No Hook, all these drugs for heartworm do not affect the adult worm, just a small window, I think from memory, when the heartworm larvae goes between the 3rd to 4th stage. With modern drugs there is about a 6-8 week period when they work which is about 3mths after being bitten by the mosquitoe. From being bitten to an adult heartworm is 6mths. The adult hearworm can live for many years. Monthly prevention is needed although it can be stretched a bit but your safety margin is reduced.
On saying all this, I come from Brisbane in Australia and worked when heartworm was endemic, ie if a dog lived outdoors and was not on prevention by 4yrs of age if tested it would stand a 95% chance of having heartworm. Treatment is expensive, complicted and not without risk to the animal, I have treated many. However in the last 10 years I have not treated any, it has disappeared, mainly due to these newer more convenient forms of heartworm prevention. I would not suggest people to not keep up the prevention as if a susceptible population of dogs increases then it would most definately reappear.
In essence you need to get an idea of the heartworm incidence in your area and the need for prevention. My guess is that there is a lot less out there now than what the drug companies would have you believe. It is a bit like vaccinating kids, some people won't have their kids vaccinated, the only way they get away with it is that most people vaccinate and the diseases don't exist or are at a very low level now. In other words they are bludging off the majority. Sorry I'm getting a bit carried away here, just one of my pet subjects. Hope it is of help.
Ken