Entomologists at the University of Guelph in Canada have confirmed European corn borer (ECB) resistance to Cry1F Bt trait (Herculex 1) in corn, marking the first case of ECB resistance to any type of Bt corn.
“Assay (analysis) results were described to me as showing ‘100% resistance,’” said Michigan State University Field Crops Entomologist Dr. Chris DiFonzo. “The registrant for the trait independently confirmed the results from additional field locations.”
The resistant populations were collected at the end of the 2018 field season from multiple sites in Nova Scotia and Prince Edward Island in Eastern Canada, and screened for resistance in the lab this past winter.
Single-trait hybrids were still being sold and planted where the resistance was identified in Canada. In a perfect world, those hybrids were supposed to be phased out by now in favor of pyramided hybrids with multiple Bts (i.e., multiple “hammers” to kill ECB), according to DiFonzo.
The planned phase-out was part of what enabled the switch from inconvenient 20% block refuge requirements to simpler 5% Refuge in a Bag (RIB). To reduce the refuge to 5%, both Bts in a pyramid are supposed to function as separate hammers to kill corn borer.
Since there was a greater chance for insects to develop resistance to single Bt traits in corn, the single Bt hybrids were supposed to go away to protect the two hammers in the 5% RIB.
So why were single-trait hybrids still being sold and planted?
“Good question,” said DiFonzo, adding that pyramids with 5% RIB are pretty much the only Bt hybrids available in the central U.S. Corn Belt.
“But in smaller markets, appropriate pyramids apparently are in short supply or perhaps not economically viable,” DiFonzo said. “Nova Scotia and Prince Edward Island are smaller markets, as are many states in the northeast, as well as states and provinces in the Great Lakes region.
“Depending on the location, growers may be able to buy single-trait hybrids, increasing the chance of developing resistance. To date, I’ve been unable to get stats on the acreage of single-trait hybrids sold in Michigan, particularly the UP and northern Michigan.”
University of Guelph Extension staff entomologists are recommending that growers should not purchase or plant single-trait Bt hybrids for ECB. Hybrids containing only Cry1F (Herculex 1 and Herculex XTRA) or Cry1Ab (some Agrisure hybrids) should be avoided as the risk of resistance increases if only one trait is being used to control ECB.
Canadian growers are also being advised to avoid planting stacked hybrids that contain only two Bt traits if one of those traits is Cry1F. The confirmation of resistance to Cry1F results in these two trait hybrids becoming a single trait hybrid in those regions where Cry1F resistant ECB populations exist. If still planting these hybrids, use caution and scout for unexpected damage.
MSU’s DiFonzo recommends Michigan growers follow the same advice.
“First, don’t plant single-trait hybrids,” DiFonzo said. “If you do use a single-trait hybrid this year, be sure to plant the required 20% refuge and scout your fields.”
Additionally, DiFonzo said corn producers should monitor and report unexpected corn borer damage in any type of Bt field in 2019.
“Consider trapping for corn borer and reporting counts in the Great Lakes and Maritimes Pest Monitoring Network,” she said. “Finally, we are rapidly needing to go back to school. Producers need to educate themselves about a pest we’ve been able to ignore for many years, but not any longer!”
Great Lakes and Maritimes Pest Monitoring Network: Is an expanded, online trapping network that is a collaboration of Manitoba, Michigan, Ohio, New York, Ontario, Quebec, New Brunswick, Prince Edward Island and Nova Scotia.
For 2019, the network will accept trapping data for western bean cutworm (WBC), European corn borer (ECB), corn earworm (CEW), black cutworm (BCW), true armyworm (TAW) and fall armyworm (FAW) in field corn, sweet corn, dry beans and snap beans. You will be able to see trap captures and report them
ECB Guide: “European Corn Borer - Ecology and Management and Association with other Corn Pests” have all you need to know. It’s being sold by the Iowa State University publications office at $8 per copy. Discounts for ordering 50-plus copies are available by contacting [email protected] or (515)294-5247. PDF versions of the report are also available for downloading online if preferred.
See more information on the Canadian resistance confirmation at https://www.canr.msu.edu/news/european-corn-borer-resistance-confirmed-to-cry1f-bt-corn
Need to confirm which Bt traits you are planting in your hybrids? Consult Chris DiFonzo’s Handy Bt Trait Table, posted at free online at https://www.texasinsects.org/bt-corn-trait-table.html
Signs of ECB Activity and Damage to Scout for in Bt Corn Fields
ECB lay eggs on the underside of the corn leaves, close to the mid-rib (Fig. 1). Fresh ECB egg masses (Fig. 1-top) are white but turn dark closer to hatching (Fig. 1-bottom) and are layered on top of each other like fish scales. Return to fields with egg masses later in the season to look for signs of feeding. ECB larvae are pale tan to pinkish-grey in color with a dark head and small round brown dots along their body.Note: The presence of ECB egg masses is not an indication ofresistance but fields with eggs should be scouted again in a few weeks to look for signs of feeding damage. If feeding damage is present in Bt corn as shown in the images below, contact your seed provider and/or extension specialist.
|Figure 1. ECB egg masses. | J. Gavloski, Manitoba Agriculture|
|Figure 2. ECB larva found in corn stalk. | A. Tenuta, OMAFRA |
Signs of ECB Feeding – Report if found on Bt Plants
ECB feeding found on Bt plants is a potential sign of resistance. Young larvae feed on the leaf surface and mine through the whorl of the younger plants. Early season signs of feeding may include window-paning, pinholes and shotholes (Figs. 3-5). These early signs of feeding are not unexpected damage as young larvae need to feed on the plant tissue to be exposed to the Bt protein and die.
|Figure 3. Young ECB larva and window-paning on a leaf. | M.E. Rice|
|Figure 4. Feeding on the whorl by ECB larvae. | T. Baute, OMAFRA|
Older larvae are able to mine into the mid-rib of the leaf, tassel or stalk of the plant and ear shank. Frass present at the leaf axils, bent leaves at the midribs, broken tassels, lodged plants and dropped ears are signs of ECB feeding (Figs 6-9). If the damage continues to progress beyond pinholes and window-paning, this would be considered unexpected damage.
|Figure 5. Pin-hole or shot-hole feeding pattern as the leaf unrolls from the whorl. | T. Baute, OMAFRA|
|Figure 6. Bent or broken tassels are signs of ECB feeding. | David Handley, U of Maine|
|Figure 7. ECB larvae may tunnel from the mid-rib of the leaf down to the stalk. | T. Baute, OMAFRA|
|Figure 8. Entry hole and frass at leaf axil as ECB enters the stalk. | J. Obermeyer, Purdue|
|Figure 9. Bent or broken corn stalks due to ECB tunneling. Damage to ear shank due to ECB tunneling. | E. Bohnenblust, Penn State University|