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Can you manage your way out of corn rootworm problems?

 Bruce Potter, Extension IPM specialist

Parts 1 & 2

Part 1: Tool selection

Corn rootworm management was one of the crop pest problems I puzzled over this past winter. Recently, some warm weather got me thinking about changing the oil and blade on my year-old lawnmower. It’s a replacement for an old machine that suffered from a lack of Zen during my maintenance efforts. Most repair efforts involved a couple of screwdrivers and pliers ‒ the only tools I could count on finding on the garage bench. Multi-purpose, they did duty as crude hammers and pry bars in a pinch. The sets of socket and box end wrenches, including those special metric jobs, and multiple sizes of screw bits, reside safely in the tool chest back at the house, but those precision tools don’t get much use. The few tools on the bench could usually be made to work, and who has the time to walk back from the yard to get the right-sized wrench? The old mower had bolt heads where an active imagination could still find six sides and there were broad valleys that no longer resembled an opening for a screwdriver bit. Overuse of a few simple tools had gradually rendered some parts nonfunctional. Those parts damaged by poor tool choices needed replacement, but other issues sent the old mower down the road. I was convinced that more attention to detail would provide long-term benefits whilst wrenching on my new mower.

Most of you are better mechanics than I am and use pliers and screwdrivers only for the tasks they were engineered for but, what does my lawnmower have to do with corn rootworms anyhow? Like my mower’s rounded bolt heads and stripped screw slots, there are quite a few fields of corn in SW Minnesota (and elsewhere), where prolonged, imprecise use of simple, effective management tools such as crop rotation and Bt means they might not have the grip on corn rootworms that they once did.

Part 2: How we got where to where we are and where are we heading?

Transgenic corn hybrids with Bt traits resistance to corn rootworm larvae (Bt RW) have been planted in Minnesota farm fields since 2003. Previous experience with the Bt traits for European corn borer and some dramatic early Bt RW performance in sites heavily infested with corn rootworms encouraged a rapid and widespread adoption of the technology, often without regard to the level of risk from rootworms. Here was a tool that was more effective, easy to use, and relatively economical compared to insecticides. The inclusion of Bt RW traits in high-yielding hybrids meant it could work in fields with a 1/4-inch, 3/8-inch, 12-mm, or whatever level of risk from corn rootworm. Bt RW hybrids quickly replaced at-plant insecticides as the primary corn rootworm management tool on many farms.

Crop rotation - an early, effective tool

Figure 1. Western and northern corn
rootworm beetles feeding on a
late-emerging corn tassel. 
Before Bt RW tools were available, crop rotation was a simple and very effective tool for managing corn rootworms ─ and for the most part, it still is. Corn rootworm larvae feed on the roots of grasses and cannot develop on the roots of broadleaf plants. Rootworm beetles lay their eggs in the soil of cornfields, their preferred host. Although rootworm larvae feeding and survival have been observed on some prairie and weedy grasses, nearly all of these appear to be lesser hosts than corn. Hypothetically, small larvae might be able to feed on the roots of grass weeds such as foxtails and barnyard grass with larger, less susceptible larvae moving to Bt corn. Weedy, volunteer corn plants can allow larvae to survive within a soybean field and late-pollinating volunteers may attract egg-laying females.

Within Minnesota, the western corn rootworm (WCR) has been an economic pest problem in fields of continuous corn, and rotation to a crop other than corn crop has been highly effective. In eastern corn belt landscapes dominated by a corn-soybean rotation, a behavioral variant of WCR arose. This variant, not yet identified in Minnesota, will lay its eggs outside of cornfields. Unlike the eggs laid in cornfields, likely to hatch in corn and starve, the rotation adapted variant eggs laid in soybean fields are likely to hatch in a suitable corn host.

Extended diapause and Bt resistance change the toolbox

Figure 2. Severe root pruning by corn rootworm larvae.
The northern corn rootworm (NCR) has historically been the most common Minnesota species, particularly in the more northerly rootworm infestations. For many years, they too could be managed when corn was rotated with a nonhost crop. The eggs of both WCR and NCR diapause (suspend development) during the winter. Over time, adapting to the predictable, corn-soybean rotation that was prevalent on many Minnesota farms, the proportion of an extended diapause biotype increased within NCR populations. Extended diapause biotype larvae find themselves in a suitable corn host because egg hatch is delayed through an extra winter (occasionally even longer). However, not all NCR individuals have extended diapause genetics and both biotypes do well in continuous corn. During the mid-1980s and again in the early 2000s ‒ coincidently, just before the first Bt RW hybrids were released ‒ extended diapause caused significant problems in parts of southern Minnesota. Growers were encouraged to employ insecticide and subsequently, the new Bt RW traits on rotated acres. Bt selection pressure in both rotated and continuous fields was now widespread.

