Fine-scale temperature fluctuation and modulation of Dirofilaria immitis larval development in Aedes aegypti

•We compare a degree-hour to a degree-day model of Dirofilaria immitis development.•D. immitis development in Aedes aegypti was observed under fluctuating temperature.•The standard degree-day model underestimated development in colder temperatures.•Our hourly temperature model accurately predicts D. immitis development.•Hourly temperature models can prevent underestimating heartworm risk around 19°C. We evaluated degree-day predictions of Dirofilaria immitis development (HDU) under constant and fluctuating temperature treatments of equal average daily temperature. Aedes aegypti mosquitoes were infected with D. immitis microfilariae and parasite development was recorded at set time points in dissected mosquitoes. Time to L3 development in Malpighian tubules and detection in mosquito heads was shorter for larvae experiencing a daily regime of 19±9°C than larvae at constant 19°C; larval development rate in Malpighian tubules was slower in fluctuating regimes maintained above the 14°C developmental threshold than larvae under constant temperatures. We showed that hourly temperature modeling more accurately predicted D. immitis development to infective L3 stage. Development time differed between fluctuating and constant temperature treatments spanning the 14°C development threshold, implicating a physiological basis for these discrepancies. We conclude that average daily temperature models underestimate L3 development—and consequently dog heartworm transmission risk—at colder temperatures, and spatiotemporal models of D. immitis transmission risk should use hourly temperature data when analyzing high daily temperature ranges spanning 14°C.