Onion thrips are of major concern in onion production. Thrips populations are influenced by various climatic variables, different transplanting dates, and spacings. Considering this, an investigation was undertaken to find out the influence of climatic variables on Thrips tabaci Lindeman under three different planting dates in West Bengal, India (15th Aug, 30th Aug, 14th Sept during kharif season and 30th Nov, 15th Dec and 30th Dec during rabi season) and three different spacings viz., 15cm × 10cm,15cm × 15cm and 20cm ×15cm(plant to plant by row to row)for two consecutive years. Thrips populations were not observed on Kharif planted onion which might be due to frequent rainfall which acts as a controlling factor of thrips. Closer spacing with dense foliage supported more built-up of thrips population than the widely spaced crop with sparse vegetation. Out of three dates of planting of rabi season crop, the 30th November planted onion crop was comparatively less infested by T. tabaci. The study unveiled that 30th November would be the best time to transplant the rabi onion crop with a spacing of 15cm × 10cm in the Gangetic plains of West Bengal for better yield. Also, the ambient temperature and canopy temperature were found to have a significant positive correlation while canopy humidity was negatively correlated with the thrips population. Besides the mentioned microclimatic variables, incidental and reflected photosynthetically active radiation also revealed a correlation with thrips population fluctuations on a few occasions.