After looking at rootworm damage in several Iowa Soybean Association On-Farm Network trials last week, where soil-applied insecticides are being tested, it became evident that in continuous corn, soil-applied insecticides are giving additional control of corn rootworm larvae, even when the hybrid planted contains a Bt rootworm trait.
Tristan Mueller, On-Farm Network operations manager, says these observations, combined with last year's findings, suggests that there was either sufficient rootworm pressure to overwhelm the Bt trait and the chemical insecticide or there is resistance to the Bt trait in the rootworm population
"We're definitely seeing heavy rootworm feeding and a significant amount of lodging in strips where the soil insecticide was not used," Mueller says. "Since rootworm control using the Bt trait requires that rootworm larvae feed on roots, we would expect to see some root damage in the untreated strips in second year or later corn.
"But we're also seeing quite a bit of feeding in the treated strips. In fields we've inspected so far, we haven't seen severe lodging in the treated strips, but there has been some," Mueller notes.
Mueller says if you have not taken a look at rootworm control in continuous corn, now is the time to do it.
Replanted areas and late-planted fields of corn may be especially vulnerable to rootworm damage, both from larvae and from adult feeding on silks and leaves. Mueller says it's a good idea to watch these fields into mid-August. The late planted corn may draw in large numbers of adult beetles looking for something to feed on. That could give those areas of the field even higher numbers next year.
"Look for root damage to get an idea of how much feeding there has been in the field. Don't just look at lodged areas. Damage to two root nodes can mean an economic yield loss," he says.
"Also look for adult corn rootworm beetles. Leaf damage caused by beetles feeding may be apparent in fields, but it seldom causes yield loss. Silk feeding by adults can interfere with pollination, so can cut into yields. And the high population of adults we're seeing in many fields in Iowa right now means massive amounts of eggs deposited in the soil," he says.
"If you have seen root damage and a significant number of beetles in fields where you intend to plant corn again next year, now is the time to decide on what management steps you should take," Mueller adds. "That may include spraying to control adult beetles before eggs are laid. If you are using a hybrid with a single Bt rootworm trait, you may want to switch to one with multiple Bt rootworm traits, add insecticides to you management program or both."
Erin Hodgson, Iowa State University Extension entomologist, says to also watch for beetle damage in moisture stressed areas of the field. "Plants do not tolerate as much feeding during pollination in hot and dry weather. For example, under ideal moisture conditions, plants could tolerate 15 beetles per plant, but that number is reduced to just five per plant under drought stress," she says.
Rootworm populations can grow exponentially in continuous corn, even if soil insecticides and rootworm resistant Bt traits have been used. "There will always be some larvae that escape control, but we're seeing high numbers of adults in some fields planted to corn hybrids with BtRW traits," Mueller continues. "Some growers have expressed a concern that the insects may be developing resistance to the Bt trait. We're working with Iowa State University entomologists to collect adult beetles in some of these fields to be tested for resistance to the Bt traits. We'll report the results when we can," Mueller says.
"In the meantime," says Aaron Gassman, Iowa State University research entomologist, "if you're serious about getting a rootworm problem under control, putting soybeans back into the crop rotation is the best way to break up the cycle in a field and help keep the Bt corn rootworm traits we have effective."
For a look at Mueller's summary of 2012 On-Farm Network soil applied insecticide studies, visit the On-Farm Network website. See last week's newsletter for a list of additional rootworm scouting resources.