A device (1) for applying a treatment agent to plants, in particular trees, comprises a collar (2) formed by an annular body and surrounding a trunk (3). is placed. By inflating the sleeve formed as an annular hollow body (2) this puts with their annular surfaces (9) sealingly against the trunk (3). A treatment agent introduced by a line (5), such as a gaseous or liquid pest control agent, can not escape from the hollow space (4) which is delimited from the concave inside of the sleeve (2) and sealed outwards by abutment of the annular surfaces (9). but enters through holes (8) in the trunk (3) and there unfolds its pest control effect.
