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Population Explosion

Populations of Western corn rootworm in first-year corn in east-central Illinois and northern Indiana exploded in 1997 in a continuation of an outbreak that first was noticed in intense soybean/corn rotations four to five years ago. As the infestation expands beyond its epicenter, researchers are certain that the outbreak is no fluke. Although the mystery deepened in 1997, the burgeoning numbers also shed some new light on soil insecticide management.

Soil insecticides, used as they were across the region, did not work satisfactorily to stem the population explosion last season. As data become available, researchers are more certain that the ineffective insecticide control was due to earlier-than-usual at-planting application coupled with a later-than-normal egg hatch. The chemicals were not capable of persisting for the longer period.

Extended feeding season. Throughout the region, 1997 planting generally was completed in early to mid-April. Then a cold spell delayed the egg hatch in most areas until mid to late June. In fact, Purdue researchers have detected early June egg hatches in each of the last five years in Tippecanoe County, IN. "That gives you two to two-and-a-half months of separation from when the soil insecticides were going down, so we feel the larvae were getting sublethal doses of insecticides," says Larry Bledsoe, Purdue University entomologist.

Jim Wilkinson, a farmer in Oxford, IN, started planting in early April and didn't see egg hatches until the second week of June. "Ninety-nine percent of the chemicals won't handle that," says Wilkinson, a pioneer of the problem, who first noticed beetles in his soybean fields five years ago. If data confirm his early suspicions, Wilkinson believes that his 1997 population numbers may soar as high as 105% above '96 numbers.

Pocket droughts across Indiana also may have affected the efficacy of soil insecticides. When timely rains don't fall, corn plants have difficulty regenerating from rootworm damage. "Across the state in a dry year, you get complaints about insecticide performance, when it comes down to the fact that farmers didn't get key rains to move the product where it needed to be," Bledsoe says.

Larval damage in Indiana in 1996 was similar to that in 1997. However,with timely moisture, it was not as visible, and plants had a better chance for root regeneration. In Purdue test plots in 1996, says Bledsoe, "we did have rootworm injury to untreated roots, but harvest yield checks showed only a one-bushel difference between treated and untreated areas. The effect of the weather completely erased the effect of larvae in '96 in the heart of rootworm territory. It was a completely different story in '97 because of the reduction in moisture and lack of regeneration, and the effects of rootworm shine right through."

Adaptation. The mystery that continues to puzzle researchers, though, is why the corn rootworm beetles ever began laying their eggs in soybeans to begin with. One of the earliest theories, which received a lot of media attention, attributed the activity to extended diapause. Researchers have disproved this theory where the eggs were thought to overwinter two seasons before hatching. Experts also no longer accept the idea that conservation tillage practices may have triggered the outbreak.

Scientists now believe that adult female rootworms have simply adapted to years of strict soybean/corn rotations. "After 25 years of soybean/corn rotation, we have set up a monoculture system for the beetles," Wilkinson says.

Somewhere along the line, a certain type of "eastern phenotype" of Western corn rootworm developed in soybean/corn rotations because larvae hatching into soybeans from eggs laid in corn could not survive on soybean root tissue. Eggs hatched into corn roots, on the other hand, enhanced the survival of those larvae, and that behavior passed down from generation to generation.

"We don't think it's an attraction to soybeans because we see rootworm beetles laying eggs in nearby alfalfa, too," explains Mike Gray, University of Illinois entomologist. "It's a selected behavior that seemingly causes them to leave corn. In east-central Illinois, we have an intense corn/soybean rotation. It's a selection of behavior (for the beetles) to leave corn and lay eggs elsewhere."

Expanding territory. In 1996 the first-year rootworm problem was confined primarily to an eight-county area of east-central Illinois and about 16 counties in northern Indiana. In 1997, sweep net surveys of soybean fields found adult beetles in central-Illinois counties west of the previous epicenter.

Virtually every Indiana county north of Interstate-70 is infested. Sweep surveys did not indicate a problem in Indiana south of I-70 last season, but adult beetles in smaller numbers have been spotted in soybeans in western Ohio to the I-75 north-south corridor between Toledo and Dayton, and there is some evidence that they may be creeping into southern Michigan.

Soybean field sweeps last summer conducted by the University of Illinois in Iroquois County, the heart of the Illinois problem, produced 400 to 500 beetles per 100 sweeps, whereas densities of only 25 beetles per 100 sweeps were found two counties west of the epicenter.

