none The results of experiments performed in the laboratory showed that the host-searching time of the female Macrocentrus linearis in the area treated with an extract of the frass of its host Ostrinia furnacalis was apparently longer than that in the untreated area, though it did not increase proportionally with the amount of the kairomones. It was also found that the use of kairomones could decrease the host-finding time of M. linearis, but when the amount of kairomones was applied in excess, the difference of host-finding time between treatment and control disappeared. Host density had an important influence upon the efficiency of the kairomones. The results of experiments in outdoor cages indicated that spraying the extract from host frass on corn plants resulted in an increase in parasitization of M. linearis on the corn borer. The rate of parasitization was increased 13-18% on the sprayed corns as compared with the untreated corns.