Yoda doesn’t even have a screen door to keep the bugs out. And we do see some bugs on planet.
Can the Force be used as bug repellent and was it? Can lightsabers double as bug zappers? What is really going on here?
It's likely that creatures like the insect-eating Sleen (seen in Star Wars: The Empire Strikes Back) do an excellent job of keeping the swamps clear of biting insects.
We know that there are both insects and insectivores in and around Yoda's hut (courtesy of "Meet the Creatures of Dagobah") and we know that larger animals are attracted by his Force-essence and tendency to provide shelter to non-carnivorous species.
...being so close to the lagoon, with open doors and windows, means Yoda's home is visited by many creatures. Yoda frequently finds snakes, lizards, butcherbugs and spiny bograts scampering across the floor. He doesn't mind the company and only sweeps the venomous ones back out the door.
The book also makes note that when he was still living in the E3 escape pod, he was subject to frequent attacks from
but these seem to have stopped when he moved into his hut, presumably as a result of sharing his living space with numerous insectivores.