(see explanation below map)

Development of infective filarial larvae of the heartworm in mosquitoes is temperature dependent. The ambient temperature has to be above 14 degrees centigrade (58 degrees Fahrenheit) before the larvae will develop in the mosquito and it requires an accumulation of 130 degrees centigrade above 14 degrees centigrade for complete development. The number of days required to achieve that accumulated 130 degrees centigrade above 14 degrees centigrade is the minimum amount of time before infective larvae will be in the mosquito's mouth parts and hence capable of being transmitted.

Although the microclimate temperatures are the ones that most effect development such data is not easy to collect because this involves measuring temperatures under leaves, eaves of houses, crevices of rocks, or under tree bark, places where female mosquitoes hide after a blood meal. However, temperature records collected at weather stations in different communities can be used as a rough guide to how long development of invective larvae will take and hence how many months out of the year the outdoor dog is exposed to potentially infective mosquitoes.

If the mean monthly temperature is only a few degrees above 14 degrees centigrade it can take so many days for infective larvae to develop that the likelihood of the female mosquito living that long is remote. Under those circumstances, transmission intensity is very low and the risk of acquiring infection is also very low even for a dog that is outdoors and not on prophylaxis. Records of mean monthly temperatures in California communities have been collected for at least 30 years. Using these figures the equivalent degrees centigrade have been calculated and the minimum number of days necessary for a mosquito to have infective larvae develop in her has been calculated for communities throughout California. 

I have made the assumption that if it would take longer than 2 months for a female mosquito to develop infective larvae, based upon the mean monthly temperature, the numbers of female mosquitoes alive at the end of that time will be few. Consequently the exposure risk will be very low. At that point a dog owner could cease giving the dog prophylaxis without markedly increasing the risk of infection. The map of prophylaxis schedules (above) for California shows the month that the first and last dose of the once a month heartworm prophylaxis should be given to prevent heartworm infection based solely upon mean ambient temperatures.  You should note that although some communities have the same total number of months of risk, they start and end at different months. 

Using this schedule will allow owners to adopt a cost effective control strategy that will protect their outdoor, non-travel dogs from heartworm infection. If the dog is taken to areas where transmission is possible for longer periods than that in which the dog normally lives, then the dog should be treated while in the new area if there longer than one month or should be treated upon return to their home for an additional month.