Full length article Effects of repeated cottonwood leaf beetle defoliation on Populus growth and economic value over an 8-year harvest rotation David R. Coyle a,1, *, Elwood R. Hart a,b , Joel D. McMillin a,c , Lita C. Rule a , Richard B. Hall a a Iowa State University Department of Natural Resource Ecology and Management, Ames, IA 50011, USA b Emeritus, Iowa State University Departments of Entomology and Forestry, Ames, IA, USA c USDA Forest Service, Forest Health Protection, Flagstaff, AZ, USA 1. Introduction Defoliating insects have varying impacts on tree growth, often depending on the timing, duration, and intensity of feeding (Kulman, 1971; Mattson and Addy, 1975). Plants have several ways of coping with defoliation, including nutrient reallocation, altering leaf size or canopy structure, or delaying senescence, and can exhibit positive or negative responses depending on the severity of defoliation (Trumble et al., 1993 and references therein). Plants often can withstand short periods of intense defoliation; however, repeated defoliation events may negatively impact plant growth, defense, and reproduction (Crawley, 1983; Solomon, 1985; Trumble et al., 1993; Reichenbacker et al., 1996; Kosola et al., 2001). Deleterious effects of repeated defoliation have been shown in jack pine (Pinus banksiana)(Volney, 1998; Conway et al., 1999), balsam fir (Abies balsamea)(Piene, 1989), mountain birch (Betula pubescens ssp. czerepanovii)(Kaitaniemi et al., 1999), quaking aspen (Populus tremuloides)(Churchill et al., 1964), and several other hardwood species (Leininger and Solomon, 1995). Populus selections are capable of significant yearly growth, and this trait makes them desirable for use in short-rotation woody crop systems that, in turn, feed into important industries such as wood products, fiber, and energy. Over 22,000 ha have been developed into Populus plantations in the US (Stanton et al., 2002). One of the most important challenges for successful Populus cultivation is repeated defoliation by insects, particularly the cottonwood leaf beetle, Chrysomela scripta (Coleoptera: Chry- somelidae) which is a major defoliator in young Populus (Salicales: Salicaceae) plantations (Morris et al., 1975; Burkot and Benjamin, Forest Ecology and Management 255 (2008) 3365–3373 ARTICLE INFO Article history: Received 9 October 2007 Received in revised form 12 February 2008 Accepted 17 February 2008 Keywords: Biomass Chrysomela scripta Economic analysis Insecticide Poplar ABSTRACT 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. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author at: University of Wisconsin, Department of Entomology, 345 Russell Labs, 1630 Linden Drive, Madison, WI 53706, USA. Tel.: +1 608 262 4755; fax: +1 608 262 3322. E-mail address: dcoyle@entomology.wisc.edu (D.R. Coyle). 1 Current address: University of Wisconsin Department of Entomology, Madison, WI, USA. Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco 0378-1127/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2008.02.023