In order to be able to make an informed decision as to whether or not chemical insecticides need to be used against a specific type of insects, the invention proposes a device for determining how many of these insects there are in the field. This device comprises a rigid plate which is covered, on at least one side, with an adhesive layer and is provided with attachment means for attaching this plate to a support which has been fitted into the ground. According to the invention, the plate comprises a plate-like holder which can releasably support a sheet of transparent plastic material which contains the layer of adhesive and covers the plate-like holder on one side. The transparent sheet is preferably attached to the holder between two slide guides which are arranged on opposite edges of the holder.
