The data presented in this paper were obtained during preliminary tests of acaricides that have been carried on in the greenhouse since 1947. Special attention has been given to the duration of residual action because this is one of the most critical indications of the potential value of an acaricide under field conditions. Residual toxicity in these experiments may he defined as the toxicity of a dry deposit to mites introduced on foliage subsequent to spraying.

The modes of action of the various acaricides are uncertain. In most cases it is undoubtedly contact action, although the more volatile materials may have some fumigant effect. Toxicity by direct ingestion is unlikely because of the feeding habits of the mites, but it is possible that some of the compounds tested may have a systemic action, being absorbed by the plant and remaining within the tissues for an extended period.