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WCR: One resilient pest

By Staff | Mar 21, 2013

COON RAPIDS – In 1963, western corn rootworms were documented in Iowa for the first time. They’ve made their way slowly from Colorado, where they were first spotted in 1867, but have now found their way well into Maine.

According to Erin Hodgson, an Iowa State University entomologist, rootworms are an extremely resilient insect, capable of developing resistance to Bt corn traits, chemicals and soil management practices in corn-on-corn system in just three generations.

Hodgson spoke to an estimated 50 farmers on March 15 during an ag fair sponsored by the Extension offices in Audubon, Carroll and Guthrie counties.

She said that in 2011, rootworms were proven to have developed resistance to at least one Bt trait – Cry3Bb1.

She said rootworms were a big problem in the Carroll County area last season and blamed the high population of rootworm beetles on producers not varying their pest management practices, which included repeatedly planting the same Bt trait in continuous corn fields.

To counter rootworms, she said, fields need to have at least one season of soybean rotation in three years, to allow for the larvae that hatch to starve, since they won’t feed on weed or soybean roots.

In addition, she said, rootworm control has to be varied and include multiple practices.

“Insects will always try to overcome practices,” Hodgson said. “Once they lose susceptibility, the chemicals and (land management) practices are lost resources.”

rootworms feed on corn roots, inhibiting the plant’s nutrient uptake. Beetles feeding on stalks, makes the stalks susceptible to soil diseases and will show itself in goose-necking and lodging.

Hodgson recommends farmers switching up their integrated pest management and insect resistant management practices.

IPM practices includes watching hybrid selections and possibly planting later in the season to allow early-hatching larvae to starve, rotating crops and constant scouting to assess the economic thresholds of treating for pests.

IRM practices include Bt trait rotations in continuous corn fields and maintaining non-Bt refuge management.

The goal, she said, is to keep numbers low and prevent resistant beetles from mating with other resistant beetles.

“There will always be survivors in any Bt corn field,” Hodgson said. Left unchecked, a resistant colony can develop quickly. One colony was developed in a lab in three generations she said.

Since the beetles mate where they eat, they have a low dispersal rate, about 130 feet in a lifetime. “So the genetics are not being diluted,” she said.

She said researchers, in 2011, found that continuous corn, using the same Bt trait three years running, have very high survival rates of rootworm beetles. Yield loss in a stressful year, she said, can be seen if just one-fourth of a root node is damaged.

“But we were finding two nodes damaged,” she said, which constituted a catastrophic failure in those plants.

“The honeymoon period of ‘plant it and forget it’ are over,” Hodgson said. “You have to understand that every year in corn-on-corn larvae numbers will increase.”

She said the best investment for confusing rootworm beetles is rotating crops with soybeans.

Following that, she said, they should rotate their traits and use soil insecticides on their non-Bt refuge acres.

“Seed traits, rotations and soil insecticides are all the tools we have,” Hodgson said.

Soil insecticides will protect primary roots, Hodgson said, which will keep plants sturdy, but larvae will feed on secondary roots, which will cause yield loss.

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