Reports of Corn Rootworm Adults and Larval Injury Common in Illinois

July 9, 1999
During the past week, reports of western and northern corn rootworm adults became commonplace across much of Illinois. By now, rootworm adults of both sexes can be located in cornfields and in east-central Illinois, and in soybean fields as well. In addition, we are beginning to receive numerous observations of performance problems with soil insecticides. At this point, providing adequate root protection has been a problem for several products. Beginning next week, we intend to begin evaluating our insecticide efficacy trials in Urbana and the following week in Monmouth and DeKalb. We will let you know the results of these experiments in upcoming issues of the Bulletin. We are a bit surprised with the reported failures of some products and the numerous adults in some soybean fields and cornfields. Recall that last year, densities of rootworms were very low due to the wet spring conditions. Overall, we estimated that densities of western corn rootworms were about 90 percent less in 1998 compared with 1997. Yet, surprisingly, they seem to be rebounding very quickly.

How can rootworm densities resurge so rapidly?

The biotic potential of northern and western corn rootworms is impressive! Research published in 1957 indicated that female western corn rootworms were capable of laying slightly more than 1,000 eggs during a 2-week period. The total life span of an adult western corn rootworm female ranged from 16 to 84 days. In 1975, a researcher field-collected mating pairs of western corn rootworm beetles and returned them to the laboratory. The average life span of these field-collected females was roughly 80 days. A pre-oviposition period of roughly 12 days was followed by the production of 1,087 eggs deposited in 13.5 clutches. Male western corn rootworms were found to have longer life spans, lasting on average for approximately 100 days. Female western corn rootworms were found in this study to mate only once; however, males were noted to mate with several females. Additional research published in 1977 indicated that male western corn rootworms are able to mate several times. Research published in 1990 suggested that life span and egg production were dependent on temperature. The greatest number of eggs laid by a female was 602 at approximately 80°F. The fewest eggs laid was 295 at 61°F. Because of this impressive biotic potential and the favorable environmental conditions at egg hatch, corn rootworms have seemingly rebounded much sooner than most would have guessed.

When should I begin to assess the performance of my soil insecticide?

Now. Chris Pierce, a graduate student in the Department of Crop Sciences, is conducting research on the seasonal egg-laying habits of western corn rootworms in cornfields and soybean fields in east-central Illinois. On July 6 he reported lodging in untreated (no soil insecticide used) rows within some of his research fields. As indicated previously, we are beginning to receive more reports of lodging across the state. So grab your shovel; now is the time to dig some roots and check them for rootworm injury. We are routinely sent root samples in September and October of each year and asked to examine them for rootworm injury. Don't wait until this fall to attempt to assess the performance of your chosen soil insecticide. By fall, root rots begin to make the task of properly diagnosing rootworm injury very difficult, if not impossible in some instances.

When should I begin to scout soybean fields in east-central Illinois for rootworm adults?

Throughout east-central Illinois, we recommend that growers determine western corn rootworm densities in soybean fields 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, distribute 12 unbaited Pherocon AM traps evenly throughout the interior of each soybean field (regardless of field size). The traps should be positioned just above the soybean canopy on metal fence posts. Each week, count all western corn rootworm (male and female) found on each trap in each field. Be sure to replace the traps every 7 days to ensure accurate counts. Producers who are outside the east-central Illinois area and are interested in determining the presence or absence of western corn rootworm adults in soybean fields can follow the same trapping procedures, but use only four traps per field. If adults are detected in soybean fields that have four traps deployed, we recommend the use of 12 traps for a more thorough analysis regarding whether or not the use of a soil insecticide may be necessary next year.

An economic threshold of seven beetles per trap per day may result in a root injury rating of 4.0 (one node of roots destroyed) the following season. A density as low as two beetles per trap per day may result in a 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 consult the following web address:
http://www.aces.uiuc.edu/ipm/field/corn/insect/wcr.html

When should I begin to scout cornfields for adult corn rootworms?

As cornfields begin to tassel and silk, they should be scouted to determine if silk clipping by rootworm adults is occurring. Densities of at least five beetles per plant are typically required to reduce pollination in commercial fields. Seed-production fields and high-oil corn may be more susceptible to economic losses caused by silk clipping of corn rootworm adults. In seed corn, an insecticide treatment may be justified if the silks on 20 percent of the plants have been clipped to a length of 3/4 inch or less, pollination is still taking place, and rootworm beetles are present.


Silk clipping by western and northern corn rootworm adults.

A second objective of scouting for rootworm adults is to determine the potential for rootworm problems next year. By mid-July, you should be committed to scouting for rootworm beetles at least once each week through early September. Determine the average number of beetles per plant by counting beetles on two plants selected at random in each of 25 areas of a field. Count all western and northern corn rootworm beetles each time. 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 ear tip, tassel, leaf surface, and behind leaf axils. Record the number of beetles you find per plant. If the average is greater than 0.75 beetle per plant in corn after corn, or 0.5 beetle per plant (first-year corn) for any sampling date, consider the following options: (1) plan to rotate away from corn, (2) be prepared to use a soil insecticide at planting during 2000, or (3) initiate a program for the prevention of egg laying this season. If densities do not exceed these thresholds for any sampling date, rootworm larvae should not cause economic root damage to corn next year.

Producers involved in a beetle-suppression program may find an insecticide application warranted if beetle thresholds are reached and 10 percent of the females are gravid (with eggs). If more than 10 percent of the females within a field are gravid, significant egg laying may have occurred already. An attempt to reduce beetle densities to decrease root injury in 2000 may prove less than satisfactory.

As the season progresses, we will continue to provide updates on the corn rootworm situation and offer continuing management suggestions.--Mike Gray

Author: Mike Gray