Greenhouse studies were conducted to examine the effects of light intensity on pac choi (Brassica rapa L. var. chinensis cv. ‘Mei Qing Choi’) and the corresponding plant-mediated effects of light intensity on the diamondback moth (Plutella xylostella L.). Plants with and without diamondback moth larvae were exposed either to ambient light or shade in 4 experiments conducted at different times of the year, resulting in a range of light intensities. The plant parameters measured were shoot biomass, primary nutrients, and phenolic content of leaves; for diamondback moth, larval consumption, cohort development, and male and female body weights were measured. In the light treatments, plants tended to have higher levels of phenolics and exhibited greater shoot biomass with higher carbon:nitrogen ratios under ambient versus shade. However, diamondback moth responses were not correlated with any plant responses, with one exception. Higher concentrations of ferulic acid under higher light intensities were associated with lower adult male body weights. Larval consumption was not different among the 4 months. However, cohort development was more rapid on plants during August, which had higher light intensities, compared with July, where light intensity was lower. Based on the responses measured, growing pac choi in the greenhouse under different light intensities is unlikely to affect diamondback moths through changes in pac choi chemistry unless decreased male body weight confers a fitness cost. However, plant-mediated effects associated with changes in light intensity on P. xylostella survival should be evaluated.