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Another problem with mosquito zappers that indiscriminately attract insects using light is that they... well, attract insects. Most of the insects that are attracted to the light will not actually fly into the trap but will sit on the plants, walls, people, etc. around the trap. So, in effect, you can actually increase, instead of decrease, the number of insects in your yard.
A method of controlling mosquitos around a pond that I have found very effective is to scrub rocks with a brush to dislodge the larvae attached there, so as to allow them to float freely and be eaten by fish. Large smooth rocks becone covered with the mosquito larvae and can be easily dislodged in this way. I also dislodge them fron small rocks that can't be effectively scrubbed with a brush by useing a garden hose with a sharp spray nozzle to spry the small stones and pebbles thereby flushing them into the pond.
Try it, it works very well.