Populations of western corn rootworm and its variant counterpart continue to increase in fields throughout western and northwestern Ohio, and are even beginning to spread east, according to Ohio State University Extension sampling.

In preliminary data of the first year western corn rootworm variant, 26 of 93 fields reported have reached or come close to the threshold of five adult corn rootworm beetles per trap per day, says Ron Hammond, an OSU Extension entomologist with the Ohio Agricultural Research and Development Center. “This year we have seen populations similar, if not slightly more, to what we saw last year with regards to the variant,” he says. “Most of these fields are still in the northwest and west-central portions of the state, from Fulton County down to Darke County, and as far east as Crawford County.

“In terms of the normal western corn rootworm, we are seeing higher populations throughout the area, as well,” adds Hammond. “Unlike five years ago, when the pest was something we had to just simply be aware of, now it’s getting to be a concern.”

Entomologists are also continuing to see high levels of corn rootworm injury this past summer, both in corn following corn and in first-year corn following soybeans. Larvae hatching in cornfields following corn, or, in the case of the variant, hatching into cornfields that had been in soybeans the previous season, cause the most significant damage by feeding on corn roots. In severe cases, corn lodging can occur and reduce yields.

Fields reaching threshold numbers will need a preventive treatment for rootworm if corn is planted into those fields next spring, advises Hammond. There are many management options available to growers to help control populations and reduce feeding injury. They include:

• Granular insecticides: Several granular insecticides are available, and all do a good job against corn rootworm populations. “The most important thing about using granules is to make sure the granular equipment is properly calibrated and the granules are applied properly at planting,” says Hammond.
• Liquid insecticides: Like granule insecticides, these products are effective in controlling rootworm larvae.
• Seed treatments: Currently there are two seed treatments that are effective against the corn rootworm: Cruiser and Poncho. “They do a good job against low to moderate populations, but they may not do a good job against high populations,” says Hammond. “If populations are severe, then we don’t recommend using the products.”
• Crop rotation: In areas not impacted by the western corn rootworm variant, crop rotation is the best management practice. “Crop rotation still does the job in breaking the cycle,” says Hammond. Continuous cornfields are most at risk for corn rootworm problems, since most adults lay eggs in cornfields for larvae emergence in the following year’s corn crop.
• Transgenics: Corn hybrids with single or stacked traits can be used, especially in fields impacted by the corn rootworm variant. “There are several Bt-rootworm transgenic hybrids that can be used to control the rootworm,” says Hammond. “If using a transgenic hybrid, a grower must plant a 20% refuge within or adjacent to the transgenic corn.” The refuge prevents the corn crop from losing its resistance to corn rootworm.

Sampling is the first step in determining the level of corn rootworm populations in a given field, notes Hammond. “Unless you have sampled your field, you don’t know for sure if you will have a problem with corn rootworm,” he says. “If growers haven’t sampled their fields for next season’s management, then they should check with their Extension office to see where the county is in terms of populations. It will at least give growers an idea as to whether or not they should treat.”

Extension specialists have been tracking corn rootworm populations in Ohio for the past decade. For more information on corn rootworm, refer to the OSU Extension Agronomic Crops Team Web site at agcrops.osu.edu. Additional information on corn rootworm management can be found at ohioline.osu.edu/ent-fact/0016.html.

Source: Ohio State University Extension

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