Localized search cues in corn roots for western corn rootworm (Coleoptera: Chrysomelidae) larvae.

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 filter 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 significantly 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.

Natural variation in maize defense against insect herbivores.

Genetic mapping projects with maize (Zea mays) have resulted in the identification of numerous quantitative trait loci (QTL) that influence resistance to insect herbivores. However, the underlying genetic basis of these QTL has been confirmed in only a small number of cases. Recent advances in genome sequencing, the development of large mapping populations, and advances in reverse genetic approaches will accelerate the discovery of novel herbivore resistance genes in maize. Areas that will merit particular research emphasis are natural variation in maize resistance to rootworms and phloem-feeding insects as well as the identification of previously unknown loci involved in the biosynthesis of maize defensive secondary metabolites.