Not unexpectedly, in 2009, I saw my first SW Minnesota fields of continuous Yieldgard® rootworm (Cry 3Bb1 Bt RW protein toxin) with unexpected injury from WCR. The lodged plants, pruned roots, and clouds of rootworm beetles were big hints that the same tool had been used too often. By 2012, some fields planted to continuous Bt RW hybrids saw poor control from multiple Bt RW traits, including a pyramid of two Bt RW traits. Complicating management, cross-resistance to three forms of Cry3 protein (Cry 3Bb1, mCry3A, eCry3.1Ab ) has been documented in WCR. One or more of these proteins are present in all Bt RW pyramids. Pyramids of two Bt RW proteins were intended to slow the development of resistance. Unfortunately, Cry3 cross-resistance and increasing resistance to Cry 34/35AB1, often paired with a Cry3 protein, has created some very uncooperative rootworm populations.

Along with overall lower NCR populations, Bt resistance issues in NCR have lagged those in WCR. The northerns are on the board now too with resistance confirmed in some 2018 North Dakota fields. See The Handy Bt Trait Table (2022) for more details on Bt traits.

In many fields, WCR and NCR populations ‒and depending on the hybrids planted, resistance to individual and pyramided Bt RW traits ‒ have been increasing for several years. However, developing corn rootworm problems are masked during years of low rootworm populations with minimal root injury and lodging.

WCR populations in southwest Minnesota were relatively low for a few years, presumably due to egg mortality during the bitterly cold winter of 2013-14. The survival of newly hatched larva can be reduced by excessively wet early-June soils. WCR populations had rebounded in long-term corn research plots at the UM SWROC by 2019, and by 2021 large WCR populations and Bt resistance became widespread in southern MN continuous corn. Populations of NCR in Minnesota had been depressed since the mid-2000s but they, and associated extended diapause, are also on the rise.

Select the right tools for the job

Although less effective than a Bt RW pyramid on a susceptible rootworm population or rotating out of corn, at-plant insecticides remain a useful option for rootworm management. Adult corn rootworms resistant to some foliar insecticides (e.g., chlorinated hydrocarbons, organophosphates, pyrethroids) and larval resistance to at-plant insecticides (e.g., chlorinated hydrocarbons) have been documented in some areas of the corn belt. However, currently labeled at-plant insecticides protect a limited portion of the corn root system and are persistent and effective only for a limited time. In effect, the part of the root system without insecticide can act as a built-in refuge. As a result, at-plant soil insecticide resistance in rootworm larval populations has been less of a concern than with Bt or foliar insecticide applications. Research at the SWROC and other MN locations indicates that at-plant insecticide options vary in efficacy. In general, granular insecticides are the most consistent, liquids are more variable, and rootworm rates of seed-applied insecticides are least effective. At-plant insecticides can reduce root injury, but they do not reverse Bt or rotation resistance and often do not reduce populations of beetles emerging from the field.

Both rootworm species tend to become more abundant in a field with increasing years in corn. Areas of Minnesota with a large percentage of continuous corn fields tend to be where overall rootworm populations are highest and unexpected injury to Bt RW hybrids occurs most often. Rotation from corn is the best option in these cases, particularly where Bt-resistance is suspected. Rotation will eliminate WCR in the field and reduce non-diapause NCR populations in the field, but will it not affect an area’s overall level of Bt resistance. Therefore, high overall rootworm beetle populations in the area can quickly reinfest a field with Bt-resistant rootworms. Previously, Bt resistance problems were most often found in fields that had been planted to corn for several years but in 2021, I’ve heard reports of WCR causing unexpected injury to 2nd-year corn.

Adding an at-plant insecticide to Bt-RW hybrid in a field where Bt resistance is suspected is like using vice grip pliers to turn a bolt with a rounded head. Even though you might accomplish the task, you probably have made the underlying problem worse.

Use rootworm management that fits the field. New technology was designed to prevent ham-fisted operators from destroying the heads of bolts and screws. Sure, those Allen and Torx head screws minimize damage from my efforts, but I’ve managed to wreck those too. New RNAi technology (e.g., SmartStax Pro) to control corn rootworm larvae is becoming available. Yes, it is effective and offers another rootworm management tool. However, because it kills more slowly than insecticide or Bt, the RNAi trait is stacked with a Bt RW pyramid. It's possible for yield-limiting rootworm injury to the roots of these new hybrids if they are planted in a field with high populations of Bt-resistant rootworms. I can fit a small flat-bit screwdriver into a Torx or Allen head, but the results have been disappointing every time I’ve tried it.

One-size-fits-all approaches seldom live up to lofty expectations. They may have worked acceptably back in the day of square-headed bolts, but underemployed hex-head bolt salesman surely encouraged the use of those darn adjustable crescent wrenches. While many agriculturalists seem intent on trying, the rootworms keep telling us that there is still no one-size-fits-all, long-term corn rootworm management strategy.

Part 3 will focus on developing a management plan.

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