Continuous Corn and Bt Corn Rootworm Resistance
Unexpected damage on Bt rootworm (Bt-RW) hybrids – hybrids with below-ground protection – has been found in several fields and new counties this month. The damage includes significant stunting, root clipping, goosenecking, silk clipping and abundant rootworm adult populations mating and feeding (Figure 1). All of these are likely signs of rootworm resistance to Bt-RW proteins. This damage can be significantly amplified with lack of rain and/or following windstorms that result in significant plant lodging.
All cases have been in fields of continuous corn for three or more years, where producers are relying solely on Bt- RW hybrids to manage corn rootworm. There have also been neighbouring fields adjacent to these fields experiencing similar issues, even in second year corn, due to resistant rootworm adults moving into their fields too.
What producers might not realize is that they are not only losing quantities of feed but also quality in these problem fields. It is not just an agronomic problem; this is also a feed supply problem. Plants with significant root clipping are not able to effectively transport nutrients, reducing the amount and quality of plant material harvested. Yield loss is already happening a few years before the damage becomes very evident. In addition, poorly pollinated ears from adults clipping the silks results in fewer kernels and smaller ears. If the field lodges before harvest, the flattened plants take more time to harvest; leaving some plants behind to avoid soil in the feed. By continuing to grow corn on corn and solely relying on Bt rootworm hybrids as the rootworm management tool, we are neglecting to manage the resistance issue, while also producing less and less quality feed for livestock.
Switching to another Bt-RW hybrid is not the solution. Once there is resistance to one of the Bt-RW proteins, rootworms are resistant to all other Bt-RW proteins used in other hybrids and brands. Moving to the new RNAi traited hybrids is also not the solution. The RNAi traits are paired with Bt traits as RNAi is not effective on its own. If populations are already resistant to the Bt-RW proteins that are paired with the RNAi trait, the RNAi trait is then a single mode of action and resistance can develop within one to three years. If we also lose the RNAi traits, we have no more transgenic corn options to turn to. Period. There are no new transgenic solutions coming down the pipeline in the next 5 to 10 years for rootworm management. We must work to maintain what durability we have left for the Bt-RW hybrids, and not misuse the RNAi traits too.
Moving to soil insecticides is also not a good solution. It is costly to have insecticide boxes added to planters after market. When soil insecticide becomes the single form of control in fields with Bt resistance, we will very soon have resistance to the insecticide. This has been well documented in the US, when they also turned to soil insecticides to try to “protect” the Bt-RW hybrids in these fields. What is the solution? Rotation out of corn once every four years (Figure 2). We simply can’t be growing corn on corn for more than three years now that Bt resistance is here in Ontario. We also need to be more selective as to when we use Bt-RW hybrids. Adults lay their eggs in early fall, which overwinter in the soil. Rootworm larvae hatch in early summer. Larvae live exclusively in the soil and don’t move around much so if they don’t find corn roots when they hatch, they die. By rotating a field out of corn, populations crash and take at least two to three years to build back up enough for the crop to need protection again. That means first and second year corn won’t need (and shouldn’t be using) Bt-RW hybrids. Save the Bt-RW and RNAi hybrids for third year corn. Then rotate out of corn again to “reboot” the system.
What crop to rotate to depends on your needs and equipment. There are alternative feed crops that could be considered. More details on alternative feed options can be found on the Canadian Corn Pest Coalition website (www.cornpest.ca). Some producers grow soybeans while others move to alfalfa to improve soil health of the field. Or they swap land with a friendly neighbour who follows a good three-year crop rotation. They plant their non-corn crop into your continuous corn field for a year while you plant a hybrid without Bt-RW or RNAi traits in theirs. So long as their field wasn’t in corn the previous year, it won’t need rootworm protection and won’t enable resistant rootworm populations to continue to build in the area. Custom planting and/or harvest are also options to consider.
These short-term hurdles will pay in the long run when you are able to knock back your rootworm populations and no longer need to solely rely on Bt-RW hybrids to manage rootworm. It will result in a more stable feed supply and significantly reduce the risk of resistance developing and spreading. It is our best approach to maintaining the durability of Bt-RW and RNAi traits.
If you haven’t checked your Bt-RW hybrids in continuous corn fields yet this month, do so. Report any unexpected damage to your seed provider and Tracey Baute, OMAFRA and Chair of the Canadian Corn Pest Coalition.
