A laboratory study of the influence of various combinations of light, dark, and temperature on adultChrysoteuchia topiaria(Zeller) revealed that the rhythm of emergence was exogenous and was initiated by changes of temperature or by the transition from dark to light corresponding to dawn in nature. Thus, the time of emergence in nature is probably determined by an interaction of photoperiod and temperature. In a field test, virgin females were attractive to males in a rhythmic manner whereby 71% of the males were attracted to the females between 0700 and 1100. Cool and cloudy weather reduced the number of responding males.