The sticky traps in Iowa corn fields last summer were an open invitation to rootworm beetles. However, scientists are still scratching their heads over the abundance that showed up for the party.
The Iowa Soybean Association (ISA) On-Farm Network placed sticky traps in 176 corn fields to collect weekly adult corn rootworm beetle counts. With four traps per field, 50 to 60 beetles were collected in each field during the course of a week. According to Iowa State University, the threshold for corn rootworm beetles is two per trap per day.
Most of the fields in the trapping program had been using the same corn rootworm control methods for several years. “But, that was not always the case,” says Rich Stessman, ISA operations manager. “We also saw high numbers in fields that used a variety of practices, from rotation to traits to insecticides.”
Higher rootworm numbers were expected based on conditions last winter and spring, but they didn’t show up in the fields where they were anticipated, Stessman explains. “Fields with higher numbers did not necessarily correspond with trapping numbers we saw in 2015 and 2016. And, we saw the most severe infestations in areas where we also had drought, meaning that larvae survival likely played a factor in the counts.”
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Confusing Counts. Erin Hodgson, Iowa State University Extension entomologist, also found trapping in 2017 to be highly variable. Some fields had little to no activity on the sticky traps, while a field nearby would see a nearly filled card. “Not every field had the same amount of activity, even at a local level,” she says.
The extended 2017 planting season may be part of the cause. “There tends to be more adults in late-planted fields because they are feeding on silks,” Hodgson says. “But the adult beetles are mobile, so just because you see a lot of trappings in a field, that doesn’t necessarily mean you have rootworm injury.”
While traps are a good way to identify beetle activity, they don’t tell the whole story. Digging and assessing root injury in-season is the best way to evaluate if current rootworm products are holding up, Hodgson explains. “Seeing adult beetles in a field would more likely point to possible increased numbers next year, because the beetles are laying eggs.”
Plan Ahead. Joseph Spencer, University of Illinois research program leader in insect behavior, urges growers not to jump to conclusions. “It is possible to have a rootworm beetle population and not have significant economic injury,” he says.
Illinois research from the 1980s showed that use of soil applied insecticide (SAI) to protect corn roots can actually result in higher rootworm populations than observed on unprotected corn roots. “By protecting the area around the seed, the roots have a chance to grow and proliferate so that, by the time they extend beyond the protected zone, the mass of roots available to feeding larvae is greater than what is able to grow on an unprotected root. Thus, the SAI-treated plants can produce more beetles than untreated, they have good root protection and don’t lodge,” Spencer says.
He adds the first choice for growers who saw Western corn rootworm pressure last year should be to rotate to soybeans. “I’d then suggest follow-up beetle monitoring in soybeans using sticky traps (two-corn-rootworm-per-trap-per-day threshold) to determine whether soil insecticide would be needed on first-year non-Bt corn,” he says.
“If sticky trap rootworm counts in rotated corn indicate a potential problem, then it might be a good time to use a pyramided Bt hybrid that expresses Bt traits that have worked on your farm or go back to soybeans.”
Iowa growers seeking additional information can volunteer to work with the On-Farm Network’s trapping survey in 2018.
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