IMPORTANCE OF SUPERCOOLING POINTS IN THE OVERWINTERING OF THE HORN FLY AND STABLE FLY (DIPTERA, MUSCIDAE)

Supercooling points were determined for eggs, 3rd instars, pupae. newl y emerged unfed adults and 3-d-old engorged laboratory reared adults o f Haematobia irritans (L.) and Stomoxys calcitrans (L.). Wild nondiapa using and diapausing pupae of H. irritans also were tested. Mean super cooling points ranged from -28.0 degrees C for H. irritans eggs to -6. 8 degrees C for H. irritans larvae. Mean supercooling points of all H. irritans developmental stages were lower than those of comparable S. calcitrans developmental stages, with the exception of larvae where th e reverse was true. The mean supercooling point of diapausing H. irrit ans pupae (-23.5 degrees C) was significantly lower than those of nond iapausing laboratory pupae (-20.8 degrees C) or nondiapausing wild pup ae (-20.2 degrees C). Developmental stages of both species were freeze intolerant, with no survival following exposures to temperatures belo w the supercooling points. Results are discussed with respect to the d isparate overwintering strategies of these species and in relation to typical climatic minima experienced in south central Texas. The cold t olerance of H. irritans and S. calcitrans pupae was compared at 4 degr ees C, a temperature below their developmental threshold of 11.5 degre es C and above their mean supercooling points. The survival of H. irri tans pupae was significantly greater than the survival of S. calcitran s pupae. Cold injury was a significant mortality factor for both speci es.