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What to do about corn rootworm
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What to do about corn rootworm

May 10, 2021

It used to be that growers could rely almost entirely on Bt corn hybrids to protect against corn rootworm (CRW) damage. Sadly, Bt-resistant CRW populations have already been found in Ontario, and Quebec may not be far behind. So it’s time to step things up a notch in terms of CRW scouting and prevention.

Start with identification

There are two corn rootworm species to look for, western and northern, and together they are known as the corn rootworm complex.

The western corn rootworm adult is yellowish with black striped wing covers and a black head. The adult of the northern corn rootworm is a uniform yellow-green to green colour.

Figure 1: Western corn rootworm adult (left), northern corn rootworm adult (right).

Both species have a very similar lifecycle, producing a single generation per year. Overwintered eggs hatch in spring when soil temperatures reach 10 degrees Celsius – usually around late May to mid-June.1

Emerged larvae are whiteish with brown heads and butts. As they move through three growth stages, they feed on plant roots. The whole larval lifetime lasts only three weeks, and it is during this period where the most critical plant damage occurs.

Figure 2: Corn rootworm larvae. Note the brown head capsule and brown anal plate. Photo courtesy of UNL Entomology.

Adult beetles emerge from the ground in late July to early August1 and, after mating, females lay eggs in the soil, most coming to rest within the top six inches of dirt.

Look in your soybeans, too

In the U.S., there is a variant of CRW that is a soybean pest. Corn crops planted after soybean are threatened by this variant because the lifecycle is just the same.

So far, this variant has not been confirmed in Ontario or Quebec, but if you’re interested in finding out if it’s in your soybean fields, you can use sticky traps just as you would in corn (see below). Place them at least 18 inches above the soybean canopy and if your average daily “catch” 1.5 CRW adults or more, some kind of control tactic may be necessary if you choose to plant corn in that field the following season.

While this variant doesn’t harm soybeans, the fact it can survive in a soybean field means continuous soybeans is not a good idea. The most effective control tactic is to rotate an infested soybean field to winter wheat.

Assess risk by trapping adults

The best way to assess potential damage from CRW next year is to find out how many adults you have flying around this year.

You can scout for larvae, but all that tells you is that corn rootworm is in your field. Experts say that two larvae per plant is enough to warrant some action, but by the time you find larvae, it’s past the point where you can do anything to prevent some economic loss. So focus on adult counts and coming up with a plan for next year.

You can directly count adults hanging out on corn plants or use sticky traps, which is likely easier and more accurate. The best time to do this is from late July to early August as they are emerging and mating.

This count will give you an idea of what kind of damage level you may be facing next spring, then confirm that finding with a root dig the following year to assess actual damage levels.

How to deploy sticky traps in corn

  • Select two rows of corn for every four to 20 hectares (10 to 50 acres). These rows must be separated by at least 100 metres (330 feet) from each other and at least 50 metres (165 feet) from the field margin.
  • At silking, place six sticky traps in each selected row. Attach them directly above the corn ear, stripping away any leaves that could get stuck on the trap. Make sure the traps are 50 metres apart along the row.
  • Mark the row so you can find it again!
  • Once a week count the number of CRW adults per trap. Do this for four weeks and replace old traps with new ones after each count.
Figure 3: Sticky trap used to sample corn rootworm adults. Photo courtesy of Cory Tilstra.

Determine economic threshold
To see if you’ve reached the economic threshold, you need to work out the average number of CRW adults per trap per day. Here’s an example:

A total of 100 beetles caught on 12 traps over one week = an average of 1.2 beetles per trap per day. (100 ÷ 12 = 8.3; 8.3 ÷ 7 = 1.2)
If you hit two or more beetles per trap per day during the four-week sampling period, start thinking about what CRW control measures you’ll be using the following season.

How to protect corn from CRW

There are three management tactics that work against CRW: Cultural, chemical and genetic.

Rotations. Corn (and sometimes soybeans) is the only crop where CRW can complete its lifecycle. If you find sufficient adult populations in this year’s crop, rotate to something else the following year. Soybeans may still be safe, but be prepared to scout, just in case. Winter wheat is the best choice.

Be sure to manage corn volunteers in that rotational crop to break the CRW lifecycle. If you can, don’t plant corn for at least two years.

If you have to plant corn on corn, then go with a non-CRW hybrid in the second year. This will reduce selection pressure on the trait and you can use a seed treatment to control the bug.

It should be noted that crop rotation is the only proven cultural control for CRW. Tillage has no effect on egg survival and winterkill, while not a cultural method, is not to be counted on. In fact, to get a decent winterkill of CRW eggs, soil temperature has to remain a constant -10 degrees Celsius or colder for a minimum of two to four weeks.2

Insecticides. The most effective approach is to use an insecticidal seed treatment, and/or use a soil-applied insecticide just before the eggs hatch in late May. The goal is to stop the larvae from feeding on the crop’s roots.

What about controlling the adults with a foliar insecticide? Again, experts don’t recommend this as there’s no economic benefit – the damage has already been done by larvae earlier in the season, so a foliar insecticide is rather like closing the barn door after the horse is gone.

Traits. The fact is that CRW has developed resistance to multiple Bt-RW proteins – meaning even pyramided Bt corn hybrids may be susceptible to this insect.

It means resistant CRW populations are here and here to stay. It’s imperative that corn growers do what they can to reduce these populations and thereby protect the usefulness of current Bt-RW hybrids for as long as possible.

So what can you do? OMAFRA has developed an excellent list of mitigation measures for corn rootworm, including a decision tool to help you decide if you need to do something and what you can do.

Check with your seed partner representative to talk about what Bt proteins are at risk, which ones still have good activity against CRW and how they should be used.

1 Ontario Ministry of Agriculture, Food and Rural Affairs. Corn rootworm in Ontario CropIPM.

2 Gustin, R.D. 1981. Soil temperature environment of overwintering western com rootworm eggs. Environmental Entomology. Vol. 10, pp. 483-487.