Abstract: Tetranychus urticae is the most important pest on vegetable crops in greenhouses inPoland and successful control of this pest is very difficult. The present registration requirementfor biological control agents in Poland states that macro-organisms such as nematodes andbeneficial organisms do not need to be registered. As a result, a lot of new natural enemies arenow commercially available and nine species can be used against spider mites.Research work was conducted on the interaction between natural enemies introduced intogreenhouse-grown tomato crops to reduce the number of pests such as spider mites and the sideeffects of insecticides and fungicides on predators. This interaction was studied with reference tothe predatory mite species: Amblyseius swirskii and Phytoseiulus persimilis T which are used inlaboratory and greenhouse tests to control the two-spotted spider mite Tetranychus urticae. Thegreatest efficacy (86%) in limiting the number of T. urticae was obtained through the use ofP. persimilis and A. swirskii. The results of this experimental treatment were statistically differentfrom other treatments involving both the separate and combined use of specific predatory mitespecies. The study also revealed competition between the two species A. swirskii andP. persimilis, with the predatory mite A. swirskii being the dominant species.The insecticides Abarex, Spin Tor and Nissorun were safe for use with predatory mites andcould be recommended for IPM (Integrated Pest Management) programmes for greenhousegrowncrops. The result of the studies carried out revealed that the species P. persimilis was themost susceptible to the different fungicides used, based on laboratory experiments, and especiallywhen the predator was released 1 and 3 days after applying these fungicide treatments. Ingeneral, the fungicides applied showed a lower level of toxicity to all the beneficial organismswhen the predatory mites were released 5 days after its application. The fungicide Topsin 500 SCappeared to be selective with respect to the species A. swirskii.