University of Illinois

No. 15/July 3, 1997

Corn Rootworm Larval Injury to Become More Apparent after the 4th of July

Chances are, by the time you read this article, the fireworks, barbecues, and picnics will be over. However, corn rootworms will likely be continuing to feast upon the roots of your cornfield. By now, you know that the 1997 corn rootworm egg hatch got off to a very late start. Consequently, we anticipate corn rootworm larval feeding will stretch well into the second week of July in most central and northern Illinois locations. As mentioned previously, soil insecticides could be put to a severe test this season.

How can I estimate the value of my soil-insecticide investment? If you have not left at least one untreated strip within your field, you may never answer this question satisfactorily. Soil insecticides work very well when corn rootworms are not present at economic levels within a cornfield. Research conducted in the early 1990s in northern Illinois indicated that economic infestations of corn rootworm larvae occurred in only about 50 percent of continuous cornfields. By leaving an untreated strip within your field and digging roots from treated and untreated areas, you will be able to assess more accurately the root protection afforded by a soil insecticide. The final measure of your soil insecticide's worth will obviously come at harvest when yields are compared in treated and untreated strips.

Can't I assume a soil insecticide paid for itself if I don't have any lodging? This assumption is not a good idea. A variety of environmental factors may cause plants to lodge (lean from upright angle) other than corn rootworms. For instance, plants may lodge because heavy rains and wind cause plants to lean in saturated soils. By contrast, under extremely dry soil conditions, plants may remain upright despite significant pruning by corn rootworm larvae. Spend a little time examining root systems in treated and untreated strips, and consider the following questions. Do you see any rootworm larvae? Do you see any signs of larval feeding (brown feeding scar lesions)? Do you see any root pruning? Are there differences in root injury between treated and untreated strips?

I've been told not even to try finding corn rootworm larvae in my cornfield. Is this good advice? No. However, the observation that looking for rootworms is "not a lot of fun" is accurate. Corn rootworms have three larval instars (Figure 1). An instar is the stage between molts, that growth process by which insects shed their skins. By the time rootworms reach the final growth stage (probably around the 4th of July this season), they can be found with some organized effort. This effort involves digging up a 7-inch cube of soil around the base of a corn plant and sifting through soil for larvae. Another technique requires that 7-inch cubes of soil be placed in 5-gallon buckets of water and counting larvae as they float to the surface. Either approach requires considerable effort. The number of cubes required to estimate accurately densities of corn rootworm larvae has prevented this approach from being popularly adopted. However, removing 5 to 10 cubes in each treated and untreated strip is suggested as a minimum number in any attempt to find corn rootworm larvae. If you find three or more larvae per plant, you may want to dig up additional plants and rate them for larval injury on the Iowa State 1-to-6 scale.

Figure 1. Body lengths and head capsule widths for larval instars of the western corn rootworm.

What is the Iowa State root rating scale, and are root ratings good predictors of potential yield? The Iowa State root rating scale was developed in Iowa during the early 1970s and uses a 1-to-6 scale to assess the level of root injury caused by corn rootworm larvae: 1, no visible damage or only a few minor feeding scars; 2, some roots with feeding scars but none eaten off to within 1-1/2 inches of the plant; 3, several roots eaten off to within 1-1/2 inches of the plant, but never the equivalent of an entire node of roots destroyed; 4, one node of roots destroyed or the equivalent; 5, two nodes of roots destroyed or the equivalent; and 6, three or more nodes of roots destroyed. A root rating of "3" has been viewed traditionally as the economic injury index. Research in the early 1990s suggested that for some hybrids a rating of "4" may be required to trigger an economic loss. However, it's not that simple. A variety of factors interact with root injury to influence yield, most important of which is availability of soil moisture. Also, hybrids differ in their abilities to regenerate lost root tissue. In wet seasons, some hybrids apparently continue to regenerate root tissue after larval injury and pay a penalty with respect to yield. In dry seasons, those hybrids that are better equipped to replace lost root tissue seem to be at a competitive advantage. So, you see it's not just as simple as rating roots!

If I used a soil insecticide at planting and find rootworm larvae, does this mean trouble? Not always. First of all, an insecticide treatment is not 100 percent effective. You're not going to "get them all." From a resistance management viewpoint, you shouldn't try to get them all. Recall that soil insecticides are generally applied in a band or in the seed furrow. Rootworms also exist outside these treated areas. So don't be too alarmed if you find some rootworms. Research conducted in the early 1980s indicated that emergence of corn rootworm beetles was often greater in insecticide-treated plots than in those where a soil insecticide had not been used. Remember, when you purchase a soil insecticide, you are buying a product designed to protect roots, not manage rootworm populations. The presence of larvae does not confirm an insecticide failure; however, finding larvae and severely pruned roots should prompt a telephone call to your local product representative.

Let us know how your soil insecticide performs this season.
Mike Gray, Extension Entomology, (217)333-6652