New rating system doesn't change insecticide rankings Corn rootworms will never know the difference, but the havoc they create will be rated differently from now on. At least by Iowa State University (ISU).

After 30 years of rating rootworm damage on a scale of 1 through 6, ISU entomologists have developed a new rating system that more accurately defines the damage done by rootworms, according to ISU entomologist Marlin Rice.

"The old system used a linear numbering system to explain exponential damage," he says. "A root rating of 2 didn't mean that the damage was half that of a root rating of 4. But, with the new Iowa State node-injury scale, there is a straight linear relationship. A rating of 3 means there's twice as much injury as a rating of 1.5."

The new rating system ranks root damage on a scale of 0.00 to 3.00. To compare the Iowa 1-6 scale to the Iowa State node-injury scale, you can go to the Internet and check the Web site, www.ent.iastate.edu/pest/rootworm/nodeinjury/nodeinjury.html .

"Injury by corn rootworm larvae can result in a significant amount of the roots being injured or removed from the plant. A low Iowa State node-injury root rating (0.25) is highly desirable," says Rice. "This is essentially the same amount of injury as 2.5 on the old, Iowa 1-6 scale."

This low root rating indicates that the insecticide adequately protected the roots from economic injury, according to Rice. "Each insecticide was measured under moderate-to-heavy feeding pressure. Roots in the 2000 experiments from the untreated plots averaged 1.68. That indicates that 1 21/43 root nodes have been eaten away."

Rice believes the new rating system will be generally accepted by the industry. "Pioneer has already accepted it. That's a start," he says. "Several universities have contacted us also. We hope it becomes the new standard. It's more intuitive."

Regardless of the rating system, last year's ISU tests of rootworm insecticides showed that chemical effectiveness remains the same.

"No insecticide was 100% consistent in providing adequate protection (a rating of 0.25 or less) during 2000," says Rice. "Even the best products sometimes failed. But this was rare. Most products gave very good root protection most of the time based on the percentage consistency."

Every product in the table from Force 3G down to Capture 2EC provided similar levels of consistency from a statistical standpoint, says Rice. "However, a number of products did not perform well and those can be found near the bottom of the table.

"The top five products really haven't changed much over the years. Force 3G and Aztec 2.1G are always on top," Rice says. "The products at the bottom have been at the bottom."

One new product tested last year, Capture 2EC, fared well, according to Rice. "Statistically, it worked just as well as Force 3G," he says. "It's a pyrethroid product that's applied the first week in June. It can be applied with an herbicide or layby cultivation."

Seed treatments ProShield and Prescribe proved "less than stellar" in the 2000 ISU trials, according to Rice.

"Entomologists at other universities have seen similar results. Farmers buying the seed treatments solely for rootworm control will be disappointed. The products may have a place with secondary pests."

The ISU results show how many complete nodes were destroyed by rootworms in the third column and how much partial node damage was done (fourth column). For example, with Force 3G no node was completely damaged and only 9% of a node was injured, on average, for a final rating of 0.09. The check plots averaged one full node destroyed and 68% of another node damaged for a rating of 1.68.

In the final column, the data lists the percentage of times a product rated a 0.25 or less on the Iowa State node-injury scale. For example, Force 3G-treated plots received a rating of 0.25 or less 96% of the time. The check plots, on the other hand, reached that rating only 13% of the time.

"An easy way to understand percentage consistency is to think of it as being similar to a baseball batting average," Rice says. "The higher the number, the better the performance."

Consistent performance is only one factor you need to consider when using a corn rootworm insecticide, according to Rice. "Other factors worthy of consideration might be cost, pounds of active ingredient applied per acre, ease of handling, application equipment needed, other pests controlled, restricted-use labeling, and potential hazards to surface water," he says.

For ISU's full report on rootworm rating systems and chemical control results, check its Web site www.ipm.iastate.edu/ipm/icm.