University of Illinois

No. 16/July 10, 1997

Two Girls for Every Boy: Sex Ratios of Western Corn Rootworms in Soybean Fields

As the corn growing season enters July, you might be concerned about this year's population of corn rootworms. Last year was the first season of a three-year project to develop an economic threshold for western corn rootworms captured with Pherocon AM yellow sticky traps in soybeans. This threshold will help us predict the potential level of damage growers might expect in the next year's corn crop. As we have reported previously for 1996, the number of adults captured in corn was lower than the number captured in soybeans, with the peak occurring near the second week of August.

We have suggested that western corn rootworms in east-central Illinois have developed a new behavior of laying eggs in soybean fields, which would explain the recent increase in the incidences of damage to corn planted after soybeans. We do not have an explanation for this new behavior, but our trapping efforts last year will allow us to compare this new behavior (egg laying and movement into soybean fields) with what we know about behavior of western corn rootworms in cornfields.

During the past few months, we have been examining about 2,000 yellow sticky traps from our study last year and recording the numbers of western corn rootworm males and females. Male western corn rootworms emerge from the soil before females. Traps placed in both continuous and rotated cornfields capture males first. As adults continue to emerge, the ratio of females to males increases. Studies by Purdue entomologists Larry Godfrey and Tom Turpin reported in 1983 showed that the percent of females was greater in first-year corn than in continuous cornfields (slightly less than 60 percent in first-year corn). The authors of this study stated that "the presence of more female beetles in FC (first-year corn) fields could result in higher WCR [western corn rootworm] egg density in the soil in the fall and a higher larval damage potential the following year."

As you might have expected, we have learned that the percent of females in soybeans is similar to the percent of females in firstyear corn. Data gathered from a soybean field in Vermilion County in 1996 revealed that females in the western corn rootworm population were 57.6 percent during the last week of July, increased to 71.0 percent the next week, and decreased to 61.8 percent during the last week of trapping (ending on August 19). Growers who continued to put out traps after August 19 captured 60 to 75 percent females.

At the end of 1997, we should be able to report a preliminary threshold for western corn rootworms in soybeans. Information to date suggests that we should expect the trends of increased populations of western corn rootworms in soybeans and increased incidences of reports of larval injury to corn planted after soybeans to continue.

The similarity in sex ratios of western corn rootworms in soybean fields and in first-year corn after soybeans suggests that the pest may be selecting soybeans as another egg-laying site. We will use yellow sticky traps again this year in both cornfields and adjacent soybean fields to monitor the behavior of the insects. This information will provide some insight into the factors that are influencing the change in egg-laying behavior of western corn rootworms. Stay tuned as we continue to study this dynamic pest.
Matt O'Neal, Entomology Graduate Research Assistant, (217)244-2637