Yes yes yes..I know I live in FL and it is very prominent down here, but it is so important. All it takes is ONE bite. The preventation is a h*ll of a lot cheaper than the treatment. Treatment for heartworms takes YEARS off of a dogs life, unfortunately. They use immiticide, which is basically a form of arsenic. It's much better to just give the heartguard and pay the $5/month.

If you miss a month, you shouldn't be overly worried, some of the studies have shown that within a month or two of the microfilaria (critters that cause heartworms) developing, if you give the heartguard it will kill it.