One Tough Bugger
By Gene Johnston
Nature always wins. Want proof? Consider the corn rootworm. “This bug eventually has found a way to resist nearly every control technique we throw at it,” says Bruce Hibbard, a USDA-ARS research entomologist based at the University of Missouri. This pest doesn’t give up easily. The latest weapon, Bt technology, promises to deliver a knockout punch bred inside the plant itself. It’s showing vulnerability, however. Corn in some Corn Belt fields is succumbing to yield-robbing root damage, even when a Bt hybrid is planted. Bt will play a strong role in rootworm control for years to come, says Hibbard. Still, if we want to maximize its long-haul effectiveness, we may need to revert to previous technology in multiple-mode warfare.
Here’s his checklist of rootworm control technologies – some from the past – that can help you reduce the overall levels of infestation.
1. ROTATE YOUR CROPS For the first 50-plus years of the last century, this was the only rootworm defense. “It breaks the cycle of the pest,” Hibbard explains. Rootworm pupates into the adult stage late in the growing season. Females then lay eggs that overwinter and hatch again the next spring. Corn-on-corn really plays into their game plan. If no corn roots exist in the field due to planting soybeans or another crop, the larvae starve and the population is greatly reduced. In many places, rootworm has found a way to get around crop rotation – but not completely. Usually, rotation helps to reduce root feeding. If it is possible to rotate crops, you may have to do that to help maintain high corn yields, Hibbard says.
2. USE GRANULAR SOIL INSECTICIDES This second-generation control technology started back in the 1940s and 1950s. Initially, farmers broadcast chlorinated insecticides on as a seed treatment. Resistance developed relatively quickly. Organophosphates and carbamates were then developed, but they didn’t offer an easy system to manage. They also were relatively expensive, so farmers banded them in the row furrow with the planter. This technology and the chemicals are as effective as ever, says Hibbard. Rootworm has shown little or no resistance to the carbamate class of chemicals in 40 years of use. Banding provides a natural refuge where rootworm can survive between rows outside the insecticidal bands. “The fact that resistance has not developed to this man- agement tactic documents that refuges work for delaying resistance,” he says. “There are places where you can use it to advantage now.”
3. USE DIFFERENT MODES OF ACTION This tactic is similar to rotating weed chemicals to prevent resistance development. With Bt corn, there are now second- and soon-to-be third-generation Bt modes of action that work in different ways than the original Bt. Thus, rootworm that resists one Bt mode is susceptible to another. If you are rotating crops and soil insecticides where possible, Hibbard advises also rotating the type of Bt corn that comes in your seed package.