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Corn+Soybean Digest

Wrangling Rootworms

The more things change, the more they stay the same, or so the saying goes. The corn rootworm is a constantly evolving pest, one that somehow seems to change just enough every couple of decades to make farmers adapt or incur big losses on their corn crop.

During the past few years, you've heard about the western corn rootworm (WCR) evolving into two biotypes. The first one lays its eggs in corn while the second lays eggs indiscriminately no matter what crop is present. If enough eggs are laid into a field that's rotated to corn the following year, you could see continuous-corn type rootworm pressure. If a farmer is unprepared, the result can be severely lodged corn and significant yield loss.

The WCR variant is indistinguishable from the “regular” WCR and it's changing the way farmers protect their corn, especially in areas close to the variant's epicenter in east-central Illinois.

“It seems evident the variant is spreading to the northwest and to some extent the southwest of that area,” says Mike Gray, entomologist at the University of Illinois. “This year in Illinois we've concentrated our on-farm survey efforts to the south and southwest of that area because of farmer reports indicating lots of beetles in soybeans. And we also surveyed counties that hug the Mississippi River — from St. Louis up to northwestern Illinois.”

The variant has been found west of the Mississippi, but while some farmers in Iowa have the pest, Marlin Rice, entomologist for Iowa State University, says there's no reason to panic.

“We certainly don't have a disaster on our hands,” Rice says. The key is scouting. If you don't scout, don't treat, he adds.

“Farmers will spend money for insect control when it's really unneeded because they hate to harvest lodged corn,” Rice points out. “In the absence of scouting information from the previous year, making an economical and practical decision on rootworm control is extremely difficult. If you didn't scout, I don't see how you could justify spending money on a pest that may or may not be in your field.”

Gray agrees that scouting is a good idea, and recommends any farmer on the edges of known variant rootworm areas use sticky traps to determine the economic threshold necessary for the following year.

“Generally, (on a 12-trap test) when you begin to get one beetle/trap/day it's an indication the variant is probably beginning to establish in that area,” says Gray. “Our economic threshold is five beetles/trap/day — that indicates the potential for economic problems the following year.”

In Illinois, the recommendation is to place sticky traps in the field during the last week in July, replacing them once a week through the first three weeks of August.

“Over a four week monitoring period you'll use 48 traps per field. So you're looking at a cost of about $50 plus your time. Then you can make a more judicious, informed decision regarding whether a soil insecticide is needed the following spring,” says Gray. “When you consider the average cost for soil insecticide is $15/acre, with 100 acres you're looking at a cost of $1,500.”

If you want to cut down your costs and time, Gray says you can use a four-trap test.

“If you're on those fringe areas, probably one of the best things you could do is put out four yellow sticky traps,” says Gray. The actual threshold is figured on 12 sticky traps per field, but four will determine presence or absence of the variant and that may be enough to guide your treatment decision the first year, Gray points out.

“Do not gamble with this insect,” Gray warns. “If you make a mistake and guess wrong because you didn't use traps, the consequences can be devastating.”

The five beetles/trap/day threshold will help you determine if you should treat next year's rotated corn like you would continuous corn. It's not to determine if you should treat your soybean field for the egg-laying females, says Gray.

He recommends that if the threshold is met, farmers should treat fields with either a soil insecticide or a plant a transgenic hybrid the following year. Gray urges caution when deciding whether or not to use a seed treatment such as Cruiser or Poncho because often they don't hold up well in high-pressure rootworm situations. Lack of consistency under high rootworm pressure has characterized the main problem with these seed treatments in Illinois trials.

Iowa also has another variant of the rootworm family, the extended-diapause northern corn rootworm (NCR). The NCR has adapted to rotation in some areas of the country by hatching during the second season after egg laying, skipping the year during which soybeans are in the field. Rice says the delayed hatch behavior seems dependent on cold weather, which means the extended-diapause problem lessens as you move east and south through the state. However, the extended-diapause NCR is found statewide.

In Nebraska, where the extended-diapause NCR has historically been a problem north of the Platte River, the pest appears to be causing damage further to the south, says Robert Wright, Extension entomologist at the University of Nebraska.

The only way to experimentally confirm the presence of the extended-diapause NCR is to collect an egg and see if they hatch two years later. Because that's not practical, Wright says they assume that extended-diapause NCT is responsible for some of the damage reports to first-year corn.

Data from Minnesota and South Dakota shows that in affected areas around 50% of the eggs in a field can be extended-diapause NCR, he adds. Wright's recommendation is if people have a history of problems, they may want to consider using a soil insecticide at planting. Iowa State's Rice says they use emergence cages over the ground in first-year corn fields to diagnose extended-diapause NCR.

Farmers shouldn't panic because rootworms — even variant rootworms — are manageable, says Rice. He points out there are areas of the state where the WCR variant and the extended-diapause NCR overlap.

“If this becomes widespread, both species will have conquered rotation as a pest management tool,” he says. “With what we know about rootworms, that's not surprising. They've conquered some insecticides and they continually evolve based on the selection pressure we place on them.”

Rice cautions growers to be aware of what's in your fields, and use integrated pest management strategies. “I give credit to the seed companies,” he says. “They have embraced the refuge concept. Farmers have to follow the guidelines or we will place tremendous selection pressure on the insect and it could evolve again.”

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