Effects of ozone and flooding on leaf chemistry and growth of Rio Grande cottonwoods (Populus deltoides ssp. wislizenii) and herbivore responses in the lab and field

The cottonwood leaf beetle, Chrysomela scripta Fabr. is the major defoliator of cottonwood trees (Populus deltoides ssp. wislizenii) in the middle Rio Grande valley of New Mexico, where urban areas experience high levels of ozone (Eichhorst 1999). Ozone damages leaves at a cellular level which I proposed results in increased production of chemical defenses (phenolic glycosides) by cottonwoods. C. scripta sequesters phenolic glycosides for defense against predators, and thus, may use elevated phenolic glycosides as a feeding cue. My hypothesis was that cottonwoods would increase chemical defenses in response to ozone damage, and therefore C. scripta would be attracted to urban areas with high ozone. As flooding may relieve stress, I proposed that flooding results in decreased levels of phenolic glycosides and subsequent decreased recruitment and feeding by C. scripta.; In the field, six sites were continuously monitored from 1998 through 2002 to determine herbivory levels and population cycles of C. scripta and other insect defoliators on cottonwoods. Experimentally I grew cottonwoods from urban, rural and intermediate sites in greenhouses with treatments of high (average 40–90 ppb) and ambient (10–20 ppb) ozone and I tested the effects of flooding. I ran chemical analyses testing for differences in phenolic glycosides (salicin, salicortin, tremuloiden, tremulacin), carbon, and nitrogen and I performed feeding trials with C. scripta to determine feeding preferences. I also conducted feeding trials with cottonwoods from various field sites.; There were no feeding preferences due to treatment (elevated ozone or flooding) or site of origin. This was most likely due to the lack of significant differences in leaf chemistry, so it remains possible that C. scripta does prefer leaves with elevated phenolic glycosides. This experiment needs to be repeated with longer exposure times (5 to 10 years) for the cottonwoods in elevated ozone. This would capture chronic effects and more closely simulate the urban field conditions before leaf chemistry and accurate feeding preference is captured.