You should have your dog on HW prevention in the lower 48 of the United States. The American Heartworm Society (the veterinary organization that studies HW) now advises all dogs to be on it year round. Some vets in low-risk northern areas with very cold winters allow clients to "take a break" during the winter -- but that's very climate specific (and can even vary within the same state).
Heartworm disease is expanding as the climate warms and dogs from high-HW areas get transported to areas where few dogs are on prevention. If you look at the changes in the disease incidence maps that the HW Society publishes (based on clinical data), these changes are very clear. So even if your area didn't have cases last year, it might this year!
Years ago, when I lived in California, there were almost no HW cases in my area at first, but I put my dogs on inexpensive, generic prevention just in case...and then the Katrina dogs arrived, and suddenly we had dogs being diagnosed in my neighborhood with HW. My dogs were safe because of those cheap monthly pills. You can't predict or control when a load of rescue dogs might show up from a place hit by a hurricane!
In your area, cheap, generic ivermectin-based products (Iverheart, TriHeart3, PetTrust, etc.) are analogs for the more expensive brand-name drug Heartguard, but the generics may cost as little as $5/month. The dose is so low that they're extremely safe (and have been used safely for years and years). Some of the newer products like Trifexis are more likely to have serious side effects reported (and they cost more).
Heartworm disease can easily cost over $1000 to treat. The treatment is risky, and has a significant mortality rate from side effects (with GSDs possibly being even more at risk, due to genetic risks not well understood). Untreated, it kills the dog -- and it's a very miserable, unpleasant death.
There are no "natural" heartworm preventatives. Mosquito repelling herbals can help, but it only takes ONE mosquito that happens to be carrying HW larvae (from biting an infected host) to bite your dog and transmit. One single bite really can be all it takes, so unless you are 100% sure mosquitoes can never contact your dog, even when it goes out out to potty, you cannot eliminate the risk.
This thread may be helpful: