Bt-trait failures are leading to more spraying of adult corn rootworms.

“As resistance to Bt traits has become more prevalent, this management tool has come back on the table,” says Bruce Potter, a University of Minnesota Extension pest management specialist.

But Midwest entomologists warn that this tactic is no substitute for sound integrated pest management (IPM) and should not be used as a standard practice. Rather, adult control is “one tactic in the toolbox to be used sparingly to help bring difficult rootworm density situations back under control,” says Lance Meinke, University of Nebraska entomologist.

“If you’re spraying beetles,” Potter says, “you are trying to fix a problem that has already occurred.” Namely: “Root damage and high levels of beetles that are potentially resistant to Bt-CRW traits.”

Beyond that, managing corn rootworm beetles is much more complex than managing larvae, Meinke says. Adult control is greatly complicated by such factors as extended beetle emergence, insect migration, treatment timing, a limited arsenal of foliar insecticides and even the proliferation of herbicide-resistant weeds. And because broadcast insecticides may affect every beetle in the field, there’s more potential for corn rootworms to develop resistance to an insecticide treatment.

Reducing egg loads is difficult, expensive

Performance problems with Bt traits are being reported in many Corn Belt states. In some continuous cornfields – especially in regions with many continuous-corn acres – rootworm populations are so high that growers have resorted to spraying foliar insecticides during the growing season to prevent females from laying eggs. “It’s being used as a complementary tactic, so next year’s tactics have a better chance of working,” Meinke says.

But it’s tough to pull off.

One of the main challenges is treatment timing. In the Bt-corn era, beetle emergence periods have lengthened to more than 30 days, says Erin Hodgson, Iowa State University entomologist.

The reasons for this are not well understood, but the combination of sub-lethal exposure and larval feeding behavior are likely factors, says Joe Spencer, University of Illinois entomologist. The lag between when beetles emerge from non-Bt refuge corn and Bt-corn is also widening, Spencer says, adding that soil insecticide can delay emergence, too.

Adults also move readily from field to field, adding to scouting challenges. Late-planted or late-maturing cornfields are a magnet for beetles, which flock to feed on green silks and pollen – a phenomenon known as the trap crop effect. Beetles “prefer young reproductive tissues,” Meinke says.

Adults also forage on uncontrolled weeds such as herbicide-resistant waterhemp and Palmer amaranth, which may provide extra pollen for beetles and increase egg-laying, he says.

For all these reasons, intensive scouting is necessary to determine the density of gravid females in a field. A single, well-timed application is not enough, Hodgson says. “It would likely take multiple applications, over several weeks, and that gets very expensive.”

“Documenting populations that emerge or immigrate late in the season can cause a labor crunch,” Potter observes.

Foliar insecticide choices are limited and have short residual activity, generally less than a week. There’s growing concern that corn rootworms are becoming resistant to pyrethroid insecticides, which are used to control rootworms and many other pests.

Adult rootworm management used to be common in the western Corn Belt before beetles became resistant to encapsulated methyl parathion, a long-acting insecticide, in the 1990s. Today, “It is very difficult to implement a successful adult control program” to suppress egg-laying, says Nebraska agronomist Mark Kottmeyer, Central States Agronomics, Kearney.

“Beetles are present in a field for a long time, into September. Control lasts only three or four days. You can spray, and within 10 days, populations are back to where they started,” he says, noting that this problem also occurs in rotation-resistant western corn rootworms.

Prevent silk clipping

Adult control can be a useful tool to protect pollination if large numbers of beetles are chewing off green silks during pollen shedding. “That’s the only time adults will have a direct impact on the current year’s yields,” Spencer says.

Severe silk clipping is mainly a threat in dry years, Potter says. Drought slows pollination, and beetle feeding compounds the problem.

During the 2012 drought, for example, western corn rootworm beetles pounded hot spots in southern Minnesota, “usually in continuous cornfields where there was a breakdown of traits,” says Al Van Grouw, an independent crop consultant from Springfield, Minn. Growers had to spend $12 to $15 per acre for an aerial insecticide application to knock down beetle populations long enough to finish pollination.

If moisture is adequate, spraying is not recommended to cut down on silk clipping unless beetle densities reach 15 to 20 insects per plant, or silks are clipped back to within 1/2 inch of the husk and pollination is less than half complete. It’s rare to reach these economic thresholds, Meinke says, “but it does happen.”

However, action thresholds drop down to one to five beetles per plant if corn is heat or drought stressed and pollination is incomplete. Likewise, in seed corn, economic damage thresholds are one to five beetles per plant feeding on green silks.

Spraying beetles to protect pollination may also be justified in fields with a significant number of delayed plants. If 10% of plants in a field are delayed, three or four beetles per plant are enough to trigger a treatment, because of the trap crop effect. “The beetles all move to those later pollinating plants, so at that point we will spray,” Kottmeyer says.

But “if pollination has already occurred, we don’t recommend treatment,” he adds. Adults can feed on leaves or brown silks without damaging yields.

Entomologists are disturbed by a trend toward “insurance” treatments of rootworm beetles at silking time. “Many growers have been treating corn with fungicide at about the time of pollination,” Spencer says. “Applicators point out that for a few extra dollars, you can throw in an insecticide and kill some beetles, too.”

But treating before insect densities reach economic thresholds rarely pays off, Spencer says, and it raises selection pressure needlessly. “Insurance” treatments can also cause other insect pests – such as spider mites – to flare up, Potter says.

Growers should use adult control only as “a last resort,” Hodgson says. Instead, focus on vigilant scouting and a sound IPM approach to larval management.

“We’re recommending a holistic approach,” Meinke adds. “Think about it as a system, considering multiple insect and weed pests at the farm level when making management decisions.”

Adopt an IPM approach to corn rootworm control

A sound integrated pest management (IPM) approach to corn rootworm control is the best way to prevent injury to this year’s crop and keep rootworm densities low over time, says Lance Meinke, University of Nebraska entomologist. Growers should focus on the following:

“Crop rotation is our first line of defense,” says agronomist Mark Kottmeyer, Central States Agronomics, Kearney, Neb. “When we see high populations of corn rootworm beetles, we rotate to soybeans. That works very well for us.”

In first-year corn, above-ground Bt-traits are sufficient in central Nebraska, he notes. In regions of the Midwest where corn rootworms have evolved to overcome rotation, first-year rootworm controls may be needed. “We try to use corn rootworm traits only when necessary, only where pressure is high,” he says.

In second-year or continuous corn, Bt-traits or soil insecticide will be needed if the previous year’s scouting shows that beetle densities averaged one to two adults per plant, says Erin Hodgson, Iowa State University entomologist.

Growers should avoid throwing the “kitchen sink” at corn rootworms, Hodgson says. If the Bt trait is working, a soil-applied insecticide provides no yield benefit. If the trait is beginning to fail, the insecticide may protect the main root ball and preserve yield, Meinke says. But this tactic doesn’t significantly reduce rootworm populations or lower Bt trait selection pressure.

Corn rootworm management “is not an easy system anymore,” Hodgson says. “We have to go back to traditional methods, including scouting. Every field, every year, deserves some attention for this pest.”