Weather Data – August 15 – 21, 2022
| Location | Year | Highest Temp (°C) | Lowest Temp (°C) | Rain (mm) | Rain (mm) April 1st | GDD 0C April 1st | GDD 5C April 1st | CHU May 1st |
| Harrow | 2022 | 27.9 | 13.4 | 17.8 | 374 | 2478 | 1796 | 2753 |
| | 2021 | 28.4 | 11.8 | 9.1 | 438 | 2481 | 1784 | 2655 |
| | 10 YR Avg. (2011-20) | 26.7 | 15.9 | 27.3 | 440 | 2459 | 1747 | 2738 |
| Ridgetown | 2022 | 29.5 | 10.9 | 30.5 | 268 | 2340 | 1665 | 2541 |
| | 2021 | 28.5 | 9.4 | 16.6 | 414 | 2345 | 1657 | 2505 |
| | 10 YR Avg. (2011-20) | 26.6 | 13.5 | 14.2 | 397 | 2314 | 1606 | 2549 |
| London | 2022 | 29.2 | 12.9 | 12.6 | 306 | 2280 | 1610 | 2473 |
| | 2021 | 29.8 | 9.3 | 6.5 | 334 | 2330 | 1646 | 2470 |
| | 10 YR Avg. (2011-20) | 26.3 | 13.9 | 20.6 | 409 | 2294 | 1591 | 2533 |
| Brantford | 2022 | 30.0 | 12.5 | 3.1 | 270 | 2289 | 1613 | 2426 |
| | 2021 | 31.7 | 7.7 | 10.2 | 302 | 2299 | 1614 | 2424 |
| Welland | 2022 | 30.4 | 13.0 | 15.4 | 291 | 2343 | 1658 | 2556 |
| | 2021 | 30.1 | 11.0 | 2.7 | 298 | 2311 | 1622 | 2468 |
| | 10 YR Avg. (2011-20) | 26.6 | 14.3 | 19.3 | 371 | 2308 | 1604 | 2554 |
| Elora | 2022 | 29.6 | 10.5 | 4.4 | 228 | 2097 | 1433 | 2224 |
| | 2021 | 28.7 | 10.3 | 1.1 | 266 | 2131 | 1456 | 2252 |
| | 10 YR Avg. (2011-20) | 25.3 | 11.1 | 22.1 | 409 | 2072 | 1382 | 2268 |
| Mount Forest | 2022 | 28.3 | 10.9 | 3.6 | 310 | 2091 | 1434 | 2259 |
| | 2021 | 28.4 | 7.9 | 32.7 | 360 | 2136 | 1463 | 2264 |
| | 10 YR Avg. (2011-20) | 24.9 | 12.5 | 30.8 | 426 | 2057 | 1374 | 2278 |
| Peterborough | 2022 | 30.2 | 6.6 | 38.6 | 305 | 2106 | 1428 | 2239 |
| | 2021 | 30.5 | 6.7 | 0.8 | 301 | 2138 | 1452 | 2240 |
| | 10 YR Avg. (2011-20) | 26.4 | 10.4 | 17.0 | 365 | 2106 | 1415 | 2275 |
| Kemptville | 2022 | 31.3 | 9.9 | 0.2 | 407 | 2255 | 1558 | 2437 |
| | 2021 | 33.3 | 8.7 | 2.0 | 266 | 2297 | 1605 | 2388 |
| | 10 YR Avg. (2011-20) | 27.4 | 13.0 | 37.4 | 407 | 2214 | 1524 | 2429 |
| Earlton | 2022 | 28.2 | 10.6 | 31.2 | 326 | 1905 | 1276 | 2091 |
| | 2021 | 31.8 | 8.0 | 0.0 | 451 | 2004 | 1337 | 2072 |
| | 10 YR Avg. (2011-20) | 24.4 | 10.0 | 16.5 | 350 | 1796 | 1190 | 2020 |
| Sudbury | 2022 | 27.0 | 10.7 | 6.5 | 296 | 1928 | 1290 | 2143 |
| | 2021 | 31.1 | 9.0 | 0.0 | 375 | 2031 | 1365 | 2136 |
| | 10 YR Avg. (2011-20) | 24.4 | 11.9 | 22.8 | 382 | 1925 | 1295 | 2170 |
| Thunder Bay | 2022 | 28.7 | 7.6 | 23.2 | 445 | 1688 | 1088 | 1816 |
| | 2021 | 34.3 | 9.4 | 0.2 | 228 | 1899 | 1246 | 1977 |
| | 10 YR Avg. (2011-20) | 25.9 | 9.0 | 18.3 | 355 | 1710 | 1092 | 1872 |
| Fort Frances | 2022 | 27.5 | 6.0 | 21.6 | 555 | 1771 | 1173 | 1974 |
| | 2021 | 33.1 | 7.1 | 11.2 | 210 | 2012 | 1353 | 2126 |
| | 10 YR Avg. (2011-20) | 26.5 | 8.6 | 17.8 | 358 | 1833 | 1197 | 2020 |