Can you manage your way out of corn rootworm problems? It's complicated!

Bruce Potter, Extension IPM specialist

Parts 3 & 4

Part 3 - Developing a management plan

A yellow sticky trap used to monitor corn rootworm
beetles.

Early corn planting and dry springs tend to favor the survival of corn rootworm larvae. Those same conditions tend to reduce the effectiveness of at-plant insecticides. Rootworms do have some weak points. NCR eggs are relatively cold-tolerant but prolonged cold, open winters can cause mortality to WCR eggs wintering in the soil. Larval mortality can occur if soils are saturated when small larvae are trying to enter corn roots. By late July and August, you will know how successful both corn rootworms and your corn rootworm management strategies were.

Monitoring rootworm populations  

For several years, Dr. Ken Ostlie and I have been examining methods to monitor rootworm populations using yellow sticky traps and a network of farmer, industry, and Extension cooperators. The cooperators receive traps and management information for the fields they monitor, and pooled cooperator data is starting to provide ground truth observations on WCR and NCR interactions with crop rotation and Bt. Several seed companies have contributed their trap data to the project in the past and participation is welcome again in 2022. This work is supported, in part by the farm families of Minnesota and their corn checkoff investment. If you would like to participate in this project during the 2022 growing season let me know!

Cooperators tend to place most traps in fields where they suspect rootworm populations are high‒ a logical approach from a management standpoint. As a result, these data likely overestimate overall populations. Nonetheless, based on examining trapping results from over 200 fields, some expected and some disturbing information on Minnesota corn rootworms started to surface during the high rootworm populations of 2021 (Table 1).

• Rootworm populations and the proportion of WCR to NCR increase with the number of years of corn with NCR tending to dominate in northern locations.
• There are fields of 1st-year corn with large NCR populations ‒presumably extended diapause (Figure 3).
• Large numbers of WCR beetles were captured in a few 1st-year cornfields. Concerning was the in-field beetle emergence observed in one case. These were in areas with large overall WCR populations but whether this was due to egg-laying in the previous year’s soybean crop, or another reason has not yet been determined (Figure 3).
• NCR beetle captures tend to be lower in fields with Bt RW but unfortunately, there are exceptions. However, there is little to no difference in WCR between fields with and without Bt RW (Figure 4).

Table 1. Captures of western (WCR) and northern (NCR) in yellow sticky traps as they relate to crop rotation and Bt-RW corn. Minnesota 2021

Trait	Years in corn	Fields	WCR maximum	NCR maximum	All CRW maximum	Proportion WCR
		#	mean number/trap/day			%
Bt-RW	1	21	0.5	1.4	2.0	37.4
	2	21	5.9	2.2	8.0	61.0
	>3	40	30.1	1.7	31.7	84.0
	All	82	12.2	1.8	13.9	60.8
No Bt-RW	1	106	2.1	2.1	4.2	51.4
	2	6	6.0	4.5	10.5	58.5
	>3	12	38.8	4.1	42.8	78.4
	All	124	15.6	3.6	19.2	62.8

2021 data source: Extension, growers, crop consultants, and seed industry.

Figure 3. Relative northern corn rootworm (left) and western corm rootworm (right) abundance in yellow sticky traps in rotated corn (2021).

Figure 4. Relative northern corn rootworm (left) and western corm rootworm (right) abundance in yellow sticky traps in continuous and rotated Bt RW corn (2021).

Scout to determine risk!

Individual fields vary in rootworm species composition, rootworm population density, and Bt RW resistance. Adjacent fields may have completely different rootworm populations and risks for injury. In other words, there is a mosaic of rootworm populations and genetics within a field and across the landscape. Each field is unique because of hybrid, planting date, and other cropping history factors.

How can you determine the risk of corn rootworm injury to corn roots in a field? It requires a considerable labor investment in the jungle-like conditions of tasseled corn during August, but scouting is the only way to select the best rootworm management tool for a field.

