Most common are tiny wasps called trichogramma. They lay their eggs in moth and butterfly eggs that become plant-chewing caterpillars. These wasps are perfectly harmless to people and animals. Also available commercially are parasitic nematodes that target insects which spend part of their life below ground. Lawn grubs, Japanese beetle larvae and weevil larvae are examples of pests nematodes control.
Nematodes work best in loamy soil in mild climates.
They are least effective in extremely sandy or clay soils, in dry areas and at extreme temperatures. You'll find them sold in either in a paste form or on a sponge, under such names as Grub-Away and Grub Guard. Mix with water and spray on moistened soil or lawn. Insects, like humans and other animals, get diseases. Some disease-causing organisms are sold to gardeners to target specific pests.