Sprays designed to destroy or repel house flies fill a certain need in
connection with the house-fly problem. No very satisfactory repellent
substances for this insect have been found which are at the same time
adaptable to general use about the home, or places where foods are
handled. Extracts of pyrethrum flowers are now generally available
commercially, and these give fairly good results in the destruction of
house flies in buildings. Most of the sprays of pyrethrum extract
contain kerosene oil as a carrier, and undoubtedly the kerosene has much
to do with the toxicity of the spray. Such materials are most applicable
to buildings which become infested with flies and which can be readily
closed up at night and the air within thoroughly saturated with the
spray by means of an atomizer. Under such conditions the flies are
rather quickly overcome by the spray and if a sufficient quantity is
used they will not revive.