ECOLOGY AND BEHAVIOR Localized Search Cues in Corn Roots for Western Corn Rootworm (Coleoptera: Chrysomelidae) Larvae E. J. BERNKLAU, 1,2 L. B. BJOSTAD, 2 L. N. MEIHLS, 3 T. A. COUDRON, 4 E. LIM, 5 AND B. E. HIBBARD 1,3 J. Econ. Entomol. 102(2): 558Ð562 (2009) ABSTRACT Cues that elicit a characteristic localized search behavior by neonate larvae of the western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), were extracted from living corn, Zea mays L., roots with acetone. Larvae were exposed to corn roots or to an acetone extract of corn roots and then transferred into a bioassay arena where their movements were tracked and recorded. After a 5-min exposure to live corn roots, larvae produced highly convoluted tracks that were indicative of a localized search behavior, and these distinctive tracks were also produced by larvae exposed to an acetone extract of corn roots. Larvae exposed to a Þlter paper control moved in relatively straight paths that were indicative of ranging behavior. Larval tracks were recorded by means of a videocamera and tracking software, and four parameters of movement were used to quantify the tracks: mean turn angle, mean meander, total distance, and maximum distance from origin. For every parameter measured, tracks resulting from exposure to the control were signiÞcantly different from tracks resulting from exposure to live corn roots and to all doses of the corn root extract. In a separate experiment, larvae exposed to corn root pieces or corn root juice exhibited the localized search behavior, but larvae exposed to oat root pieces and oat root juice (nonhost) exhibited ranging behavior. KEY WORDS Diabrotica virgifera virgifera, Zea mays, host search behavior, bioassay Strnad and Dunn (1990) reported that the behavior of neonate western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), lar- vae immediately after removal from the roots of corn is distinctly different from the behavior of larvae re- moved from the roots of a nonhost plant. After expo- sure to host plant roots, a larva exhibits a localized search movement pattern, whereby it travels rela- tively slowly, with increased turns and path crossings and little overall area covered. In contrast, a larva exposed to the roots of a nonhost plant exhibits a broader ranging behavior characterized by an in- creased rate of travel, with fewer turns and path cross- ings and more area covered. Oyediran et al. (2004) used this difference in behavior to evaluate responses of northern corn rootworm, Diabrotica barberi Smith & Lawrence, to alternate hosts. The Strnad and Dunn (1990) bioassay has recently been used to assess larval responses to Bacillus thuringiensis (Bt) corn as well as to native sources of resistance to western corn root- worm feeding damage (B.E.H., unpublished data). The goals of the current study were to use behav- ioral bioassays to determine whether the localized search cues were chemical versus physical in nature and to recover the chemical cues from corn roots. Materials and Methods Insects. Western corn rootworm eggs (nondiapaus- ing strain) were obtained from the USDAÐARS Plant Genetics Research Unit in Columbia, MO, and from the USDAÐARS Laboratory, Brookings, SD. Survivor- ship of these laboratory rootworm colonies has been compared with that of wild-type insects in Þeld studies (Hibbard et al. 1999). The source insects were reared on corn plants grown in soil using methods described by Jackson (1985) and modiÞed by Hibbard and Bjos- tad (1988). The eggs were incubated in soil at 27°C, and larvae were used for bioassays within 16 h of hatching. Plant Tissues and Juices for Bioassays. For all test materials, seeds were washed in soapy water, rinsed well, and then germinated on moist blotter paper (Steel Blue, Anchor Paper Company, St. Paul, MN) at 26°C in a closed polyethylene tub (4 Ð5 d for corn and This article reports the results of research only. Mention of a proprietary product does not constitute an endorsement or recom- mendation for its use by the USDA, Colorado State University, Uni- versity of Missouri, or Gyeongsang National University. 1 USDAÐARSÑMWA, BSPM Dept., Colorado State University, Ft. Collins, CO 80523. 2 Department of Bioagricultural Sciences & Pest Management, Colorado State University, Fort Collins, CO 80523. 3 205 Curtis Hall, University of Missouri, Columbia, MO 65211. 4 Biological Control of Insects Research Laboratory, 1503 S. Prov- idence, RES. PK., Columbia, MO 65203. 5 Division of Applied Life Science (BK21 Program), Graduate School of Gyeongsang National University, 900 Gajwa-dong, Jinju 660-701 Korea.