Parasitic wasps are very effective predators, though each species typically preys on one particular kind of pest. Typically, adult parasitic wasps find host insects and either lay eggs into or onto their prey. Larvae can develop singly or - depending on the species - number in the thousands, killing the target host. Note that these wasps are tiny - often not visible to the naked eye - and not aggressive, though some may sting if being crushed by an unwary gardener.
Parasitic wasps range in size from under a half a millimetre (0.02 inch) to about 2 centimetres (3/4 inch), depending on the species. They can be extremely effective: braconid wasps against aphids, caterpillars, sawflies and greenflies, Encarsia formosa against whiteflies, etc. There are parasitic wasps for most insects - including those preying on beneficial insects, including other benificial parasites.
Some parasitic wasps are available commercially. Many can be attracted by growing small-flowered plants nearby, as adult wasps require food - nectar and pollen - as well as moisture. Alyssum, chamomile, yarrows and tansy, dill and fennel, lavender, lemon balm, parsley, statice and wild carrot especially are known to attract beneficial wasps to the garden.
Moisture best comes from shallow standing water, or from moisture on leaves and stems, i.e. from rain, dew, or sprinkler-type irrigation. Still, rain and dew are unpredictable; dew burns off quickly on a summer day; sprinkler irrigation is inefficient. Saucers of water or shallow puddles or ponds, with partially-submerged pebbles for take-off and landing, work well. However, such can also be breeding grounds for mosquitoes. Experiment to determine what works well for you.
Parasitic wasps also require protection from sun, rain, and other predators. Strive for a diverse local ecosystem with a good variety of plants to provide the shelter they require.
A few of the commercially available wasps:
Note that parasitic wasps can be very sensitive to insecticides, though they typically complement other parasitic insects quite well, i.e. ladybugs for aphids.
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