From ThriftyFun: DIY Mosquito trap - Cut the top off a 2 liter bottle. Invert the cone and place it inside the straight part of the bottle. Glue the two pieces together. Add 1 tsp yeast and 1/2 cup sugar to some luke warm water, and pour the mixture into the bottle. Mosquitoes are attracted to the carbon dioxide that you exhale. The yeast feeds off the sugar and emits the same gas, so the mosquito enters the bottle, thinking she will find food there.