The cottonwood leaf beetle, Chrysomela scripta F., is a serious defoliator of plantation Populus in the United States. Current control methods include biorational and synthetic chemicals as well as selecting Populus clones resistant or tolerant to C. scripta defoliation. Specific ratios of long-chain fatty alcohols to alpha-tocopherylquinone (α-TQ) on the leaf surface of Populus spp. act as phagostimulants to adult C. scripta. The chemical concentrations and ratios vary among Populus clones; however, the effect of defoliation on the subsequent production of these chemicals is unknown. We investigated the effects of defoliation, clone, and season on Populus leaf surface chemical production. Chemical concentrations and ratios were monitored in 1998 and 1999 on eight Populus clones with and without larval C. scripta defoliation. Chemicals were extracted from the leaf surface and analyzed via gas chromatography. Larval C. scripta defoliation rarely caused changes in leaf surface chemistry at the defoliation levels tested; however, the production of these phagostimulants did vary by clone and season. Foliar alcohol and α-TQ concentrations and α-TQ:total alcohol ratios differed significantly among clones. Furthermore, α-TQ concentrations and α-TQ:total alcohol ratios varied temporally in some, but not all, clones. In general, foliar alcohol and α-TQ concentrations either did not vary or increased, but α-TQ:total alcohol ratios declined throughout the growing season. This research illustrates that the production of leaf surface phagostimulants is not a function of defoliation, but is most likely controlled by genetic and physiologic processes. Additionally, because Populus clones vary in their foliar chemistry, this variation could be exploited in tree breeding programs.
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