Rootworm Biology and Impact
Corn rootworm (CRW) is a serious pest of continuous corn in Ontario. CRW larvae are the main concern, as they clip roots and inhibit root growth, resulting in poor nutrient and water uptake in the plant. Significant yield loss occurs before these symptoms are even noticed. For every node of roots clipped, there is a 15 to 18% yield loss. Additional crop loss occurs when portions of the field lodge due to this root clipping, making harvest difficult to impossible.
There are two species of CRW in Ontario. Western corn rootworm adults are yellow with three wavy black stripes on their wings, while northern corn rootworm beetles are green. The adults emerge from the soil in mid summer and are attractive to corn when in the fresh silk stage, feeding on pollen, silk and leaves. Once their native field is advanced, they move to nearby later planted corn fields. In areas where most of the landscape is planted to corn, populations thrive, flying from field to field to feed and lay eggs. So resistant populations are often shared among neighbouring fields.
The majority of adults prefer to lay their eggs in corn fields, hence why continuous corn production increases rootworm populations. If these same fields rely solely on Bt hybrids for rootworm management, they are at increased risk of resistance developing. If corn is not planted into the same field the following year, the rootworm larvae starve and populations crash.
Bt Resistant Rootworm in Ontario
The extensive reports of injury on pyramid Bt rootworm (Bt-RW) hybrids in several Ontario counties in 2020 suggests that cross resistance to multiple Bt rootworm proteins has developed under continuous corn production practices here. These observations are very similar to what US growers experienced back in 2013 before widespread cross resistance occurred.
There are only four Bt-RW proteins to control CRW. Three of these proteins are closely related, so once CRW has developed resistance to one protein, they are able to tolerate the other two closely related proteins, even if never exposed to them before. Most Bt-RW hybrids have two Bt rootworm proteins, known as pyramid hybrids containing two Bt proteins specifically targeting CRW. With only four Bt-RW proteins available, many of the Bt-RW hybrids share similar pairings of closely related Bt-RW proteins. Repeated use of these hybrids in the landscape has resulted in multiple field failures, regardless of which company supplied the Bt-RW hybrid. So rotating Bt-RW hybrids is not going to solve this resistant CRW issue.
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