Sex-biased herbivory by Altica subplicata on Salix cordata and Salix exigua, and its potential mechanisms, were investigated. In the laboratory, beetle larvae survived better on female than on male S. cordata plants. In S. exigua, larvae achieved higher pupal weights on female than on male plants. Salix exigua supported better larval development than did S. cordata in terms of pupal weight, proportion burrowing and proportion pupating. In the field, S. cordata hosted greater densities of beetles than did S. exigua, but no sex-bias was detected; a trend toward female bias was suggested by presence/absence data. Plant traits differing significantly between species were stem diameter, number of shoots, water content, trichome count, total phenols, and distance from shore. Within and between species, the number of available leaves (as a result of plant architecture) and microclimate (influenced by distance from shore) likely exerted the greatest impact on beetle distribution.