What Does ECB Resistance to Cry1Ab Mean ?

Last week, we announced that a population of European corn borer (ECB) likely resistant to Cry1Ab was found in Nova Scotia last fall. In case you missed it, here is that announcement: Suspected Cry1Ab Resistant European Corn Borer Found in Nova Scotia. This finding is concerning, as the ECB populations in the area were previously found to be resistant to Cry1F in Nova Scotia in 2018, which was the first occurrence of field evolved resistance of ECB to any Bt protein in the world. Since then, another ECB population resistant to Cry1F was found in a field in southern Quebec in 2019 and another in Manitoba in 2020. This indicates that ECB is becoming less susceptible to Cry1 proteins as they share similar modes of action against ECB. If ECB can overcome one of them, it is much easier for them to become tolerant to the other Cry1s too. There are only four Bt proteins that are currently available in above ground Bt corn for ECB control, 3 of which are Cry1 proteins. The fourth (Cry2Ab2) must be paired with Cry1A.105 to be effective. With Cry1A.105 being the only remaining Cry1 protein effective against ECB in Nova Scotia, resistance to it could happen more quickly. Viptera (Vip3A) does not work on ECB and is used against the other above ground pests (black cutworm, corn earworm, true armyworm, fall armyworm and western bean cutworm).

Bt Protein for ECBEventTrade Name
Cry1AbBt11Agrisure CB
Cry1AbMON810YieldGard Corn Borer
Cry1FTC1507Herculex I
Cry1FDP-4114part of Qrome
Cry1A.105/Cry2Ab2MON89034YieldGard VT Pro
Bt cry proteins used in above ground Bt hybrids for ECB control in North America.

An effective insect resistance management program includes using corn hybrids that contain more than one Bt protein that are effective against the same pest (called a pyramid hybrid). For ECB populations that are becoming resistant to Cry1 proteins, there are no hybrids left that would meet the criteria for having two effective proteins working against the same pest. Here is a modified version of the 2023 Bt Trait Table with all of the Cry1 proteins against ECB highlighted in yellow. There are also still some single trait hybrids with Cry1Ab on the market that should be avoided. Single trait Cry1F hybrids were removed from the market in 2020 following the discovery of the Cry1F resistant populations.

No new Bt proteins are coming in the near future for ECB control, so we need to work together to reduce resistance development and preserve ECB Bt corn as best we can. Resistance of ECB to Bt corn also places other host crops at risk of having to increase insecticide applications, if ECB populations begin to build again. Host crops like potatoes, peppers, green beans and others have benefited indirectly from the significant reduction in overall ECB populations across North America due to widespread use of Bt corn. ECB resistance to Bt corn could also place organic production at risk for any ECB host crop, as they heavily rely on foliar Bt insecticidal sprays that contain some of the same Cry proteins used in Bt corn.

What can we do to manage this risk and impact?

  1. Don’t use any single trait hybrids. Use hybrids that have at least two effective Bt proteins that work against ECB.
  2. Consider not using Bt corn hybrids if ECB is not a pest of concern for you. Corn hybrids containing only herbicide tolerant traits don’t contain Bt proteins and therefore don’t contribute to ECB resistance development.
  3. Monitor for ECB in all Bt corn hybrids this year and report any unexpected damage found to your seed provider and provincial specialist (Ontario – Tracey Baute). Here is what to look for: Signs of ECB Activity and Injury
  4. Shred stalks shortly after harvest, using flail mowers. ECB overwinter in the corn stubble left on the soil surface. By shredding the stalks, followed by burying them, a large portion of the ECB population is killed and won’t carry over into next season. This is especially important and must be done in any Bt fields found to have unexpected injury from ECB.
  5. Monitor for ECB activity in other host crops too. We are always looking for ECB from other host crops to determine if the three ECB pheromone races (E, Z or hybrid) prefer certain hosts, and if Bt resistant populations move to and from corn to other crops. This helps us determine if other host crops can play a role as refuge to help dilute the resistance or if they add to the resistant issue by enabling ECB populations to spread. Organic growers using Bt foliar insecticides should also report to their provincial specialist if they notice that the sprays are not effectively managing ECB like they used to.