"Densities in these outside counties are not as great as what we are seeing in eastern Illinois, but we still have to raise the question of why they are there to begin with," Gray says. "Is this the beginning of what may be in '98 more Western corn rootworm problems outside the (eight-county) area?" Illinois entomologists expect to determine threshold populations after a thorough review of the 1997 data.

Need for economic thresholds. Their data are based on trials the university conducted with 17 farmers with infested fields. Those trials began in 1996 when researchers evenly distributed sticky traps throughout selected soybean fields and the perimeters and checked them weekly for one month. In the spring of '97, they returned to those same fields, now planted in corn, and conducted side-by-side trials with untreated strips next to strips treated with soil insecticides. Corresponding the root injury ratings against beetles caught on traps will help researchers to arrive at economic threshold numbers.

Purdue researchers believe that where rootworm beetles were seen in soybeans, or other crops, such as alfalfa going into corn, application of a soil insecticide is probably justified. When beetles can be found within a few minutes of searching soybean fields, economic damage is likely.

To check for rootworm larvae in corn, Purdue researchers suggest random selections of plants from 10 representative sites across the field to examine the root system and surrounding soil for infestation. Either check by hand (crumbling the dirt) or rinse-wash the root system into a pail of water (the larvae float to the top). If the average number of larvae is two or more by hand-sorting or eight or more by washing, you may want to consider soil insecticides.

Insecticide saturation? Prior to the outbreak of infestation in first-year corn, Illinois farmers were treating only about 13% of corn following soybeans. "We've been told by dealers that their soil insecticide sales jumped a lot (after the outbreak)," Gray says. "We've heard estimates that jump all the way to 60 or 65 percent, though we don't have any hard numbers to verify that yet."

That means unprecedented amounts of soil insecticide are going on the ground, particularly in the northern two-thirds of Indiana where almost every county is affected. Accurate, well-managed application is imperative. "Spreading this amount of soil insecticide across northern Indiana is a huge deviation from the past, so let's be sure we're doing the right thing," Bledsoe says. "The potential increases for hazardous situations, and farmers don't want to see that or pay for it. The amount of insecticide used in the next few years will be unnecessarily high. Many low-risk category farmers who choose to use them without evaluating their particular situations will be the principal losers, the environmental risks notwithstanding."

Bledsoe urges farmers who suspect a problem to plant untreated as well as treated check strips to ascertain their level of economic damage. Additionally, Gray warns producers to make sure their planters are very carefully calibrated. "Most of the planters out there have ancient technology for delivering these granular insecticides. It could be a more precise science than it is," he says.

Generally, Gray says, most soil insecticides perform best in a 7-in. band ahead of the press wheels where some of the insecticide is incorporated. In-furrow treatments targeted solely for corn rootworms don't perform quite as well as band treatments, with one notable exception, Gray says: "Fortress performs better in-furrow because it tends to be a more volatile compound."

Bledsoe adds that the chemicals with very low solubility and very little movement in the soil tend to fair worse than others, particularly in a drier season when timely rains are of the utmost importance.

Due to environmental, as well as resistance concerns, broadcast treatments are frowned upon. Soil insecticides have a track record as a good resistance management tool because only a portion of the field in the 7-in. band is treated, therefore ensuring that untouched survivors, with no resistance and still vulnerable to chemicals, remain in the field.

SmartBox technology is an ideal strategy for Western corn rootworm control, scientists say. Indiana farmer Wilkinson, an IPM advocate, endorses the SmartBox because of its accurate metering, but he also believes some farmers using the boxes may have seen unsatisfactory results in 1997 due to calibration inaccuracies. Wilkinson, a self-described "high-residue" farmer, used the boxes on his planter last year. He is considering moving the boxes to his cultivator for later-season incorporation (using row cleaners to throw residue back over the chemical) to try to extend rootworm control.

"Product selection is important," Gray advises. "Some have a history of not being as consistent as others. You can look at any university trial and you'll see some products perform on a more consistent basis than others." He recommends growers check with their state's entomologist to obtain consistency ratings.

No matter what strategy farmers take with their fields this year, Bledsoe urges them all to plant check strips. "It's just a matter of turning insecticide boxes off for four passes across the field that are fairly well separated. It may convince them not to treat. Without a check row, they have no basis to make an evaluation if the money they are spending - and the pesticide they are putting into the environment - are necessary."

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