Expect corn rootworms in full force this year

By Charis Prunty
Contributing Writer

This winter may have seemed long, cold and especially snowy to many in Minnesota. But for the rootworm eggs laid a few inches below the surface in corn fields last year, experts say, it hasn’t been all that bad.

Southern Minnesota farmers battle two types of corn rootworms: The northern corn rootworm (NCR) and western corn rootworm (WCR). These rootworms damage corn as their larvae feed on the roots and the adult beetles eat corn silks and leaves. Root damage can make corn plants misshapen, hindering both pollination and harvesting, and sites of rootworm damage lend themselves to infection by pathogens. All of this contributes to yield loss.

According to Bruce Potter, Extension integrated pest management specialist out of Lamberton, an early blanket of snow helped to insulate fields this winter, while air temperatures did not dip cold enough for long enough to kill rootworm eggs. To kill all WCR eggs, the ground temperature needs to be .5 degrees or lower, or stay at 14 degrees or lower for four weeks. Any lesser amount of time at this temperature means only a percentage of eggs would be killed.
“Without that frost going deep and the females laying their eggs deeper (because of drought), in the months of August and September, before we caught fall rains, I don’t feel there’s much frost out to damage any eggs,” agreed Matt Salentiny, a crop consultant for Centrol Crop Consulting out of Tyler.

However, there’s still a chance those eggs could drown, if the area receives strong rains in mid-June.

“Nobody wants a 4 to 5 inch rain, but those rains will drown the hatching larva,” Salentiny said, adding that this happened locally in 2016, 2017 and 2018. “That (rain) was drowning the rootworm below ground. It was hurting our crops, for certain, but we were crashing populations all over.”

The good news, Potter said, is sticky trap counts last year showed that rootworm populations were down a little bit throughout the state. Yet the rootworm population was high enough to cause injury and was still fairly high overall — especially in southern Minnesota. The slight decrease in summer 2022 could be due to lack of snow cover on fields the previous winter and because it was so dry for a couple years. In general, rootworms like it dry, Potter said. But if they have to lay their eggs lower than 2 to 4 inches in the soil, this can put them in a bad position.

While WCR may die off from the cold, NCR is native to Minnesota and is more cold tolerant. For its part, NCR employs a tactic called “extended diapause” to stay alive. Extended diapause means the rootworm eggs, which were laid in corn fields, don’t hatch when the field is planted to soybeans the next year. They wait.

“It’s just an adaptation to a corn-soybean rotation that, if you delay your egg hatch for another year, you’ll be back in corn,” Potter explained. “If you don’t, in a corn-soybean rotation, you hatch in soybeans and you can’t survive.

“They have to have at least grass roots for a good host,” he added, “to help them get through their development. Corn is by far the best host. If they hatch in a soybean field or another non-host, they starve to death.”

So, how can farmers protect their corn against rootworms? Jeremy Smidt, who lives in rural Pipestone County and sells for AgriGold, said selecting the proper seed for each field can help. His seed sales have been “a lot heavier” in favor of double-stacked corn this year, which is genetically cultivated to resist pests both above-ground and below-ground.

“Especially if you’re going to do corn-on-corn, it’s kind of a must to have some rootworm protection,” Smidt said. “You’ve got to plant a stacked corn that’s got both an above- and below-ground insect pressure.”

However, some modifications and treatments that have worked in the past are starting to lose their effectiveness. Potter said researchers are seeing quite a bit of resistance to the Bt traits in the rootworm population. According to Colorado State Extension, Bacillus thuringiensis (Bt) is a soil bacterium that produces insecticidal toxins. Genes from Bt can be inserted into crop plants to make them capable of producing insecticidal toxins and thereby resisting pests like rootworms.

Some rootworm populations have become resistant to protections like Bt and certain insecticides. All the experts noted that the worst rootworm problems are found in fields planted to corn every year. In southern Minnesota, some farms rely on corn to feed cattle or to fulfill an ethanol contract.

“There’s some growers that I have as customers, that feed a lot of cattle and they’re buying corn besides what they raise,” Smidt explained. “But in the last couple years, they’ve had such rootworm pressure that they’ve been forced to take a few acres out and put into soybeans to try to get a little bit of a rotation going, to try to eliminate some of that pressure.”

In fact, taking problem fields out of corn is the most effective remedy, Potter said. Fortunately, just one year of planting an alternate crop can suppress the rootworms in that field for multiple years to come, if conditions are right. The field might then have several years with no major rootworm management needed. Or, if infestation is high in the area, it might only get a couple years before needing to rotate again. This partly depends on the rootworm population of neighboring fields. If neighbors work together to rotate crops in a specific area, they can effectively kill off the pests in just one year.

Everyone stressed that knowing what rootworms are doing in each individual field is critical to keeping them under control.

“Mid-March, seed would be your biggest thing,” Salentiny said. “Talk to your seed dealer, your agronomist, and make sure they have the proper seed. And also, if you had good history notes of your fields in the past, that will help you make good decisions now.”

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