COLONIZATION AND EXTINCTION IN A POPULATION OF THE SHOOT-GALLING SAWFLY, EUURA-AMERINAE

The shoot-galling sawfly, Euura amerinae, usually attacks only young trees of Salix pentandra. Similar patterns of attack by other herbivores on plants have generated four alternative hypotheses to account for these patterns: The hypotheses on induced defenses in attacked plants, plant age. plant vigor, and regulation by carnivores. This long-term study followed all individual trees in a population from establishment of sawflies to their local extinction. Trees colonized the site in 1978, they were first attacked by sawflies in 1983 and the population of sawflies became extinct in 1990, after a brief flush to peak densities of galls in 1988. Mean productivity of adults declined steadily from 1983, at two per gall, to zero in 1990, while willows grew very rapidly. Some trees were attacked each year for 4 to 7 years suggesting lack of an induced defense in the plant, and carnivores were unimportant in generating the sawfly population dynamics. Only the Plant Ape Hypothesis was supported by the data, indicating the probable importance of ontogenetic aging in the increasing resistance of trees to sawfly attack.