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Issue No. 14, Article 5/June 24, 2005

We Probably Are in for Tough Times with Corn Rootworms

On June 17, I wrote an "Alert" article about corn rootworms for the Web version of the Bulletin. That article was prompted by the numbers of reports of significant corn rootworm damage being observed in many cornfields as well as the emergence of western corn rootworm adults. Those reports have continued into this week.

I'll start with reports close to home. On June 20, Ron Estes, coordinator of the Illinois Insect Management Program, and some graduate students and summer employees dug up four roots from an untreated check row in our standard corn rootworm efficacy trial near Urbana. They found 27, 15, 5, and 6 larvae on the four plants, respectively. They also observed serious rootworm larval damage on one plant. In addition, the general lack of soil moisture is exacerbating the problem. All corn plants are showing signs of moisture stress, but severely injured plants are suffering the most. Plants with less rootworm larval injury are not yet showing obvious symptoms of moisture stress, although a continued lack of rainfall will change that.

Corn root system from an untreated check row in the standard corn rootworm efficacy trial near Urbana (University of Illinois).

Corn plants on left showing signs of moisture stress, exacerbated by corn rootworm larval injury. Note the plants without symptoms of moisture stress in the row labeled 310 (University of Illinois).

Reports of poor performance by soil insecticides and insecticidal seed treatments in cornfields planted in April have been numerous and widespread. Quite honestly, the reports are too numerous to summarize effectively. The same is true for observations of emerging western corn rootworm adults: we have received numerous reports of people finding western corn rootworm adults. Right now, people can find first-, second-, and third-instar larvae as well as pupae and adults in cornfields.

As I indicated in the "Alert" article, the dry soil conditions have not benefited the efficacy of soil insecticides and insecticidal seed treatments. However, I strongly encourage people to examine both treated and untreated root systems from the same field before jumping to conclusions. Just yesterday (June 21), I examined two roots from an untreated check and two roots from a plot treated with a soil insecticide (not to be named at this time). Although pruning by corn rootworm larvae was evident on the roots from the treated plot, the rootworm larval injury was far worse on the roots from the untreated check plot.

We will be using the newer 0-to-3 node-injury scale created by entomologists at Iowa State University to assess rootworm injury in most of our trials this summer. We will use the 1-to-6 root-rating scale in situations when historical continuity is appropriate. As you assess rootworm injury in cornfields or in research or demonstration trials, you should use one of these rating scales for comparative purposes. The 0-to-3 node-injury scale and the 1-to-6 root-rating scale are compared here.

The 0-to-3 node-injury scale allows you to indicate the number of nodes pruned by corn rootworm larvae (the whole number to the left of the decimal point) and the percentage of nodes chewed off (the numbers to the right of the decimal point). So an average node-injury rating of 1.5 indicates that 1-1/2 nodes of roots have been pruned by rootworm larvae.

It's also important to report that significant corn rootworm larval injury in corn planted after soybeans has been observed in counties south of I-70. Randy McElroy (Monsanto Company) and Dan Beckman (Effingham Equity) observed noticeable rootworm larval injury in three to four fields of corn planted after soybeans in Crawford County. They found six to eight larvae per plant, significant feeding, and some lodging. The larvae ranged from first instars in the roots to large second or newly molted third instars. To continue to monitor the distribution of the variant western corn rootworm, we will conduct a survey of rootworm larval injury in counties primarily south of I-70 later this summer.

So once again, we must hope for rain. Rainfall will not benefit the efficacy of soil insecticides and seed-applied insecticides at this time, but it will alleviate some of the moisture stress that corn plants are suffering. In addition, some hybrids may be able to compensate for some rootworm injury by growing new roots. However, that won't happen much if dry conditions prevail.

One final comment. The emergence of rootworm adults also reminds us that as corn begins to tassel and silk, both rootworm adults and Japanese beetle adults may interfere with pollination. A recent report from Ron Hines, senior research specialist at the Dixon Springs Agricultural Center, indicated captures of more than 600 Japanese beetles per day in a trap in Pope County. We'll write an article about protecting the corn pollination process from insects in next week's issue of the Bulletin.--Kevin Steffey

Kevin Steffey

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