We studied the capacity of one species of predator to control two major pests of greenhouse crops, Western flower thrips (Frankliniellaoccidentalis(Pergande)) and the greenhouse whitefly (Trialeurodes vaporariorum(Westwood)). In such a one-predator–two-prey system,indirect interactions can occur between the two pest species, such as apparent competition and apparent mutualism. Whereas apparentcompetition is desired because it brings pest levels down, apparent mutualism is not, because it does the opposite. Because apparent com-petition and apparent mutualism occurs at different time scales, it is important to investigate the effects of a shared natural enemy onbiological control on a time scale relevant for crop growth. We evaluated the control efficacy of the predatory mitesAmblyseius swirskii(Athias-Henriot) andEuseius ovalis(Evans) in cucumber crops in greenhouse compartments with only thrips, only whiteflies or bothherbivorous insects together. Each of the two predators controlled thrips, butA. swirskiireduced thrips densities the most. Therewas no effect of the presence of whiteflies on thrips densities. Whitefly control by each of the two predators in absence of thrips wasnot sufficient, yet better withE. ovalis. However, whitefly densities in presence of thrips were reduced dramatically, especially byA. swir-skii. The densities of predators were up to 15 times higher in presence of both pests than in the single-pest treatments. Laboratory exper-iments withA. swirskiisuggest that this is due to a higher juvenile survival and developmental rate on a mixed diet. Hence, better controlmay be achieved not only because of apparent competition, but also through a positive effect of mixed diets on predator populationgrowth. This latter phenomenon deserves more attention in experimental and theoretical work on biological control and apparentcompetition.