Research Brief: 0 to 3 Node-Injury Scale for Corn Rootworm Rating
Eileen Cullen, University of Wisconsin-Madison Entomology Department
A recent article published in the February 2005 issue of Journal of Economic Entomology titled “Node-Injury Scale to Evaluate Root Injury by Corn Rootworms (Coleoptera: Chrysomelidae)” (Oleson et al. 2005) provides new and improved guidance for the annual corn root rating task (see related article this issue titled Time Approaching to Evaluate Corn Roots for Injury by Corn Rootworms). A summary of the newly published technique, along with article excerpts, are provided here. You may link to the following address to purchase a copy of the article online.
http://miranda.esa.catchword.org/vl=3352199/cl=44/nw =1/rpsv/cw/esa/00220493/v98n1/s1/p1. The article can also be obtained from your local University campus library, photocopied off the shelf or through interlibrary loan.
Over the past decades, field crop entomologists have developed several techniques to measure efficacy of corn rootworm larval control tactics. These have included taking larval counts in the soil, collecting adult beetles as they emerge from the soil, corn root damage ratings, root size and regrowth ratings, measuring root-pull resistance, recording the percentage of lodged plants, and measuring grain yield. Of these techniques, root damage ratings have become the standard because this method is most efficient as compared with time expended and variability of results associated with the other methods.
Corn rootworm larval feeding on corn roots can significantly reduce grain yield by interfering with photosynthetic rates, limiting the uptake of water and nutrients, and increasing the plant’s susceptibility to lodging. Over large data sets, across locations that include favorable environmental conditions allowing corn plants to compensate and prevent larval root feeding from having its maximum influence on yield, a poor relationship has been found between rootworm injury and yield. However, Oleson and his Iowa State University co-authors refined their study of nine locations over four years by focusing on a subset of three locations with similar rootworm feeding pressures, all in the range of approximately two complete nodes (circle of roots) pruned in the untreated checks. Here they were evaluating root injury on plants subjected to low, medium and high environmental stress conditions (e.g., rainfall, heat stress, wind gust mph, planting date) at the respective three locations, but all of which were significantly impacted by rootworm injury.
Historically, the primary scale used for scoring corn root injury from corn rootworm larval feeding has been the Iowa 1-6 scale developed by Hills and Peters (1971) also at IowaStateUniversity. This older method is a 6point rating scale where the amount of root injury is classified into one of six categories, ranging from no damage or only a few minor feeding scars (rating of 1) to three or more nodes of roots pruned to within 1.5 inches of the stalk (rating of 6).
The old 1 to 6 ISU rating scale is as follows: 1= no damage or only minor feeding scars; 2= feeding scars evident, but no roots eaten off to within 1.5 inches of the stalk; 3= several roots eaten off to within 1.5 inches of the stalk, but never the equivalent on an entire node; 4= one node of roots completely destroyed; 5= two nodes of roots completely destroyed; 6= three nodes of roots completely destroyed.
Oleson et al. (2005) point out that a critical deficiency of the Iowa 1-6 scale is that each increase in a root rating score does not reflect a linear increase in the actual amount of injury contained on the root. For example, although a root rating of 4 on the 1-6 scale is often thought of as just one node injured (see 1-6 scale description previous paragraph), injury can actually range from 1.00 to 1.99 nodes pruned before the rating moves up to category 5. Thus, the old scale tends to be a more subjective between different people rating roots and more qualitative than quantitative.
The Node-Injury Scale scores, by contrast, range from 0.00 to 3.00, and depend on the cumulative proportion of a node or nodes that are pruned to within 1.5 inches of the stalk. Nodes are easily identified on a corn stalk between each node. Of course there are no roots coming off of nodes above ground above the brace roots. Early in corn plant development however, (V1 through V5, up to V6 or V7) node axes do develop roots. These are the nodes that we examine during root rating. Larval injury is typically confined to the third through the sixth nodes of adventitious axes of corn because development of these roots coincides with the time of rootworm egg hatch and larval development (Riedell 1993).
