The majority were harmless insects, or beneficial insects such as parasitic wasps that help keep other pests at bay. The authors (Timothy Frick and Douglas Tallamy) calculated that the four million bug zappers that might be bought over the last four years in the USA would destroy 71 billion nontarget insects. Environmental vandalism! The corollary of this is that as well as killing beneficial or harmless insects you are starving much of the bird life in your area of their insect diets. Silent springs again.
Female mosquitoes (the ones that bite) are attracted to mammals, not the moon or lights. From a long distance, they are drawn to carbon dioxide because normally that means a breathing mammal is in the vicinity. As they get closer to the target, body heat becomes their fixation. There are some traps now that use carbon dioxide baits rather than light, and these traps will capture much higher numbers of mozzies. The carbon dioxide can be produced from dry ice, or by burning propane. Adding another lure called octenol can further boost the allure of carbon dioxide. I don't know if any studies have been conducted that have shown whether even these traps will reduce your chances of getting bitten in a domestic situation. But still, the problem with any zapper designed to attract insects and mosquitoes is that while they might get some of the insects around your house, they will also attract heaps from your neighbor's place or adjacent bushland. Most of these will not actually go into the trap, but sit on plants or walls around the house. In effect, you are only increasing the number of insects around you - they are lures after all. You should be aware of this scientific joke, as Alberto Broce (Entomology Professor at Kansas State University) said 'If you have problems with mosquitoes in your backyard, give one of these traps to your neighbors so the mosquitoes will go over there'.
You might think well OK, I will only place my zapper inside the house where only those inside will be killed, and none from outside can come in. That's fine, if you want to litter your house with clouds of insect viruses and body parts. One study found that millions of live bacteria and viruses were released when insects were disintegrated by the zap. This effect is especially dangerous around food preparation areas. Zappers will explode insects, spraying their body parts and a cloud of bacteria into the atmosphere. Insect particles can be particularly allergenic to some humans (I'm sure that clouds of any animal disintegrations would be allergenic).
Scientists studying mosquitoes often use light traps called the New Jersey light trap, or the CDC light trap (after the Centers for Disease Control), to monitor mosquito populations. These traps are used for survey purposes, not the prevention of bites. At times, these traps can capture some low numbers of mozzies using light alone. It seems that most species have no interest in light, but there are a few that will go in small percentages to the light. Usually, the traps are also placed near ponds or bushland to increase the catch. But the scientist's interest is to collect mozzies, not to avoid getting bitten - if the traps suck in mozzies from a wider area, most will not enter the trap but will search for a host instead as they approach. Scientists routinely add carbon dioxide to these traps because they are then far more useful and predictive about mozzy populations.
There are different kinds of lights. The ones in a house are mostly incandescent, producing little ultraviolet light but mostly visible light. Fluorescent lights that emit higher levels of UV, called black light or blue light are most often used in zappers. Other lights such as mercury vapor globes produce even more UV and are particularly attractive to moths and beetles.
From my own experience, I have never seen a mozzy flying around an incandescent light globe at home, only moths. This suggests that mozzies in my area are not attracted to light, even UV light. I also know of a colleague who collects insects in my area using a mercury vapor light outside, and he never sees a mozzy near them, just heaps of moths and beetles. But at Loch Sport, the mozzy capital of the world (a great place, where their tourist tee shirts have a giant mozzy logo on the front), I have seen the large 'salt marsh mozzy' being mildly attracted to incandescent lights. So perhaps that is one species that would be attracted in small numbers to zappers. But again remember, light traps will only be useful for monitoring numbers and species, not for reducing the number of bites you get.
It is interesting to take a look at some of the Bunnings stores around town. Bunnings is a chain of giant hardware stores in Australia. The store at Notting Hill in Victoria had about two zappers on display, placed high up on a shelf out of reach of all but the most determined of customers. Obviously too embarrassed to actively market them. When I asked a salesperson about them, he said they don't actually attract mosquitoes, so their store pushes citronella repellents for mozzy control instead - well done! But then I went to the Bunnings store in Blackburn, where there was about 100 zappers on prominent display, with four out of their box and turned on for demonstration (perhaps they are human lures instead?). The main distributor's name on the boxes was 'The Kelly Company', which said that the device would kill mosquitoes and flies. Flies, don't they fly at day!!. I'm sure you have similar pedlars of death in your areas.
So what should you do versus mozzies? Use repellents not attractants (Oh Der!) such as citronella and personal roll-ons like DEET. Get rid of your zapper. There is nothing more annoying on a warm summers night when you want to relax, than to hear your neighbors constant zap of another lovely moth or beetle frying in death and sending its body parts and viruses into the atmosphere for all to breath. If you don't really mind killing moths and beetles en masse, and think the world would be a better place without them, then I suggest you see someone to get over your phobia. Try hypnosis or something!
Another method sent to me by a reader, is biological control using Bacillus thuringiensis, sold for example as Dipel or Thuricide. These mosquito dunks can be added to fish ponds or standing water, and will kill the larvae. It would also kill some other insects in the pond, such as beetle and perhaps dragonfly larvae, so may not suit all situations. It should be fine in bird baths, as B. thuringiensis has no or little affect upon vertebrates.
Some emails I have received say that bug zappers do work, and attach photos of the catch to verify. However, the insects I have seen are not mosquitoes, but non-biting midges, small crane flies and other diptera. Some look very much like mosquitoes, but lack the sucking beak that gives the female mosquito its notorious abilities. (Posted January 2004)
Links showing the uselessness of zappers
Links on how to avoid mozzies in other ways