Western flower thrips (Frankliniella occidentalis) are the largest pests of fruit-bearing vegetables in protected areas. Extremely polyphagous, short life cycle, high potential of reproduction and a large number of generations enables quick development of resistance that has been observed all over the world, and also in Croatia, where it was determined for pyrethroid and neonicotinoid preparations on all tested populations, and only some of the tested populations were sensitive to the active substance - spinosad. The main goal of the study is to determine the different sensitivity of adult forms of Western flower thrips to the active substances from the pyrethroids, neonicotinoids and spinosin groups. During 2019, removing pepper and cucumber plant parts, adult forms of Western flower thrips across the four counties were collected from nine localities, and testing was performed according to IRAC Test Method No. 10. During the studies, the treated population of Western flower thrips were resistant on eight sites to the active substance lambda-cyhalothrin. High abamectin resistance was determined at almost all locations, only the population of „Ferovac“ site was being resistant in some places. Populations of Western flower thrips were resistant to the active substance spinosad, only the population of „Mlinište 1“ (cucumber) was highly sensitive. Due to insufficient efficiency of permitted preparations, the suppression of the Western flower thrips can only be carried out in its integrated production and / or integrated vegetable protection.