Herbivore deme formation on individual trees: a test case.
We examined the deme-formation hypothesis, which states that sessile herbivores on long-lived hosts become locally adapted to the defensive phenotypes of individual trees. We showed a five-fold increase in resistance by individual pinyon pines (Pinus edulis) to the pinyon pine needle scale (Matsucoccus acalyptus). Although such variation could represent a significant selection pressure favoring deme formation, two lines of evidence led to rejection of the hypothesis. First, there were no significant differences in mortality among scale populations in a reciprocal transfer experiment. Second, a seven-year experiment showed that mortality of newly founded, incipient scale populations was similar to established scale populations. While our experiments fail to support the deme-formation hypothesis, they do demonstrate significant variation in the resistance traits of a natural tree population. Although we feel that demeformation is still probable in this system, it is likely to occur on a larger geographic scale than individual trees as proposed by Edmunds and Alstad.
Department of Biological Sciences, Northern Arizona University, 86011, Flagstaff, AZ, USA.
Neil S
NS
Cobb
Department of Biological Sciences, Northern Arizona University, 86011, Flagstaff, AZ, USA.
Thomas G
TG
Whitham
Deme formation; Fine-scale adaptation; Host plant resistance; Matsucoccus acalyptus; Pinus edulis