Elevated Herbivory in Plant Hybrid Zones: Chrysomela Confluens, Populus and Phenological Sinks

Abstract

Using the free—feeding beetle Chrysomela confluens (Coleoptera: Chrysomelidae), we examined the hypothesis that naturally occurring hybrid zones between two cottonwood species are "sinks" or centers of insect abundance. Over a 3—yr period, we found that 94% of the C. confluens population was restricted to a 13—km hybrid zone, which represents <3% of the cottonwood population. Of several potential mechanisms tested, expanded host phenology in the hybrid zone, relative to pure zones, best explained the distribution of C. confluens which is dependent upon newly flushed leaves for optimal growth and reproduction. The hybrid zone is a superior beetle habitat because: (1) early leaf flush in the hybrid zone provides the first source of food for beetles in spring and (2) staggered leaf phenologies in the hybrid zone allow beetles to shift only newly flushed Fremont trees as foliage of sympatric hybrid and narrowleaf trees declines in quality. This shift by ovipositing females can result in a 600% increase in fecundity relative to nonshifting females. Additionally, 10—yr common garden experiment shows that there is a strong genetic component to the timing of leaf flush, which suggests a long—term stability of resources in the hybrid zone that beetles can rely on. For C. confluens, the hybrid zone is a "phenological sink" that increases beetle fecundity and leads to chronically high herbivory year after year. Because movement from the hybrid zone reduces fecundity, emigration into adjacent pure zones is likely hindered. This hypothesis requires staggered phenologies between overlapping host species; it probably does not require hybridization between host species. Further, this hypothesis may in part explain frequent outbreaks of insects in nurseries and plantations. Thus, two distinct mechanisms have been identified that result in increased insect abundance in plant hybrid zones; "phenological sinks" resulting from staggered plant phenologies and "hybrid sinks" resulting from increased susceptibility of hybrids to insect attack. Such studies suggest that hybrid zones are important natural laboratories for the study of plant—herbivore interactions.