Abstract: The purpose of this study was to explore the seasonal dynamics and spatiotemporal niche of Tetranychus urticae and Frankliniella occidentalis on chrysanthemum seedlings under greenhouse condition. The population sizes of F. occidentalis and T. urticae in different parts of chrysanthemum seedlings were investigated by using diagonal five-point sampling method. The quartile method was used to compare population sizes of F. occidentalis and T. urticae during the main occurrence period, and the temporal and spatial niches of these two pest species were analyzed. The results showed that the main occurrence period of adults and nymphs of F. occidentalis were from late April to early June and from late April to mid-November in 2020, respectively. In 2021, the main occurrence period of adults and nymphs of F. occidentalis were from early May to late July and early March to mid-June, respectively. Both adults and nymphs peaked in May, with the highest population being 8.76 and 5.67 ind·plant^-1, respectively. In 2020, the main occurrence period of adults and nymphs of T. urticae was from late April to late September, and the main occurrence period of eggs was from late April to early October. In 2021, the main period of adults, nymphs and eggs of T. urticae occurred from late July to late September, both adults and nymphs peaked in September of 0.52 ind·plant^-1, while the egg peaked from late July to late August, and the population size reached to 4.36 ind·plant^-1. The population size of adults F. occidenalis increased first and then decreased during the whole year, increasing from January and peaking in May. The population size of T. urticae showed a multi-peak pattern, and the population activity pattern was different from year by year. Population size increased slowly from April, and fluctuated greatly after May. F. occidenalis mainly damaged the young part of upper chrysanthemum seedlings, while T. urticae mainly damaged the lower part leaves which were close to the ground. Therefore, the control of these two pests should be started from April. The control of F. occidenalis should be focused on the upper part leaves, and that of T. urticae should be focused on the leaves close to the ground.

Key words: Frankliniella occidentalis,  Tetranychus urticae, seasonal dynamic, spatiotemporal niche, quartile method