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At 65" F, the percentage of larvae of the European corn borer entering diapause increased from practically 0 to 95 as the photoperiod was increased from 0 to 9.5-14 hours of light per day, and decreased sharply to a minimum of 3 as photoperiod was increased to 16 or more hours of light per day. Rearing and studies on oxygen consumption revealed that, to be most effective, exposure to factors inducing diapause must begin before the second day of the final instar, and must continue for a period equivalent to the duration of the final instar a t 65" F. Exposure during a n y single instar but the last was not effective in inducing diapause.