Abstract: Biological control of Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) byspontaneously appearing parasitoids (Necremnus spp.) was studied during 52 tomato cropcycles carried out in 42 greenhouses in South-East Spain between April 2017 and May 2018.Necremnus tutae (Reuter) was found in all greenhouses, reaching a successful T. absolutacontrol in 36 crop cycles (69%). Several pesticides were found to negatively affect the activityof N. tutae, e. g. sulphur applied by dusting, which is a common practice in organic crops.Consequently, many treatments were found to be counter-productive, resulting in significantcrop losses due to T. absoluta damage. Biological control by N. tutae reached high rates ofparasitism (up to 73%) and mortality (via host-feeding, up to 92%), throughout spring andsummer, most likely due to the high temperatures during this period. Since N. tutae cannoteasily be mass reared, it is not available for augmentative releases.Two key strategies should be followed to promote the contribution of spontaneouslypresent parasitoids. Firstly, the application of pesticides against other pests and diseasesshould be reduced to a minimum, with selected methods and products. Secondly, thegreenhouse environment can be made more suitable for parasitic wasps by planting selectedflowering plant species for the provision of nutritional food supplements, i. e. nectar. Theoptimisation of biological control of T. absoluta by N. tutae offers a free and sustainable wayto significantly reduce crop damage and insecticide usage in tomato crops.