ELEVATED HERBIVORY IN PLANT HYBRID ZONES - CHRYSOMELA-CONFLUENS, POPULUS AND PHENOLOGICAL SINKS

Using the free-feeding beetle Chrysomela confluens (Coleoptera: Chryso melidae), we examined the hypothesis that naturally occurring hybrid z ones between two cottonwood species are ''sinks'' or centers of insect abundance. Over a 3-yr period, we found that 94% of the C confluens p opulation 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, be st explained the distribution of C confluens which is dependent upon n ewly flushed leaves for optimal growth and reproduction. The hybrid zo ne is a superior beetle habitat because: (1) early leaf flush in the h ybrid zone provides the first source of food for beetles in spring and (2) staggered leaf phenologies in the hybrid zone allow beetles to sh ift onto newly flushed Fremont trees as foliage of sympatric hybrid an d narrowleaf trees declines in quality. This shift by ovipositing fema les can result in a 600% increase in fecundity relative to nonshifting females. Additionally, a 10-yr common garden experiment shows that th ere is a strong genetic component to the timing of leaf flush, which s uggests a long-term stability of resources in the hybrid zone that bee tles 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 reduc es fecundity, emigration into adjacent pure zones is likely hindered. This hypothesis requires staggered phenologies between overlapping hos t species; it probably does not require hybridization between host spe cies. 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 pla nt hybrid zones: ''phenological sinks'' resulting from staggered plant phenologies and ''hybrid sinks'' resulting from increased susceptibil ity of hybrids to insect attack. Such studies suggest that hybrid zone s are important natural laboratories for the study of plant-herbivore interactions.