If you don’t mind your sukkah smelling like Italian salad dressing, garlic really does work to repel mosquitos.
Get a spray bottle (the best is the large can type used to spray a garden, but a clean, well washed out windex bottle will do) put in several spoonfulls of garlic powder and water. Shake it very well. Best if you let it sit for a while first. Then spray all around the sukkah – the schach, the walls, ALL over. The natural chemical in garlic is actually the active ingredient in the mosquito repellent used by exterminators.