The Mercola article that Jeffrey gave the link for is a must read. The maps on there tell you during what months it is very important to protect against heartworms. In NJ, you should start giving heartworm prevention at the beginning of June, earlier if it is unseasonable warm(for myself, having seen too many heartworm positive dogs, I would start in May, if there is any doubt) and give the last dose of the year at the end of November, again being aware of any unseasonably warm weather.
It’s safer to skip spring than to skip fall, as far as how heartworm infections occur and how heartworm prevention works. You definitely want that last dose of the year a month after the last possible mosquito bite.
