BACKGROUND : Culicoides imicola Kieffer and Culicoides bolitinos Meiswinkel (Diptera: Ceratopogonidae) are both of
veterinary importance, being vectors of Schmallenberg, bluetongue and African horse sickness (AHS) viruses. Within
South Africa, these Culicoides species show a marked difference in their abundances according to altitude, with C.
imicola highly abundant in lower altitudes, but being replaced as the dominant species by C. bolitinos in cooler,
METHODS : The thermal physiology of field collected adults of each species was determined to evaluate whether it
could account for differences in their distribution and abundance. Critical thermal maxima (CTmax) and minima
(CTmin), as well as upper and lower lethal temperatures (ULT and LLT) were assessed after acclimation temperatures
of 19°C, 24°C and 29°C. Critical thermal limits were determined using an ecologically relevant rate of temperature
change of 0.06°C.min−1.
RESULTS : Significant differences in CTmin and CTmax were found between acclimation temperatures for C. imicola
and C. bolitinos. In C. bolitinos, the LLT of individuals acclimated at 24°C was significantly improved (LLT50 = −6.01°C)
compared with those acclimated at the other temperatures (LLT50 = −4°C). Acclimation had a weak (difference in
LLT50 of only 1°C) but significant effect on the LLT of C. imicola. When CTmin, CTmax, LLT and ULT were
superimposed on daily maximum and minimum temperature records from locations where each tested Culicoides
species is dominant, it was found that temperatures frequently declined below the CTmin and LLT of C. imicola at
the location where C. bolitinos was dominant.
CONCLUSIONS : The distribution and abundance of C. imicola is likely directly constrained by their relatively poor
tolerance of lower temperatures. Results for C. bolitinos suggest that the adult phase is hardy, and it is hypothesised
that the thermal biology of other life stages could determine their range.