Population dynamics of a tree-dwelling aphid: regulation and density-independent processes

1. A population of the Turkey-oak aphid (Myzocallis boerneri Stroyan) was s ampled at approximately weekly intervals on two Turkey-oak trees for 19 yea rs. 2. On one tree (A), the aphids exhibited a distinct seasonal pattern with a spring increase, summer decrease, early autumn increase, and late autumn d ecline. On the other tree (B) the aphids remained at low densities after th e decrease in summer. 3. On tree A, significant undercompensating density dependence occurred dur ing all periods of the seasonal population development, and their strength varied little during the course of the season. On tree B, significant densi ty dependence compensated exactly for increase, but appeared only after the decrease in summer when the population remained at very low densities for the rest of the season. 4. Density-independent weather variables affected the population dynamics v ery little. Their influence was marginally significant only at very low den sities when the aphids were regulated exactly by compensating density-depen dent factors. 5. The results suggest a curvilinear density dependence, with strong regula tion at low densities, and weak at high densities. That is, this aphid was most regulated not at the peak but at the trough densities.