Ok, they help repel before they bite. From the Advantix site: K9 Advantix® helps protect your dog from mosquitoes because it repels and kills mosquitoes before they can bite. This waterproof protection is gentle enough for puppies, as young as seven weeks of age. K9 Advantix helps reduce the risk of transmitting disease-causing organisms, and provides relief from biting.
I would think this would be helpful (better than using nothing at all), than using heartworm meds...
