Differential Colonization of Resistant and Susceptible Host Plants: Pemphigus and Populus

Plants show extensive intrapopulational variation in quality as hosts for herbivores. Adaptive discrimination among individual host plants depends on the availability of plant traits that are present when plants are colonized and that are correlated with quality. Here, we examine the ability of colonizers to select appropriate hosts when colonization occurs long before feeding, and when traits available as cues may be least associated with host quality. In such systems the precision of host selection may be limited by a lack of appropriate cues at the time of colonization. In the life cycle of the aphid Pemphigus betae, autumn migrants select among host trees (Populus angustifolia and natural hybrids with P. fremontii) far in advance of the spring feeding stages, while trees are in a very different physiological condition. In order to estimate host quality, we censused successful and aborted galls on 34 trees in a Utah canyon during seven consecutive years, thus obtaining tree—specific rates of stem—mother survivorship. We estimated attractivity of these trees by censusing autumn migrants during 3 yr. Both survivorship and attractivity varied among trees, and differences among trees persisted across years. Tree—specific colonization rates were positively related to gall establishment. These results indicate that the actual colonization pattern is more adaptive than a random colonization process; however, the ability to select better hosts is far more perfect. To determine how migrants might use cues evident during colonization and correlated with quality, we examined the relationships between tree attributes in autumn and colonization rate and between these same attributes and stem—mother survivorship. One attribute, degree of leaf retention in autumn, was positively correlated with both colonization and subsequent progeny survival. Thus, the tendency to colonize trees retaining leaves longer in autumn may contribute to P. betae's limited ability to preferentially colonize better hosts. The results support the hypothesis that, in life cycles in which colonization and feeding are separated, the evolution of adaptive host discrimination may be constrained by the availability of appropriate cues.