POPULATION-DYNAMICS OF THE YEW GALL MIDGE TAXOMYIA-TAXI AND ITS CHALCID PARASITOIDS - A 24-YEAR STUDY

1. The dynamics of three populations of Taxomyia taxi (Inchbald) (Dipt era: Cecidomyiidae) and its chalcid parasitoids have been studied over a 24-year period. Most individuals have a 2-year life cycle but some develop in 1 year. Details of within-generation mortalities in T.taxi are used for life table analyses. 2. Mortality in the period between e mergence of adult T.taxi and larval infestation of buds appears to be density-dependent and is the largest component of overall mortality. I n 2-year life cycles, this mortality and that caused by Torymus nigrit arsus (Walker) contribute equally to variance in overall mortality. In 1-year cycles, mortality caused by Mesopolobus diffinis (Walker) is d ensity-dependent and accounts for most within-gall losses. 3. T. nigri tarsus, which attacks only 2-year galls, is absent from all population s for a number of years in the middle of the study period. Its searchi ng efficiency declines as its density and that of its host increase. 4 . Densities of M. diffinis are strongly correlated with those of avail able hosts. Percentage parasitism of 2-year galls is less than that of 1-year galls, suggesting the occurrence of enemy-free space. 5. Altho ugh there are some correlations in densities and mortality between tre es, the dynamics of populations on each are frequently different. 6. A n earlier analysis of 10 years' data failed to demonstrate density-dep endent effects. The extension to a 24-year run has revealed such effec ts, but also demonstrates long-term fluctuations in population densiti es, with corresponding changes in the balance of advantage between 1- and 2-year life cycles of T. taxi.