University of Vermont

WESTERN AND NORTHERN CORN ROOTWORMS IN FIELD CORN Western and Northern Corn Rootworms in Field Corn in Vermont

Sid Bosworth, Extension Associate Professor, Department of Plant and Soil Science, University of Vermont

Do you have a problem with Western Corn Rootworm? If you don’t know or even if you “think” you have a problem, then late July/early August is a good time to find out. This is the period, during pollination time, that adult beetles emerge from the soil to feed on tassels and corn silks. Otherwise you never see this insect unless you are good at digging up corn roots in June and looking for rootworm larvae [closeup image]. Of course, that is when the most damage is being done - during the larval stage when they use your corn roots for breakfast, lunch and dinner!

Besides not seeing the insect, you may not “see” the damage either. It can be quite subtle - at least according to some Cornell research conducted a few years ago. They found that at a high enough larval population, corn silage yields could be reduced by one or two tons without showing a decline in grain yield. At some medium population of rootworm larvae, you could get these yield losses without necessarily seeing the classical visible damage of lodging or “goosenecking” that occurs when rootworm populations are high enough to cause significant damage to the brace roots. If you do see classical symptoms, then chances are that you are getting even larger reductions in yield, both silage and grain.

Of course, this all depends on the kind of year as well. At the same population of rootworms, you could get absolutely no damage or yield losses during a moderate season with adequate rainfall but significant losses during a dry year when water is limiting. During a good growing year, there will be a larger, more extensive mass of roots enabling the plant to better “mine” water and nutrients from the soil and more than perhaps rootworm larvae can keep up with.

So back to late late July/August. This is the most visible time to actually see the insects. Rootworm adults are about ¼ inch in length. Northern rootworm adults are the most common and are tan to pale green in color. You will often see them buzzing around your ceiling lights in late July and August. The western rootworm is newer to our region - first detected in Vermont in 1991. Similar in size, they are yellow with a black strip along their wing covers.

The Western corn rootworm is much more damaging to corn than the Northern. In fact, research in the early 80’s in Vermont showed very little damage from Northern corn rootworm even at high populations. But it’s Western cousin is a different beast and requires some serious attention. So, if you don’t know if you have it, do yourself a favor and go out and look.

First, look in a field that has been in corn for more than one year. Since larvae feed only on corn roots and no other crop, few problems exist in the first year of corn. Stealth is a key as you move through the corn crop to detect adults. These insects are easily startled and will literally drop off the plant if disturbed. So move through slowly and quietly.

If you do see some insects that fit the description of Western corn rootworm, try to catch a few to confirm it’s identification. If you are planning to keep that field in corn the following year, you should actually scout the field more intensely or use a crop consultant or Certified Crop Advisor before making any decisions about the use of a corn insecticide. Don’t rig your planter with an insecticide applicator just because one Western adult was found and especially if one was found in your neighbor’s corn. You need an average of one adult western corn rootworm beetle per plant (after looking on at least 50 plants throughout the field staying away from the borders) for your populations to be high enough to warrant a management decision. If you have a lot of Northern adults, you can include them in your calculations but 2 Northern adults equals one Western.

Rotation is your best control of these insects. Insecticides are available if you must repeat a corn planting in the same field. They generally cost around $15 per acre. So a two ton increase in yield could easily offset this cost provided you know you have a real problem.

For more information on the Vermont Corn Rootworm IPM  program, click here.

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Last modified May 26 2004 01:29 PM

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