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

The shoot-galling sawfly, Euura amerinae, usually attacks only young t rees of Salix pentandra. Similar patterns of attack by other herbivore s 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 19 78, they were first attacked by sawflies in 1983 and the population of sawflies became extinct in 1990, after a brief flush to peak densitie s of galls in 1988. Mean productivity of adults declined steadily from 1983, at two per gall, to zero in 1990, while willows grew very rapid ly. Some trees were attacked each year for 4 to 7 years suggesting lac k of an induced defense in the plant, and carnivores were unimportant in generating the sawfly population dynamics. Only the Plant Ape Hypot hesis was supported by the data, indicating the probable importance of ontogenetic aging in the increasing resistance of trees to sawfly att ack.