Key‐factor/key‐stage analysis of long‐term life table data for a fruit gall midge,Asphondylia sphaera (Diptera: Cecidomyiidae)

1. Population dynamics of Asphondylia sphaera Monzen (Diptera: Cecidomyiidae), a species that induces fruit galls on Ligustrum japonicum (Oleaceae), was studied from 1970 to 1996 in broad‐leaved evergreen forests in Kagoshima, southern Japan. The numbers of fruit galls and emerging adults fluctuated greatly from year to year along with alternate year flowering of the host plant. 2. To detect density‐dependent and independent forces operating on the A. sphaera population and to assess the relative strength of top‐down and bottom‐up effects, we used the method of Key‐factor/key‐stage analysis, which allowed us to avoid various problems of the conventional key‐factor analysis. 3. Five factors and seven stages were distinguished in the life tables. Key‐factor/key‐stage analysis indicated that the number of flower buds made the largest contribution to the annual changes in total survival rate by operating through the proportion of eggs and first instars that survived abortion of host flower buds and flowers, which contributed most to density‐dependency. 4. The population dynamics of A. sphaera has been strongly influenced by the bottom‐up effects of both the abundance of flower buds and the abortion of flowers of L. japonicum. 5. A top‐down effect of Bracon asphondyliae Maeto (Hymenoptera: Braconidae) was weakly density‐dependent and contributed less to the total survival rate. Abiotic factors, such as summer and winter temperatures and the effect of typhoons were negligible.