Corn rootworm beetles are mobile. Beetle populations and their offspring are usually variable within a field. Apart from those rootworm disaster fields where the result of poor rootworm management is reminiscent of the Hindenburg, looking at just a few plants or yellow sticky traps in a field will not provide a good estimate of beetle populations. Developing a sound scouting plan for counting beetles on plants or yellow sticky traps can help determine the risk for future corn injury within the field. Combining your beetle scouting with observations on lodging and root injury can help you get a handle on whether the beetles emerged within the field and potentially indicate resistance to Bt RW.

Mostly through trial and error, I figured out there was more than one size bolt on the old mower. It is risky to select a rootworm management tool based on scouting results from a different field. On the other hand, scouting information from numerous fields might provide an estimate of overall populations and their changes from year to year. Scouting corn rootworms provides details.

Part 4: It's complicated!

Consider these points when developing a rootworm management plan of your own.

• 

  Figure 5. An aggravated western corn rootworm
  larva floated from a corn root.

  Corn rootworms have shown the ability to overcome insecticides, Bt, and crop rotation.
• There are no good options to rescue a field from rootworm injury after planting.
• Eventually, a management program is likely to fail when you use the same management tool over and over.
• Corn rootworm problems are field-specific. Use beetle scouting and/or sticky traps to determine the risk for each field.
• Use root injury scores and beetle populations to evaluate your RW management success and adjust as necessary each year.
• Root injury is often more yield-limiting, and insecticides perform less well, under drought stress. You do not need to use a control tactic on low rootworm populations. Bt-RW hybrids and insecticide applications to low rootworm populations will not provide an economic benefit but they still select resistant individuals. (The management philosophy for rootworms should be different from weeds; besides, you can’t kill them all…really).
• A Bt RW trait or full rate of an effective at-plant insecticide can provide ROI when the previous year’s scouting indicates a risk of lodging and/or yield loss from rootworm feeding.
• Add a full rate effective insecticide layer to a Bt RW hybrid when populations are high and/or you are planting a Bt RW hybrid and suspect resistance. Large, Bt-resistant populations are often found in areas where continuous corn production is prevalent. As rootworm pressure increases, you might benefit from planting hybrids with larger root systems.
• Rotate out of corn where very high RW populations occur. Traits and/or insecticides may not provide adequate control of very high populations, particularly in fields with some level of Bt resistance to Bt RW is suspected. Beetle control to control silk clipping may sometimes be needed to allow pollination but economic benefits from these treatments are not common.

So… you have lots of beetles and unexpected injury to a Bt RW hybrid (more than ½ node of roots pruned to within 1 ½ inches) and you can’t or won’t rotate the field out of corn?

• Contact your seed dealer to report the performance failure. They will document the damage and perhaps more importantly check for Bt expression. I know it’s hard to believe but, sometimes a field gets planted to the wrong hybrid.
• Rotate the field out of corn! Seriously! Other management options risk unacceptable root injury and continued resistance development. You can plant a hybrid without a Bt RW trait and apply a full rate of an effective at-plant insecticide. Depending on the insecticide, rootworm population density, and the weather, the insecticide may/or may not provide adequate control. Remember, even with the insecticide applied, the field can still produce large numbers of Bt-resistant beetles to colonize neighboring fields.
• Bt RW hybrids share traits across companies and there is cross-resistance to some traits. Very high levels of resistance to Herculex® and associated Cry3 traits mean the Bt hybrids will probably perform like a hybrid without Bt. You are encouraging the development of even higher levels of resistance to Bt even with an insecticide overlay.
• Beetle control may reduce egg-laying in the field to a manageable level, but intensive, prolonged scouting is needed to properly time the multiple foliar applications that are typically needed.
• New rootworm management technologies are being introduced (i.e., RNAi) or still under development in top-secret locations. However, history and rootworm biology tell us that these too will likely be overcome eventually. The RNAi technology needs an effective Bt RW trait companion to perform well. Placing these new hybrids into fields with very high rootworm populations and Bt RW resistance is asking for trouble. Rotating the field out of corn for at least a year would be the best tool to use.

My new mower still has all its bolt and screw heads intact and judicious use of the proper tools at the proper time should keep things functioning well ‒ It seems that the bolt holding the blade to the crankshaft might need some special treatment though ‒ A piece of pipe over the wrench handle and a hammer ought to do the trick.