To some, the phrase "beneficial insect" may seem like an oxymoron but experienced gardeners know that there are a host of insects you want taking up residence in your garden. One of the most beneficial is the praying mantis, pictured above. A co-worker at Grand Lake Ace garden center took this photo yesterday and this guy had to be a good 4" long! Praying mantis will eat a whole variety of bad insects in the garden so are very valuable guardians to attract. Of course many people know that ladybugs eat aphids and other small insects, if you can entice them to stick around. But these two very common and recognizable 'good' predators are just the most familiar.
Other visitors you want to lay out the welcome mat for include dragonflies and damsel flies, which go after soft-bodied insects; assassin bugs (those commonly seen, elongated orange & black covered bugs) which feed on caterpillars & harmful beetles; the common black ground beetle, an equal opportunity predator; brown lacewings that eat aphids, mealybugs & scale insects and surprisingly yellow jacket and paper wasps, whose larva eat a variety of insects. And that's not mentioning spiders, a very beneficial predator to have around.
One thing I think is worth mentioning. No one likes ants in the house, especially large numbers raiding your honey jars and such but outside ants do no harm to your garden. They appear in numbers after rains or when you've just watered near a colony but in short order they return underground. There really is no need to poison them, especially since that leaves poisons in the soil. The only area where they can be a concern is if you have thrips or some such harmful chewing insect on say a citrus tree and the ants go to harvest the honeydew that certain insects create. These insects can sometimes hitch a ride on the ants and thus spread to other trees, thus infecting them. Generally though ants are just a benign insect that we can peacefully co-exist with.
So who are the bad guys? Beyond the aphids and mealybugs and caterpillars that everyone is familiar with, you'll want to somehow contain or rid yourselves of a few of these guys: the largish green stink bug, the aptly named orange & black harlequin bug, the destructive cucumber beetle, leaf miners and whiteflies, leafrollers, cabbage worms (that lead eventually to the pretty, white cabbage moths) and coddling moths, which go after fruit trees.
An excellent at-a-glance guide to both categories of insects is the Mac's Field Guide to Good & Bad Garden Bugs, which you can find in a single, laminated page at many nurseries.
Encouraging good predators in your garden will lead to a healthier garden and less work for you in combating the "forces of evil."