Insecticide Efficacy Shown in Roots

What to Keep in Mind When Using Insecticides

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Growers returning to planting-time soil insecticides should assess the rootworm situation in their fields before picking a product. (DTN photo by Gregg Hillyer)

LAWRENCE, Kan. (DTN) -- After nearly a decade of relying on Bt-corn hybrids to control rootworm populations, many growers in the heart of the Corn Belt are returning to the use of planting time soil insecticides. Entomologists want to make sure they do it right.

"Some farmers -- particularly younger farmers -- may not have experience with soil insecticides at all," University of Nebraska entomologist Bob Wright told DTN.

Knowing how insecticides work, picking the right product for your fields, and learning how to evaluate the product's performance are essential to using insecticides successfully, Wright said.

KNOW THE LIMITATIONS

"When using planting time insecticides, the main goal is to protect the central root system, so that we have reduction in lodging and yield loss, but we do not necessarily control a high percentage of the larvae," Wright explained in a rootworm webinar hosted by the North Central Integrated Pest Management Program in February. "A lot of the root zone can penetrate outside the treated area and rootworm larvae can survive on the exterior part of the root bulb that is not protected by the insecticide."

In places with heavy rootworm populations, or fields where Bt-trait failures have been observed, soil insecticides can have a measurable yield benefit. However, their use is not universally recommended, Wright said.

University of Illinois surveys have shown that parts of the state have low adult rootworm beetle counts, so the insecticide could be a wasted input for some farmers, particularly if they are using pyramided Bt-corn hybrids. "We've done some testing the last couple years (in Nebraska) and the one thing that was consistent was where we had rootworm hybrids with two Bt traits for rootworm, we didn't see any benefit to using the soil insecticide," Wright added.

Scientists have also expressed concerns that adding soil insecticides to Bt hybrids unnecessarily can help speed the development of resistance. Because both Bt hybrids and soil insecticides have been shown to delay the emergence of rootworms, using both can increase the likelihood of resistant beetles finding each other, one study found. See the study here: http://goo.gl/…

PICKING THE RIGHT PRODUCT

Selecting the right insecticide for your field requires getting to know the rootworm populations on your acres, Wright said.

Neonicotinoid seed treatment products such as Poncho and Cruiser can offer some protection against rootworms, but they are not the best option for growers with heavy rootworm populations, Wright said.

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"Usually the labels of those products will indicate that you have to use the highest rate of the neonicotinoids to have any effect on the rootworms," Wright said. "And even that rate is for use against only low to moderate populations. So it wouldn't provide sufficient, stand-alone protection for high populations."

Liquid insecticides containing the active ingredient bifenthrin (pyrethroid) have the advantage of convenience, because they can be added to fertilizer applications or just mixed with water and used during planting, chemical retailers told DTN.

However, granular insecticides, such as Counter, Force, and Aztec, remain the best options for growers with dense rootworm populations in their fields, Wright noted. "What we've seen in a lot of studies is that under heavy pressure, the granular insecticides generally work best, whereas liquid insecticides tend to drop off in performance," he said.

Entomologists say granules may generally release their insecticide more slowly than liquids, which means roots can stayed protected during the weeks or even months between planting and rootworm larvae emergence.

Trials run at the University of Nebraska did not show measurable differences in effectiveness between in-furrow and banding applications of insecticides. However, if you have above-ground insect problems, such as cutworms, the banding applications can help control those insects in addition to rootworms, Wright said.

Weather conditions matter, too. "If it's windy at application time, it's probably better to use an in-furrow application so it doesn't blow away," he said.

"Another factor is equipment calibration, which is probably an underappreciated issue," Wright added. "It's really important that when you're putting out insecticides, you make sure the equipment is properly calibrated. If it's under-applying the product, it could hurt efficacy."

TIMING AND SCOUTING

The timing of rootworm egg hatches is as important as it is elusive.

Typically, larvae emerge in early June, but eggs have been known to hatch as soon as early May and as late as mid-June, depending on weather conditions.

While it's convenient to add soil insecticides during planting time, growers who plant in early April should be aware that their insecticide could start to break down in the two months between application and larvae emergence, Wright noted.

Scouting can help growers who want to time their insecticide application with larvae emergence instead of planting time, and it is also an essential tool in determining the effectiveness of an insecticide program.

Generally accepted signs of rootworm problems -- lodging, yield loss, and emerging adult beetles -- are actually not reliable gauges of insecticide efficacy, Wright explained.

"Roots grow outside the treated areas and result in surviving adults," Wright explained. "Even when insecticides are working well and protecting roots, you're still going to get some adult emergence, so it doesn't mean the insecticide is not working."

Lodging can be a result of certain weather conditions, and yield loss can occur without significant lodging, so using fallen corn plants as an indicator of insecticide failure is equally unreliable, Wright added. "You have to evaluate root injury to evaluate efficacy," he concluded.

Evaluating roots during the growing season can also help growers better plan their management strategy for the next year. Wright directed growers to this interactive node-injury scale from Iowa State for measuring root damage: http://goo.gl/…

Iowa State's 2013 rootworm insecticide trials can be found here (pdf): http://goo.gl/…

Insecticide trials from the University of Nebraska can be found here: http://goo.gl/… and here: http://goo.gl/…

The University of Illinois' insecticide trials from 2013 can be seen here: http://goo.gl/…

Emily Unglesbee can be reached at emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

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Emily Unglesbee