A series of experiments were done to study the effect of simulated summer and winter temperatures on the development of Phlebotomus perniciosus (Diptera:Psychodidae) and on the survival of Toscana virus in transovarially infected insects. Sand flies maintained at 28 degrees C developed relatively fast, with adults emerging from 40 to 55 days after initial oviposition. Similar results were obtained with insects reared at 25 degrees C. In contrast, sand flies maintained at 15 degrees C developed slowly up to the fourth larval instar; at that point, further development ceased and the insects entered diapause. Diapause could be terminated by increasing the ambient temperature to 25 degrees C. The ambient temperatures at which the immature forms were reared (15 degrees C, 25 degrees C, and 28 degrees C) had no effect on the subsequent F1 adult filial infection rates with Toscana virus (49.1%, 47.5%, and 46.5%, respectively). The results of these experiments provide a model of how Toscana virus survives the winter in endemic areas by maintenance in diapausing P. perniciosus larvae. In another experiment, venereal transmission of Toscana virus was shown from transovarially infected males to non-infected virgin females. This is the first demonstration of sexual transmission of a phlebovirus by sand flies. If venereal transmission occurs in nature, it would provide an alternative method of virus amplification in the vector population, in the absence of viremic vertebrates.