Post-Takeoff Orientation of Western Corn Rootworm

Rufus Isaacs
Department of Entomology
Michigan State University
East Lansing, Michigan

Does post-takeoff orientation of western corn rootworm
to corn or soybean reveal the potential for oviposition within a soybean field?

Matthew O'Neal, Joseph Spencer*, Lance Meinke§, Chris DiFonzo and Doug Landis
Department of Entomology, Michigan State University, East Lansing, Michigan.
*Illinois Natural History Survey, Champaign, Illinois.
§ Dept. of Entomology, University of Nebraska, Lincoln, Nebraska.

Introduction

A variant of the western corn rootworm (WCR) capable of laying eggs in soybeans and other crops, and subsequently damaging first-year corn rotated into these fields, has recently moved into Michigan. In the past, female WCR beetles only laid eggs in cornfields, and since WCR larvae can only survive on the roots of corn, crop rotation was very effective for managing this pest. During the late 1980's to 1994, isolated areas of east central Illinois suffered rootworm injury to corn following soybeans. By 1995, WCR larval injury to corn following soybeans had spread throughout E. central Illinois and northwestern Indiana. Extension entomologists in both states have suggested that heavy reliance on a corn-soybean rotation selected a variant of WCR that lays at least a portion of its eggs in crops other than corn. In a strict corn-soybean rotation, larvae hatching from eggs laid in soybean fields emerge in first year corn. In these fields, WCR larvae can cause extensive root injury that leads to lodging and yield reduction.

Objective

Currently, no phenotypic or genotypic markers are available for identifying variant WCR. Naranjo (1994) observed that adult WCR in South Dakota, a region outside of the variant's current range, oriented more towards corn than soybean fields. He observed that consistently more female WCR flew towards corn than soybean (73.9% towards corn). With this in mind, our hypothesis was that variant WCR would fly towards soybeans more than other WCR. Our strategy was to compare adult WCR collected in Illinois with those from Nebraska, a region of the United States in which injury to rotated corn has not yet been reported. A behavioral difference between these two populations would be used to characterize WCR gathered in Michigan.

Methods Dr. Joseph Spencer and Dr. Lance Meinke assisted in collecting WCR from Illinois and Nebraska. We collected WCR from Ingham County, near the campus of Michigan State Univesity. We used a method for releasing and observing adult beetles between a corn and soybean field based on Naranjo (1994). An orientation platform was constructed from a 1 m metal stake placed in the middle of 2 m diameter circle (Fig. 1). The circle was subdivided into 8 equal sections marked with colored string extending from the central stake. Ten female WCR were released at a time from a modified 500 ml centrifuge tube, mounted on top of the stake. The section (1-8) and crop (corn, soybean or grass-border strip) into which each individual flew was recorded. Beetles that dis not fly outside of the 2-m circle within 10 min of release were not counted. We released Michigan and Nebraska beetles at Michigan State Univesity on 29 and 30 of July. To avoid releasing potential variant WCR into Michigan, beetles from all three states were released site at the University of Illinois on August 10, with aid from Dr. Spencer. Beetles were released in groups of ten during the morning (700-1000 hrs) and evening (1700-2000 hrs).

Results and Discussion

Figure 2. Flight of female western corn rootworms at a corn-soybean field interface

Flight into the 8 sections of the release site by Illinois (X2 = 22.94, n = 34, P < 0.05) and Michigan (X2 = 19.8, n = 209, P < 0.05) beetles deviated from a random distribution, but did not deviate from random by Nebraska beetles (X2 = 2.22, n = 72, P > 0.05). Regardless, all three populations flew more towards corn than soybeans (MI = 60%, n=174; NB = 62%, n = 51; IL = 64%, n = 25: percent of total flights toward corn for beetles choosing either corn or soybeans). Our preliminary results are consistent with those of Spencer et al. (1999) in which female WCR selected corn over soybean when released in a wind tunnel. From both our field results and the laboratory results of Spencer et al. (1999), variant WCR do not appear to be orienting more towards soybean than corn during flight. We suggest, as per Spencer et al. (1999), that variant WCR is not attracted to soybean. However, trapping results from Illinois (O'Neal et al. 1999) indicate that a greater percentage of WCR females are collected in soybean than cornfields throughout most of the corn growing season. Mechanisms responsible for the movement of WCR into soybeans are still unknown. We are currently investigating the role of corn phenology as a factor in driving WCR into soybean fields.

Bibliography

Naranjo, S. E. 1994. Flight orientation of Diabrotica vigifera virgifera and D. barberi (Coleoptera: Chrysomelidae) at habitat interfaces. Annals of the Entomological Society of America 87: 383-394.

O'Neal, M. E., M. E. Gray, and C. A. Smyth. 1999. Population characteristics of a western corn rootworm (Coleoptera: Chrysomelidae) strain in east-central Illinois corn and soybean fields. J. Econ. Entomol. (in press).

Spencer J. L., S.A. Isard, and E. Levine. 1999. Free flight of western corn rootworm (Coleoptera: Chrysomelidae) to corn and soybean plants in a walk-in wind tunnel. J. Econ. Entomol. 92: 146-155.

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