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, newly emerged unfed adults and 3-d-old engorged laboratory reared adults of Haematobia irritans (L.) and Stomoxys calcitrans (L.). Wild nondiapausing and diapausing pupae of H. irritans also were tested. Mean supercooling 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 the reverse was true. The mean supercooling point of diapausing H. irritans pupae (-23.5 degrees C) was significantly lower than those of nondiapausing laboratory pupae (-20.8 degrees C) or nondiapausing wild pupae (-20.2 degrees C). Developmental stages of both species were freeze intolerant, with no survival following exposures to temperatures below the supercooling points. Results are discussed with respect to the disparate overwintering strategies of these species and in relation to typical climatic minima experienced in south central Texas. The cold tolerance of H. irritans and S. calcitrans pupae was compared at 4 degrees C, a temperature below their developmental threshold of 11.5 degrees C and above their mean supercooling points. The survival of H. irritans pupae was significantly greater than the survival of S. calcitrans pupae. Cold injury was a significant mortality factor for both species.