Stocks ofTrioxys complanatusfrom California, Iran and Italy were compared to detect differences in climatic adaptation. Results showed the Italian stock was significantly less tolerant of extreme heat, suffering more mortality and displaying a slower developmental rate than the other 2. The Italian ecotype also failed to enter into the heat-induced, semi-protective aestival diapause that characterized portions of the other populations. Significant portions of all stocks entered a cold-induced diapause, although the Italian strain had a lower induction threshold. Daylength had no effect on diapause induction. All stocks interbred but a lower female-to-male ratio of offspring was found in all matings with Italian females. Responses reflect the climates of each stock's origin and indicate potential field success in the biological control of their host, the spotted alfalfa aphid.