Attracting Beneficial Insects
By Ellen M. Silva, Virginia Cooperative Extension
Attracting and maintaining a population of beneficial insects is important to managing pests in your garden with a minimum of pesticide sprays. Tiny parasitoid wasps are aggressive beyond their size when it comes to pursuing aphids and caterpillars. Lacewing larvae and ladybug larvae and adults make inroads on aphid populations. Ground beetles prey on a variety of ground-dwelling pests.
These various beneficial insects consume large numbers of pest insects, but their diets are not limited to other insects. In fact, many of the beneficial species have periods in their life cycles when they survive only on nectar and pollen. Therefore, planting a variety of insectary plants will ensure an adequate supply of nutrients to keep beneficial insects going strong. Insectary plants also include those plants that provide shelter for beneficial insects, another critical requirement.
At one time, hedge rows that separated one field or garden from the next provided an ample supply of insectary plants to feed and shelter a variety of beneficial insects. The wide variety of plants in a hedge row, including small trees and shrubs as well as perennial and annual weeds, typically leaf out and bloom earlier than most crop plants, providing beneficial insects with an early food supply.
Most gardens today are too small for a hedge row. An alternative is to plant a border of dwarf fruit and flowering trees mixed with flowering shrubs and perennials. Such a border could be a landscape feature and screen the vegetable garden from view. At the same time, it would provide many of the benefits of the traditional hedge row.
Plan an insectary border for successive bloom from early spring through fall, providing nectar throughout the season. This will not only satisfy the needs of many beneficial insects, but also provide color in the garden. Avoid vigorous chemical control of pests found in the insectary border; after all, you don't want to kill beneficial insects. Also, any pests in the border may become hosts for beneficial insects should prey levels be low in the garden you are trying to protect.
Including plants of different heights can be very important. Ground beetles require the cover provided by low-growing plants. Lacewings lay their eggs in shady, protected areas, so providing such places near crop plants is a good idea.
Selective weeding can encourage beneficial insects by leaving potential food sources in the garden. Allowing certain volunteers to remain in the garden is somewhat like random companion planting. Just know what weeds or volunteers are helpful. Not all blooms are equal – large, nectar-filled blooms actually can drown tiny parasitoid wasps. Tiny flowers produced in large quantity are much more valuable than a single, large bloom.
Many members of the Apiaceae (formerly known as Umbelliferae) family are excellent insectary plants. Fennel, angelica, coriander, dill, and wild carrot all provide in great number the tiny flowers required by parasitoid wasps. Various clovers, yarrow, and rue also attract parasitoid and predatory insects. Low-growing plants, such as thyme, rosemary, or mint, provide shelter for ground beetles and other beneficial insects. Composite flowers (daisy and chamomile) and mints (spearmint, peppermint, or catnip) will attract predatory wasps, hover flies, and robber flies. The wasps will catch caterpillars and grubs to feed their young, while the predatory and parasitoid flies attack many kinds of insects, including leafhoppers and caterpillars.
Of course, you also may plant species that are not noted for harboring beneficial insects. There is no rule that says an insectary border must be limited to insectary plants.