Insects as traits in plant systematics: their use in discriminating between hybrid cottonwoods

Phytophagous insects are natural bioassays that may be used to segregate closely related plant taxa. To examine the value of this method, we conducted a comprehensive survey of insects and arachnids in a natural zone of overlap and hybridization between Fremont (Populus fremontii) and narrowleaf cottonwood (Populus angustifolia). Three categories of trees were distinguished by leaf morphology and identified by previous genetic analyses as Fremont, F1 and BC1 hybrids, and complex backcrosses (BC2–BC4). These trees were subsequently reclassified by a discriminant function analysis of their associated herbivores. The level of concordance between the two methods was 98% (n = 43 trees). Because insect distributions may reflect differences in plant morphology, chemistry, phenology, and other characters, insect bioassays may be a more rigorous method of distinguishing closely related plant taxa than reliance solely on morphological or chemical analyses. In contrast with chemical and genetic analyses, insect surveys can be quickly done in the field at minimal cost. Insect bioassays will not replace current methods of distinguishing closely related plant taxa, but they may be useful for supplementing current methods.