Despite common belief, most insects are not pests. It's only those that feed on desirable plants or transmit diseases that cause problems for gardeners. The majority of insects are very useful. Some are important pollinators of fruits, flowers and vegetables, while others help control insect pests. Predatory insects eat large numbers of other insects. Some are predaceous as both adults and in their immature form, while others are predaceous only in their immature stage or only as adults. Many predatory insects feed on only certain types of insects. Lady beetles for example eat mostly aphids while others feed on a wide variety of insects. Other common predaceous insects include praying mantids, green and brown lacewings, ground beetles, minute pirate bugs, damsel bugs, syrphid fly larvae and snakeflies. Though they are not insects, spiders, predaceous spider mites and centipedes are also important predators in a garden ecosystem.
Insect pollinators include several bee and fly species. Honeybees, bumblebees, orchard mason bees and syrphid flies are the most important pollinators in our coastal gardens.
Parasitoids are insects that live in or on a host insect, feed on the host, and usually kill it in the process. Most parasitoids are small, stingless wasps or flies that lay their eggs in or on specific host insects. The eggs hatch, and the larvae feed within or on the hosts. Although these insects are not easily seen, they have an important impact in decreasing pest populations. Perhaps the most frequently seen parasitoid insect in our local gardens are the trachnid flies.
Although beneficial insects can play a role in controlling pests in your garden, don't expect them to keep your garden pest free. In general, releasing large numbers of beneficial insects has not proven to be an effective method of pest control in the home garden, especially in the case of adult lady beetles. These insects tend to move around and often end up migrating out of the garden.
It can be complicated to meet the needs of released insects well enough to keep them in your garden. All beneficial insects require a reliable food source, and some require a very specific food. Some beneficials also require specific temperatures and humidity conditions.
Invite beneficials to your yard by providing food. Pollinators are attracted by a wide variety of blooming plants. Many adult predators and parasitoids feed on nectar and pollen in addition to pest insects. Most are quite small and can reach the nectar and pollen of small flowers only. The plants that attract them are sometimes referred to as insectary plants. Some popular insectary plants in our area are zinnia, aster, cosmos, calendula, marigold, candy-tuft and yarrow.
By scattering insectary plants throughout your garden and landscape, you can attract beneficial insects. Or reserve a garden bed or border for them. Many non-flying predators such as ground beetles and spiders need a place to hide from their enemies. Groundcovers and coarse mulches such as bark dust and straw provide this habitat.
All beneficial insects are susceptible to insecticides. Most insecticides available to home gardens are broad-spectrum, meaning they kill a wide range of insects, including beneficials. If you decide to use an insecticide, try to choose the least toxic materials. Spot spray only infested plants. Don't spray plants in bloom and make spray applications early in the day when many insects are less active.