Evaluating Corn Roots for Rootworm Damage

June 23, 2000
A couple of telephone calls and office visits on June 20 prompted me to write this article. From mid-June through early July, many growers, dealers, consultants, and seed and pesticide company representatives spend some time roaming through cornfields, digging up root systems, and looking for corn rootworm larvae and signs of their feeding injury. For many, this activity is nothing more than a preliminary assessment of how well the soil insecticide applied at planting is working (or not working). For others, especially for growers who did not apply a soil insecticide, this activity might reveal enough root injury and rootworm larvae to justify application of an insecticide during cultivation if the corn is not too tall. In either case, appropriate interpretation of what you find is important.

I cannot emphasize appropriate sampling procedures enough. Selecting roots from the edge of the field or only one spot in the field does not comprise a good sampling plan. If you want a reliable assessment of the level of rootworm damage in a field, you have to extract roots randomly from several sites throughout a field. I know this is time-consuming, and often unpleasant, but sampling from the entire field is the only way to see the whole picture. You don't want to make a snap judgment about rootworm control (or lack thereof) from an unrepresentative sample.

At this time of year, the best procedure for sampling for rootworm larvae and assessing root damage is to dig up a 7-inch cube of soil around and including the root system. You can either sift through the soil by hand to look for rootworm larvae (this can be time-consuming and frustrating) or place the soil and root system in a bucket of water and watch for the larvae as they float to the top. Both methods have very tenuous thresholds associated with them: three or more larvae per root system (hand search) and seven or eight larvae per root system (water flotation). However, I urge caution if you decide to use these thresholds; neither is based on robust research that correlates numbers of rootworm larvae with amount of root injury. In fact, we have stopped using them as guidelines because sampling for rootworm larvae is unreliable for assessing the density of rootworms in a given field. In addition, soil insecticides will not kill all rootworm larvae. Most of you know that soil insecticides kill only those rootworms that come into contact with the active ingredient in the concentrated 7- to 8-inch band applied at planting time. Many rootworm larvae survive and feed outside this "band of protection." Therefore, the presence of rootworm larvae in a field treated with a soil insecticide does not constitute poor performance of the insecticide; the presence of rootworm larvae in these fields is natural.

We think it is more prudent to assess performance of an insecticide by evaluating the amount of damage to the root system. In fact, that is exactly what we do when we evaluate our rootworm research trials in July and August. It is a bit too early to assign root ratings based on the well-known Iowa State University 1­6 rating scale. Rootworm larvae will continue to feed well into July, so injury apparent right now may become a bit worse in a few weeks. We'll provide an explanation of this rating scale and a video clip explaining the procedure in a near-future issue of the Bulletin. However, it's not too early to look for signs of feeding injury. But again, don't overreact to feeding scars and a little bit of root pruning. If rootworm larvae are present, you will find this level of injury no matter what insecticide was used. You should be concerned only if you find an appreciable amount of root pruning on one or more nodes of the root system.

People who find rootworm larvae and rootworm damage in some cornfields are apt to want some information regarding "rescue" treatments. If the corn is not too tall to be cultivated, Furadan 4F and Lorsban 4E are two liquid formulations that can be applied on either side of the corn rows ahead of the cultivator shovels. Furadan will provide better control than Lorsban if the soil is dry. Counter CR, Force 3G, Lorsban 15G, and Thimet 20G also are labeled for application during cultivation, but the performance of granular insecticides in nonirrigated corn has not been reliable. If you decide to apply an insecticide during cultivation, make certain that you read the insecticide label carefully. Exceeding certain amounts of specific products per acre is illegal.

One final note: Invariably, some people insist on applying a "rescue" treatment for rootworm control in cornfields that are too tall to cultivate. We have been consistent in not supporting this approach for trying to "resurrect" a field with significant rootworm damage. Research results from all over the Corn Belt have not revealed reliable performance of insecticides broadcast over the field. Not enough of the insecticide penetrates the corn canopy and reaches the ground. In addition, unless a lot of rain falls on the field, the insecticide will not move down into the root zone to kill the feeding rootworm larvae. Therefore, we strongly discourage broadcast applications of insecticides to control rootworm larvae in corn that is too tall to cultivate.--Kevin Steffey and Mike Gray

Author: Kevin Steffey Mike Gray