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Scouting for Corn Rootworm Adults

July 21, 2000

Emergence of corn rootworm adults is in "full bloom" throughout Illinois; peak population densities and peak egg laying will occur within the next few weeks. Therefore the time for scouting for rootworm adults is upon us. There are three different reasons for scouting for adult corn rootworms throughout most of state and a fourth reason in the counties in Illinois where western corn rootworms lay eggs in soybeans. The first reason to scout for rootworm adults is to watch for silk-clipping injury that interferes with pollination. We discussed this in an article in issue no. 13 (June 23, 2000) of the Bulletin. The other three reasons for scouting for rootworm adults are to

  • Determine the potential for rootworm larval injury next year if corn is planted after corn. Typically, results from this scouting effort suggest whether a preventive soil insecticide should be applied when corn is planted next spring.
  • Determine whether you should prevent rootworm adults from laying eggs in corn this year to prevent rootworm larval injury next year. This practice has specific guidelines for corn after corn but not for corn planted after soybeans. As we have stated emphatically in the past, we strongly discourage attempts to prevent western corn rootworms from laying eggs in soybeans until we have more information about their movement and egg-laying behavior.
  • Determine the potential for rootworm larval injury next year if corn is planted after soybeans in areas of Illinois where this behavior has been identified.

Following are guidelines for scouting for rootworm adults in corn (two different objectives) and soybeans.

Corn planted in 2000, corn to be planted in 2001--Is a preventive soil insecticide warranted?

To answer this question, scout for rootworm adults at least once each week from mid-July through early September. During each scouting trip, count the number of beetles (both northern and western corn rootworms) on two plants selected at random in each of 25 areas of a field. The counts take about 45 minutes in a 40-acre field. As you approach a plant, move quietly to avoid disturbing the beetles. Count the beetles on the entire plant, including the tassel, leaf surfaces, behind leaf axils, and on ear tips. Record the number of beetles you find on each plant, then determine the average number of beetles per plant for the field. If the average is greater than 0.75 beetle per plant in corn after corn (corn planted in both 1999 and 2000) or 0.5 beetle per plant in first-year corn (something other than corn planted in 1999, corn planted in 2000) for any sampling date, a grower should plan to apply a preventive soil insecticide if corn is planted in 2001.

Corn planted in 2000, corn to be planted in 2001--Preventing egg laying by rootworm females in 2000

This approach toward rootworm management is a viable approach if one adheres to the guidelines and is committed to a rigorous scouting program. Keep in mind, however, that this approach used year after year in some counties in Nebraska eventually resulted in a population of western corn rootworms resistant to both carbaryl and methyl parathion. Although we have not encountered rootworms resistant to insecticides in Illinois for many years, sole reliance on one control tactic is seldom a good idea.

In general, the guidelines for sampling for rootworm adults to determine when to prevent egg laying are the same as discussed previously, although some consulting firms may suggest alternative combinations of numbers of plants and groupings of samples. However, virtually all recommendations for this approach for managing corn rootworms express the same thresholds. If the number of beetles reaches or exceeds 0.75 per plant, apply an insecticide when 10% of the females are gravid (with eggs). Consequently, you will need to be able to tell the difference between males and females and between gravid and nongravid females. Male and female western corn rootworms can be distinguished fairly easily. Female northern corn rootworms are slightly larger than the males. Gravid females of both species have distended abdomens. Squeezing a gravid female's abdomen will reveal eggs within the liquid exuded. Companies that support this approach for management of corn rootworms usually provide more detailed explanations and vivid graphics.

Fields that have been sprayed to prevent rootworm adults from laying eggs should be monitored weekly after treatment to accommodate immigration into the field or continued emergence within the field. A second application of an insecticide may be necessary if the number of beetles reaches or exceeds 0.5 per plant.

Soybeans planted in 2000, corn to be planted in 2001--Is a preventive soil insecticide warranted?

The saga of the variant of the western corn rootworm that lays eggs in soybeans dates back more than 5 years now. As you are well aware, the problem with western corn rootworms in corn planted after soybeans has spread from its origin in east-central Illinois and northwest Indiana. The problem has been verified from southern Michigan, western Ohio, throughout the northern half of Indiana, and in as many as 30 counties in Illinois. Consequently, monitoring for western corn rootworms in soybeans has been the best strategy for determining the potential for rootworm larval injury in corn planted after soybeans.

Scouting for western corn rootworm adults in soybeans should be conducted from the last week in July through the third week in August by using yellow sticky traps (Pherocon AM traps). In the last week of July, 12 unbaited Pherocon AM traps should be distributed evenly throughout each soybean field, regardless of field size. The traps should be positioned just above the soybean canopy on metal fence posts. Each week, remove the old trap and replace it with a new trap. Count all western corn rootworms (male and female) found on each trap in each field, and determine the average number of beetles per trap per field.

After the 1999 field season, we adjusted the thresholds to accommodate the new data we gathered. If you find an average of 10 beetles per trap per day, you can anticipate an average root-injury rating of 4.0 (one node of roots destroyed) the following season. If you find an average of 5 beetles per trap per day, you can anticipate an average root-injury rating of 3.0 (some root pruning, never an entire node destroyed) next year. The level of root injury that may result in economic loss varies according to the growing season and the hybrid selected. In general, root-injury ratings between 3.0 and 4.0 may cause yield loss. However, for certain corn hybrids during poor growing conditions, our research has shown that yield losses can occur with root ratings between 2.0 and 3.0 (minor root scarring to minor root pruning). For additional information on scouting soybean fields for western corn rootworm adults, please check out the following site:

As we begin to get a feel for the densities of corn rootworm adults in both corn and soybean fields this summer, we will report our findings in the Bulletin. In the meantime, happy scouting!--Kevin Steffey and Mike Gray

Author: Mike Gray Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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