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April 24, 2003

Entomologist: Skimping on insecticide won't root out rootworms

WEST LAFAYETTE, Ind. – Northern Indiana corn growers shouldn't cut corners on soil insecticides this spring or they could see their crop carved up by western corn rootworms.

Purdue University specialists observed near-record numbers of rootworm beetles in soybean fields this past fall. Those beetles laid eggs, which are expected to hatch in coming weeks. When they hatch, the rootworm larvae will be ready to feast on corn roots, said John Obermeyer, Purdue Extension entomologist.

"We've been monitoring soybean fields for rootworm beetles since 1995," Obermeyer said. "The numbers we found in 2002 were very close to the record-high levels of 1997."

Rootworms tunnel from corn root tips to the plant base. The pest's feeding habits can weaken corn plants, leading to stalk lodging and reduced yields.

Especially troubling is the rise in rootworm larval feeding on first-year corn following soybeans, Obermeyer said. In Indiana the phenomenon is most common in cornfields north of Interstate 70. For farmers in that region, insecticide application rates and timing of treatments is critical.

"There are many factors for the producer to consider," Obermeyer said. "For one, should they use a rootworm insecticide at planting? I think most producers north of I-70 should strongly consider the use of an insecticide because of the damage that occurred last season and the beetle numbers that we saw for egg laying.

"Some producers may be trying to cut expenses and consider reducing the rate of insecticide they use. In northern Indiana – especially in high-risk areas – we don't think that's a very good idea. When you reduce your rate you have less insecticide strength to work in your favor at the time of egg hatch, which is probably going to be sometime in early June."

Obermeyer said he urged farmers to follow product labels carefully when applying insecticides. Only a few products are known to be effective in controlling rootworms when applied in lesser amounts. Even then, producers in rootworm-threatened counties should wait before easing off on application rates.

"The labeled rates are what we recommend producers follow," Obermeyer said. "For those choosing a reduced rate, we suggest they do this only with products such as Counter 20CR and Lorsban 15G. That's because we've been able to test those particular products for an extended period of time with reduced rates.

"The reduced rate that I'm talking about is cutting the insecticide by only a quarter and using three-quarters of a full rate. Because of the rootworm pressure that we see in most northern Indiana fields, we would recommend producers not even consider reduced rates until about the second week of June."

Obermeyer said farmers should keep other points in mind when considering reduced insecticide rates:

• Reduce application rates only on fields planted after May 1 and where rootworm beetle counts from the previous crop season were at, or slightly above, the economic threshold for damage.

• Avoid reducing rates in first-year corn where rootworm beetle populations were high in fields planted to soybeans or alfalfa the previous year.

• Experiment with reduced rates on selected fields or parts of fields to determine overall effectiveness before applying the same rate of insecticide on all fields.

"Also, producers should choose hybrid corn varieties with good strength in their roots so that if some rootworm feeding occurs the stalks have a better chance of staying upright," Obermeyer said.

Although beetle numbers were high this past fall, that does not guarantee a rootworm infestation this crop season. The record beetle count and egg laying in 1997 should have produced devastating larval feeding the following spring. However, heavy rain at egg hatch caused many rootworm larvae to drown.

"The rain impeded soybean planting progress, but it did reduce the rootworm population," Obermeyer said.

Additional information about rootworms and insecticide application rates can be found in Purdue Extension publication E-49-W, "Managing Corn Rootworms – 2003," by Obermeyer and Purdue entomologists Rich Edwards and Larry Bledsoe. The publication can be downloaded online.

Writer: Steve Leer, (765) 494-8415, sleer@purdue.edu

Source: John Obermeyer, (765) 494-4563, john_obermeyer@entm.purdue.edu

Ag Communications: (765) 494-2722; Beth Forbes, bforbes@aes.purdue.edu; http://www.agriculture.purdue.edu/AgComm/public/agnews/

Related Web site:
Purdue University Department of Entomology


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