In wild bird populations nestlings are commonly infested by ectoparasites. The effects that ectoparasites exert on host fitness usually become evident after the complete elimination of parasites or a substantial reduction in their abundance. Among the most common elimination methods in ecological studies, the use of chemical insecticides is traditionally preferred. However, the application of several doses is usually needed because the efficiency of insecticides decreases over time. This results in increasing toxicity effects and disturbance of both parents and nestlings. In the present study, we tested the efficacy of Inesfly, a new insecticidal paint. Inesfly acts by releasing active water-soluble compounds progressively over a period, therefore having a longer-lasting effect than other insecticides. By applying this insecticidal paint inside nest-boxes used by blue tits Cyanistes caeruleus during the breeding season, we investigated its effect on the abundance of blackfly (Simuliidae), biting midge (Culicidae), flea (Ceratophyllidae), mite (Dermanyssidae) and blowfly (Calliphoridae) larvae. The insecticide significantly reduced the abundance of Protocalliphora azurea blowflies, while other parasites remained unaffected. Our results show that Inesfly was ineffective against ectoparasites infesting blue tit nest-boxes, blowflies excepted, at least at the dose applied. Finally, no negative effects on nestlings were reported following the use of this insecticide.