Scouting for European corn borer infestations

Fei Yang, University of Minnesota Extension corn entomologist
Tatum Dwyer, University of Minnesota Dept. of Entomology, MSc Student
David Wangila, University of Minnesota Dept. of Entomology, Postdoc Researcher

European corn borer (ECB), Ostrinia nubilalis (Hubner), has been an economically important lepidopteran insect pest in Minnesota cornfields since the 1940s. For many years, ECB was one of the most damaging insects to corn. Since the introduction of transgenic Bt corn hybrids in 1996, ECB populations have been significantly suppressed across the United States and Canada. This suppression has benefitted even those farmers who do not plant Bt corn due to the areawide suppression of ECB populations. Recently, however, farmers have started planting more acres with hybrids that lack the ECB Bt traits, and certain areas of Minnesota where a smaller proportion of fields have seen economic threshold populations of this insect.

Additionally, ECB resistance to the Cry1F Bt toxin was confirmed in Nova Scotia, Canada in 2018. Since then, resistance to other Cry Bt toxins (e.g., Cry1Ab, Cry2Ab2, Cry1A.105, and Cry1F) has been confirmed in several other regions in Canada, including Quebec and Manitoba. In 2023, ECB resistance to Cry1A.105 and Cry2Ab2 Bt traits was confirmed in Connecticut, U.S. These cases pose a severe risk for the further spread of resistance to all effective Bt traits in the market for ECB across the U.S. If we lose this powerful Bt tool, controlling ECB will be challenging, as the large, damaging larvae remain inside the corn stalk, where foliar insecticide sprays have minimal efficacy.

Timely resistance monitoring is paramount in slowing the evolution of insect resistance. Early detection provides valuable insights for devising appropriate management strategies for resistant insects. To determine if ECB is developing resistance in your area, we recommend scouting and collecting ECB larvae or eggs for Bt toxin bioassays. If you observe ECB infestation in your corn fields in 2024, we are seeking volunteers to collect these insects for testing. We will assay the insects and provide you with reports on their Bt resistance levels. To volunteer for the 2024 collection, please contact Fei Yang (yang8905@umn.edu) or Tatum Dwyer (dwyer332@umn.edu).

European corn borer description  

ECB undergoes complete metamorphosis with four life stages: egg, larva, pupa, and adult (moth). In Minnesota, there are two biotypes (univoltine and multivoltine) of ECB. The univoltine biotype has one generation per year and is predominant in the northern part of the state. The multivoltine biotype, predominant in the southern part of the state, typically has two generations per year in Minnesota. These biotypes can overlap in some areas where they coexist.

Eggs

Females lay eggs in small masses that overlap like fish scales (Figure 1A). These egg masses are deposited on the underside of corn leaves, often near the midrib and on the basal two-thirds of the leaf blade. Newly deposited eggs are creamy white, while eggs close to hatching have larvae with black heads.

Larvae

Larvae (Figure 1B) are the damaging stage for corn and hatch in 3-7 days, or approximately two weeks after moths emerge. Newly hatched larvae feed on corn leaf surfaces or pollen if available. Second instar larvae create small, round "shotholes" (Figure 1C) in whorl stage corn. Third instar larvae can tunnel into leaf midribs and stalks (Figure 1D & 1E). Feeding by ECB larvae causes direct physiological yield losses, as stalk tunnels disrupt the movement of water, nutrients, and photosynthates within the plant. Losses can range from over 6% loss per borer per plant in pre-tassel corn to 3% loss per borer per plant after pollination.

Pupae

The pupae are dark reddish brown with slightly rounded heads and pointed “tails.” Univoltine and 2nd generation pupae are found in cornstalks, corn cobs, corn residue, or weed stems.

Adults

ECB adults are about 1/2–5/8 inches long and nearly triangular in shape when at rest. Females are light brown in color, and males are darker brown (Figure 1F). Both have irregular, wavy lines across their forewings.

Figure 1. European corn borer eggs (A), larva (B), typical leaf shotholes (C), stalk damage (D), lodged stalk damage with larva (E), and adult moths (F).

Scouting European corn borer

Focus on corn fields whose developmental stages are most attractive to the egg-laying female moths. The scouting method for ECB varies depending on whether the corn is in the whorl stage or post-tassel stage. Prior to tassel emergence, scout for larval feeding and larvae in the whorl. Select five spots within the field and pull 10-20 whorls at each spot. Examine these whorls for signs of feeding and count any live larvae found when unrolling the whorl. After tassel emergence, inspect the underside of the ear leaf and the three leaves above and below the ear for egg masses. Select five spots throughout the field and examine five plants at each spot. Timing is crucial because larvae are only outside the stalk and exposed to insecticides until the 3rd instar, which lasts 10 to 14 days for each generation. Refer to the scouting guide (Table 1) for a summary of when and where to scout for the two ECB biotypes in Minnesota.

Table 1. European corn borer scouting guide for two biotypes in Minnesota.

	Multivoline biotype
	1st generation	2nd generation	Univoltine biotype
When	Late May - early June	Mid-late August	Late July - early August
Action sites	Grassy areas	Grassy corn / soybean	Grass / soybean
Corn stage	Early-planted	Late-planted	Pre-tassel - pollinating
Where	Whorl (>17" extended leaf)	Ear +/- 3 leaves	Ear shoot - tassel
What	Shotholing / larvae	Egg masses	Larvae / egg masses
% loss / borer / plant	5.5 -6.0 (early whorl) 4.4 - 5.0 (late whorl)	4.4 (pollen shed) 3.0 (post pollination)	6.0 - 6.5 (pretassel) 4.4 (pollen shed)
Insecticide  % control	80	50 (50-70)	75
DDs 1st moths**	374	1400	<911
DDS start scouting**	800	1550	1100
DDs stop scouting**	1000	2100	1300

**Degree days are based on an upper temperature threshold of 86F and lower threshold of 50F.

Funding support

We appreciate the Minnesota Corn Grower Association and Minnesota Corn Research and Promotion Council for their generous funding support for this project.

References

Hutchinson, W. D., E. C. Burkness, P. D. Mitchell, R. D. Moon, T. W. Leslie, S. J. Fleischer, M. Abrahamson, K. L. Hamilton, K. L. Steffey, M. E. Gray, R. L. Hellmich, L. V. Kaster, T. E. Hunt, R. J. Wright, K. Pecinovsky, T. L. Rabaey, B. R. Flood, and E. S. Raun. 2010. Areawide Suppression of European corn borer with Bt Maize Reaps Savings to Non-Bt Maize Growers. Science. 330: 222-225.

Potter, B. 2020. European corn borer in Minnesota. University of Minesota Extension publication. https://extension.umn.edu/corn-pest-management/european-corn-borer-minnesota-field-corn

Calvin, W., Dwyer, T., Yang, F. 2024. European corn borer: Old pest, new problems. https://blog-crop-news.extension.umn.edu/2024/02/european-corn-borer-new-pest-old.html

Smith, J. L., and Y. Farhan. 2023. Monitoring resistance of Ostrinia nubilalis (Lepidoptera: Crambidae) in Canada to Cry toxins produced by Bt corn. J. Econom. Entomol. 116: 916-926.