In short-term laboratory experiments, we studied the influence of light intensity, photoperiod, and temperature on the feeding and oviposition activity of two aphelinid parasitoids, Encarsia formosa Gahan and Eretmocerus eremicus Rose and Zolnerowich, on the greenhouse whitefly, Trialeurodes vaporariorum (Westwood). E. eremicus parasitized significantly more whitefly hosts than E. formosa in all treatment combinations of light intensity and photoperiod at 24 and 20°C. At 24°C, both E. formosa and E. eremicus parasitized approximately twice as many whitefly hosts at the high light intensity (112–114 W/m²)–long daylength (L 16:D 8 h) treatment than at the low light intensity (12–14 W/m²)–short daylength (L 8:D 16 h) treatment. In most instances at 20 and 24°C, significantly more dead whitefly hosts were recorded in leaf cages introduced with parasitoids than leaf cages with no parasitoids, indicating that host feeding occurred. Both E. formosa and E. eremicus parasitized significantly more whitefly hosts under the simulated summer (i.e., high light intensity [82.0–83.6 W/m²]; long daylength [L 16:D 8 h], 24°C) treatment than the winter (i.e., low light intensity [10.8–11.1 W/m²]; short daylength [L 8:D 16 h], 20°C) treatment. In addition, significantly more dead whitefly hosts were observed in leaf cages introduced with E. eremicus than E. formosa under the winter treatment, suggesting that E. eremicus killed more whitefly hosts through host feeding than E. formosa. This was not the case for the other treatment combinations. E. eremicus contributes more to greenhouse whitefly mortality than E. formosa by a combination of greater parasitism and host feeding and therefore is recommended for use in winter months with day temperatures ≥ 20°C, in addition to later in the season, where it is currently being used.