corn hybrids is likely to result in heavier infestations of first generation larvae, but later planting can be attractive to egg-laying females during the second generation. Non-Bt corn should not be the earliest or latest planted fields in an area without a plan to scout them regularly.

Since second generation larvae primarily feed in the lower portion of the plant and might result in stalk or ear breakage, early harvest of infested fields can reduce yield losses from dropped ears and lodged plants. Hybrids with stiffer stalks and larger shanks may also help reduce ear losses.

Tillage practices have been known to have an effect on overwintering larvae of the European corn borer; however, destruction of larvae would have to occur on an areawide scale to have a significant effect on the overall population. European corn borer adults disperse throughout the landscape after emergence in the spring, so any destruction of larvae does not result in a direct benefit to the same field. Because of the dispersal of adults, no-till fields should not be expected to have more European corn borer activity.

Abiotic mortality factors: Mortality of European corn borer larvae is very high within the first 48 hours after egg hatch (Ross and Ostlie 1990): dry winds, high temperatures, and heavy rainfall may kill 70% or more of newly hatched larvae.

Biological: European corn borer eggs and larvae are attacked by predators (lady beetles, lacewings, spiders, insidious flower bugs, and predatory mites), parasitoids (wasps and flies), and pathogens (Beauvaria bassiana, a fungus). Combined, these biological control agents can kill 7.5% of European corn borer larvae, on average, in the Midwest. Reducing foliar insecticide applications can promote these biological control agents, which are often generalists that attack other pests as well.

Plant-incorporated protectants: Corn hybrids containing crystalline Bt proteins (Cry proteins) have been very effective at managing European corn borer since their introduction in 1996, and the widespread adoption of these hybrids resulted in areawide suppression of European corn borer populations over several decades. Although Bt for European corn borer has been successful, recent resistance development in Canada and Connecticut is a reminder to scout for this pest and practice integrated pest management (IPM) and integrated resistance management (IRM).

A key concept in IRM for Bt crops is the use of a refuge, which is non-Bt seed planted within or near the Bt field. This refuge area will produce individuals that are susceptible to the Bt toxin because they are not exposed to it while feeding. These susceptible moths will mate with potentially resistant moths from the Bt portion and result in susceptible offspring, which can slow down the evolution of resistance development. Most Bt corn is now sold with refuge-in-the-bag (RIB), which makes compliance with refuge requirements easy for farmers. However, if Bt seed is used without RIB, a strip or block refuge strategy must be used and account for 5-20% of the corn acreage on a farm. In general, more refuge area is better for mitigating resistance development.

Chemical: Insecticide application timing is one of the most critical aspects of non-Bt European corn borer control for a few reasons. First, most insecticides used in field crops do not have a long residual, so they kill larvae over a short period of time and are often impacted by environmental conditions. Second, once larvae enter the plant, insecticidal control is ineffective.

Early season applications targeting first generation larvae can be effective as long as larvae are still feeding within the whorl. Late season applications targeting second generation larvae must be made before larvae enter the stalk or ear. For both generations, insecticides should be timed 10-14 days after egg laying begins (see Tracking Development) to achieve adequate control with a single application.

References

Rice and Hodgson, 2017. Ecology and management of European corn borer in Iowa field corn. Iowa State University Extension and Outreach, CROPS 3139R (free PDF download).

Ross and Ostlie, 2019. Dispersal and survival of early instars of European corn borer (Lepidoptera: Pyralidae) in field corn. Journal of Economic Entomology, DOI: https://doi.org/10.1093/jee/83.3.831

Additional Resources

European Corn Borer – Ecology and Management and Association with other Corn Pests. Iowa State University Extension and Outreach, NCR 327 ($8.00 PDF or hard copy).

Mesh Heliothis trap – Great Lakes IPM, product number GL/SC-1320-01; $84.66 per trap.

Guide to moths in Iowa field crops. Iowa State University Extension and Outreach, CROP 3174 (free PDF download).

The Handy Bt Trait Table for U.S. Corn Production: https://www.texasinsects.org/bt-corn-trait-table.html

Pest Maps and Forecasting website: https://go.iastate.edu/HXOWPQ