James D. OLESON, Jon J. TOLLEFSON1
1 Iowa State Univ., Dep. of
Entomology, Ames, Iowa 50011-3140, USA
The scale that is used most commonly to evaluate injury to corn roots by corn rootworm (Diabrotica spp.) larvae was published in 1971 by Hills and Peters of Iowa State University. Iowa State University has developed and is beginning to use a new larval injury scale that it feels is better and easier to use. The Hills and Peters scale is not linear. When larval injury is light, only very small amounts of larval feeding are sufficient to increase the scale one whole rating. From mid range to severe feeding injury a complete node of roots must be destroyed to raise the rating by one point. The Hills and peters scale also requires one to remember the definition of each rating category. The "Node Injury Scale" now being used by Iowa State University is linear and intuitive. The scale is intuitive because it ranges from 0, no injury, to 3, three nodes of roots destroyed. Three nodes of roots is the maximum typically attacked by corn rootworm larvae. The scale is linear because it is based upon the proportion of a node destroyed; therefore the scale is directly related to the amount of root tissue destroyed. For example, if the equivalent of none node of corn roots is destroyed by corn rootworm larval feeding the Node Injury rating assigned would be 1.0; if more than a node had been attacked and the additional injury was equivalent to half node of roots, the rating would be 1.5. The scale is also flexible in that the resolution depends upon the increments upon which ratings are assigned. At Iowa State University, we normally rate on increments of a quarter node, i.e., 0.25, 0.5, 0.75, 1.0, etc. The Economic Injury Level is 0.25, one quarter of a node destroyed, on the Iowa Node Injury Scale.