Relative importance of plant ontogeny, host genetic variation, and leaf age for a common herbivore

Using an experimental eucalypt forest of known pedigree and laboratory feeding trials, we examined the relative importance of plant ontogeny (heteroblasty), genetic variation among host trees, canopy height, and leaf age as potential drivers that could affect the distribution and feeding preference of a common insect herbivore, Chrysophtharta agricola. We found that ontogeny is a major factor affecting this insect. Its importance rivaled leaf physiological age, a well-documented factor, which served as our standard for judging the relative importance of other effects. Three patterns emerged: (1) In the field, beetle feeding was nine times greater in the adult zone than in the juvenile zone of heteroblastic trees (i.e., trees with both adult and juvenile foliage). (2) Laboratory feeding trials confirmed their strong preference for adult foliage. (3) Although eucalypt species, hybrid cross type (F1, F2, and backcrosses), and canopy height also exhibited significant effects on beetle feeding in laboratory trials, their relative importance was much less than ontogeny and leaf physiological age. We conclude that beetles perceive greater variation in host quality within individual plants than between different eucalypt species and their hybrids. The magnitude of these effects argues that ontogeny may rival other better studied plant traits that affect herbivores. We discuss how genetic regulation of phenotypic expression in plants may affect herbivore populations and structure communities.