For brace roots initiated from the stalk above the soil, a root is considered pruned if it is eaten back to within 1.5 inches of the soil line. When evaluating brace roots however, make sure that they actually made it into the soil where they were exposed to rootworm feeding. As you’ve likely seen, some brace roots are initiated from the stalk above the soil, but they do not reach or penetrate the soil line and remain greenish/white “nubs” never exposed to corn rootworms in the soil.
The Node-Injury scale is easy to learn and use because it is really just the percentage of each node that is injured. A root can receive a rating between zero and three. For example, a perfect root with no injury is rated a zero. If one node, or circle of roots, is eaten back to within 1.5 inches of the stalk, it receives a 1. Two nodes eaten gets a 2, and three nodes eaten gets a
3. Damage in between a complete node is noted as a percent. For example, if one quarter of the roots on a node are damaged it gets a rating of 0.25. Also, if the 4th node has ½ its roots damaged, and the 5th node is completely eaten, you would give a rating of 1.5, or one and a half nodes eaten.
We use the Node-Injury Scale to evaluate root injury by corn rootworms in our UW Madison Entomology research trials, insecticide efficacy screening program and on-farm surveys of the Variant Western corn rootworm in southeastern and southern Wisconsin. IowaStateUniversity, as well as University of Nebraska have also adopted the Node-Injury Scale in their trials. Don’t forget that the relationship between corn rootworm injury and reduction in grain yield is complex and likely influenced by many variables. Oleson et al. (2005) found that along the range of the Node-Injury Scale between 0.00 and 1.50, an average of one node of root injury caused a yield reduction of 0.59 bushels/acre, 7.6 bushels/acre, and 22.3 bushels/acre under low, medium and high environmental stress conditions, respectively.
Among universities using the Node-Injury Scale, root ratings of 0.25 or lower represent a conservative benchmark for product efficacy. That is, insecticide products or Bt CRW hybrids that keep rootworm larval injury below one-quarter of a node of roots pruned are deemed effective. It is generally acceptable to set a Node-Injury Score of 0.50 as a maximum benchmark for rootworm larval injury, no more than one-half node pruned. Node-Injury Scores between 0.25 and 0.50 correspond to the old ISU 1-6 efficacy benchmark category 3 (“several roots eaten off to within 1.5 in of the plant, but never the equivalent of an entire node of roots destroyed”).
In insecticide efficacy trials, the ability of a product or hybrid to “consistently” provide protection is important. Product consistency equals the percentage of times a treatment provided root protection at or below a set level of injury. For example, setting 0.50 Node-Injury Score or less as a consistency benchmark, a productwith individual node-injury scores of 0.50, 0.50, and
0.50 would be 100% consistent. A product receiving scores of 0.25, 0.50, and 0.75 would be 66% consistent. (Oleson et al. 2005). The old ISU 1 to 6 scale is still in use by some Midwestern universities, as well as consultants who have long used this method. Although there are quantitative and qualitative differences between the two methods, the important thing to keep in mind as you compare results between years and across locations, is product consistency. Either method will provide data on product efficacy and consistency. This article will help you decipher between the two root rating methods if you are new to root rating or are considering making the switch over from the older 1-6 scale this season as you rate roots.
The Node-Injury Scale can be viewed on an interactive node-injury scale Web page through IowaStateUniversity at
http://www.ent.iastate.edu/pest/rootworm/nodeinjury/no deinjury.html. The computerized root injury illustrated at this site demonstrates how the node-injury scale quantifies progressive feeding by corn rootworm larvae.
References Cited
Hills, T.M. and D.C. Peters. 1971. A method of evaluating postplanting insecticide treatments for control of western corn rootworm larvae. Journal of Economic Entomology. 64: 764-765.
Oleson, J.D., Y.L.Park, T.M. Nowatzki and J.J. Tollefson. 2005. Node-injury scale to evaluate root injury by corn rootworms (Coleoptera: Chrysomelidae). Journal of Economic Entomology, Vol. 98: 1-8.
Riedell, W.E. 1993. Advances in understanding corn rootworm damage effects on maize physiology, pp. 76
90. In D. Wilkinson [ed.], Proceedings of the Annual Corn Sorghum Research Conference. No. 48. American Seed Trade Association, Washington, D.C.