The greenhouse whitefly, Trialeurodes vaporariorum (Westwood) is an important pest of greenhouse vegetable crops worldwide. Management of T. vaporariorum with Encarsia formosa Gahan has been used widely in greenhouse crops since the 1970s. Eretmocerus eremicus Rose & Zolnerowich, described in 1997 from the Bemesia tabaci complex in the United States, is also commercially produced for inundative release to suppress whiteflies infesting greenhouse ornamental and vegetable crops. Both E. formosa and E. eremicus are used singly or in combination in greenhouse tomato production in southwestern Ontario, Canada. It has been reported that biological control of T. vaporariorum is less effective during winter and early spring. Lower light intensity and shorter photoperiod have been suggested as possible reasons for failures in biological control in the winter and early spring. To date, however, few studies have been conducted to evaluate the influence of light intensity and daylength on the behaviour and efficiency of natural enemies. Results from environmental chamber studies investigating the influence of light intensity and daylength on parasitism of E. formosa and E. eremicus is presented.