Effects of repeated cottonwood leaf beetle defoliation on Populus growth and economic value over an 8-year harvest rotation

The effects of 8 years of cottonwood leaf beetle, Chrysomela scripta, defoliation on growth and economic value of four Populus clones (91 × 04-03, D105, Eugenei, and NM2) was investigated in a split-plot randomized complete block design with unprotected and insecticide-protected plots. After 8 years clones 91 × 04-03 and D105 had significantly greater survival than Eugenei or NM2. This difference in survival was not attributed to chemical protection, and while not qualified, was most likely the result of disease and storm breakage. Protected trees had significantly greater height, diameter, and stem volume than unprotected trees, with clone 91 × 04-03 consistently growing the largest. Protected plots had greater basal area and stem biomass than unprotected plots, but branch biomass was greatest in unprotected plots. Differences in branch biomass were most likely due to loss of apical dominance, and subsequent increased branching, resulting from C. scripta defoliation. Aboveground biomass was unaffected by treatment because of the increased branch biomass of unprotected trees. Clone 91 × 04-03 responded the most positively to chemical protection, as stem volume of unprotected trees was over 30% less than in protected trees. A financial analysis of our data indicated that none of the clone × treatment combinations would be economically acceptable due to the high land costs in the midwestern US. However, the use of less expensive insecticides may make clone 91 × 04-03 an economically viable crop in this region.