It is my understanding that, in general, fleas do not attack a healthy dog. Not saying there couldn't be an isolated case, but just in general they infect weak or sick pets. Dr. Pitcairn's Guide to Natural Health for Dogs & Cats recommends things like natural flea shampoos (containing d-limonene - dogs only) or using a lemon rinse (thinly sliced lemon addded to 1 pint near boiling water, steeped overnight) sponged onto the skin.
As for heartworm, when evaluating the risk/benefit, you may want to take into consideration what climate conditions are required for them to be a threat. It isn't enough to just have mosquitoes, it must stay warm enough for the maturation of ingested microfilariae to infective, third-stage larvae (L3). According to the
American Heart Worm Society, "Laboratory studies indicate that development and maturation requires the equivalent of a steady 24-hour daily temperature in excess of 64°F (18°C) for approximately one month. Intermittent diurnal declines in temperature below the developmental threshold of 57°F (14°C) for only a few hours retard maturation, even when the average daily temperature supports continued development. At 80° F (27° C), 10 to 14 days are required for development of microfilariae to the infective stage." I was quite surprised to read that. It's not very often that we have a two week period here where the temp at night doesn't go below 57°F.
For heartworm prevention, Dr. Pitcairn recommends a raw diet fortified with garlic and yeast, which help repel mosquitoes. Also, a natural insect repellent: rub one drop of eucalyptus oil, diluted in one cup of warm water, over the muzzle and area between the anus an genitals. Buddy's not on a raw diet, but we do supplement with garlic & yeast tabs, which also provide good nutrtional support including B vitamins.
If you want/need to use 'insecticides', I have read that Revolution is the best because Revolution (selemectin) is not an insecticide by chemical structure but rather is an antibiotic, falling into the avermectin group of drugs, which includes ivermectin (Heartgard).