Efficiency of chemical control of Quudruspidiotur perniciosus Comst. can be assessed, 3-5 days after treatment, by the use of a colour reaction relating .to oxyreductase activity. The optimal conditions for this reaction are a 0.5-1.5 % solution of 2,3,5-triphenyl-tetrazoliurn chloride (pH 10.5) and time of 2-3 h. It is noted that use of a biochemical test for assessing mortality is possible for other insects, and is not dependent on the reasons causing their death.