Diapause of the Eastern tiger swallowtail butterfly Papilio glaucus is dependent on latitude and subspecies origin. Lower latitude populations of two subspecies, Pg. australis and Pg. glaucus, show little diapause response to a long-day exposure of larval L:D (light:dark) 16:8 at 23.5/19.5 C. Higher latitude populations of P.g. glaucus diapause at the 507o level under this long day regime. Approximately all individuals in populations of the northernmost subspecies, Pg. canadensis, diapause under L:D 16:8, longer photoperiods, even in warmer constant temperatures. This response implies that diapause in Pg canadensis is obligate. Subspecies isolation may explain differences observed in photoperiodic response. INTRODUCTION The eastern tiger swallowtail butterfly Papilio glaucus L., has a geographic range extending over much of North America. Three putative subspecies are believed to be parapatric, with each occupying a portion of this range: Pg australis Maynard occurs in the extreme southeastern U.S.; Pg. glaucus is found from the Great Lakes to the Gulf of Mexico, and Pg. canadensis R & J extends across most of Canada. The three subspecies generally appear to have geographic blend zones where apparent hybridization or introgression occurs (Scriber, 1982; Ritland and Scriber, 1985). One of the physiological factors which may contribute to the subspecies isolation in Papilio glaucus is patterns of voltinism. Papilio g. glaucus and Pg australis are both bivoltine or trivoltine (Edwards, 1897; Scudder, 1889) and Pg canadensis is described as univoltine with pupal diapause intervening in each generation (Ebner, 1970; Scriber, 1982, 1983). Numerous studies have centered on the induction of diapause throughout geographic ranges and it is accepted that the critical photoperiod which induces diapause can vary with latitude even within populations of the same species (Danilevskii, 1965; Rabb, 1969; Beck, 1980; Tauber et al., 1986). Hagen and Lederhouse (1984) found this type of variation within populations of P. glaucus ranging from Florida to Quebec. When raised at long-day photoperiods, only populations from northern latitudes diapaused. The authors concluded that populations of northern latitudes are restricted to univoltinism because of a long larval growth period and a short growing season which prevents a second generation (see also Scriber, 1975). We have examined the photoperiodic diapause response in populations of Papilio glaucus glaucus, P g canadensis and P g australis extending across a geographic range from Florida to Manitoba, Canada, with particular attention paid to populations from Wisconsin. Populations from Wisconsin are unusual in that, within a limited geographic range northward, Pg. glaucus disappears and Pg canadensis becomes the dominant subspecies. Within this small range, voltinism patterns also change abruptly, from bivoltinism to univoltinism. We now demonstrate that although the diapause response of Pg glaucus populations vary with latitude, the univoltism of Pg. canadensis is due to a strict obligate diapause. 'Current address: Competitive Research Grants Office, U.S. Department of Agriculture, 15th and Independence, S.W. Washington D.C. 20251-2200. 2Current address: S.C. Johnson and Son, Inc., 1525 Howe Street, Racine, Wisconsin 53402. 3Current address: Department of Entomology, Michigan State University, East Lansing, MI 48824.