Beneficial Insects in the Garden

A diverse population of insects is crucial for the health and productivity of vegetable and ornamental gardens.

You can help these beneficial insects find their way to your garden by planting their favorite native pollinator plants. These plants attract insects by producing nectar and pollen. Insects such as bees are responsible for pollinating many of the flowers which become our fruits and vegetables. Other beneficial species of insects like lady beetles eat insects that cause damage to our garden plants.

Bees and butterflies are important pollinators but many wasps, flies and beetles also provide this service. Many wasps, flies, beetles, assassin bugs, ambush bugs and lacewings benefit the garden by eating pests like aphids, stink bugs and Japanese beetles. Many people are fearful of insects and it is wise not to handle them. But even the bees and wasps which possess stingers and venom are docile while feeding at flowers. Only a small percentage of insects are considered to be pests.

The use of insecticides to kill insect pests will also kill beneficial insect species which are providing free and natural pest control. Awareness of the unintended toxicity of pesticides to other forms of wildlife should always be considered. Plants treated with systemic insecticides in the nursery can produce toxic pollen and nectar which may harm pollinating insects. When purchasing seeds and plants, inquire whether they are organic or pesticide-free stock.

The following beneficial insect species should be welcomed in all gardens for the important services which they provide.

Wasps

Wasps pollinate plants while feeding on nectar. They also prey on many insects and spiders which serve as food for the wasp larvae. The wasps which are most familiar to us are social wasps including the paper wasps, yellowjackets and hornets. These social species nest in colonies and may aggressively protect their nests if they are disturbed. However when feeding on nectar most wasps are docile if they are left alone.

In addition to these social species there are many wasps which are solitary nesters. The nest may be a burrow in the ground, a cavity formed in wood or a plant stem, or constructed with mud. In some species the female wasps paralyze an insect or spider with venom and then place the still living prey in their nest. Here the female lays her eggs on the immobilized prey which will serve as food for the wasp larvae after the eggs hatch. Most kinds of wasps have their own preferred prey, such as stink bugs, beetle larvae, cicadas, or spiders.

Female wasps in the Braconid family inject their eggs inside living caterpillars. The wasp larvae feed inside the growing caterpillar eventually chewing through the caterpillar's skin to spin their silken cocoons on its body. These tiny wasps are termed parasitoids, since they ultimately kill their host.

Flies

Flies pollinate plants while feeding on nectar and pollen. Some adult flies such as robber flies capture other insects in flight and many fly larvae feed on garden pests. Horse flies, deer flies and house flies have bad reputations due to their painful bites or unsanitary habits. However flies in the Syrphid family, known as hover or flower flies, are frequent garden visitors relying on pollen and nectar for food. The larvae of many Syrphid fly species are important predators of aphids.

Most members of the Tachinid fly family are parasitoids. The adult flies feed on nectar while the females deposit eggs or larvae on or inside their target insect host. The larvae feed upon and eventually kill the host. The species of insects among their victims include grasshoppers, gypsy moth caterpillars, Colorado potato beetle larvae, Japanese beetles, squash bugs and stink bugs.

An interesting note is that some plants take advantage of flies’ attraction to unpleasant odors by producing flowers that smell badly. Flies are important pollinators of such plants as our native carrion-flower Smilax herbacea and purple trillium Trillium erectum.

Beetles

The beetles form a very large and diverse insect order. Some species feed upon vegetation while others are predators of insects, slugs and worms. Adult soldier beetles are one of the families of beetles that pollinate flowers while feeding on pollen and nectar. Larval soldier beetles are nocturnal and develop under moist sheltering objects where they feed on insects and slugs or plant materials.

Some beetles can be serious pests in the garden. Adult Japanese beetles feed on vegetation and are very destructive. Larval Mexican bean beetles and Colorado potato beetles damage their namesake crops. However these are only a few of the many species of beetles and any insecticide applied to control them will also kill the other beneficial beetles, wasps, flies and assassin bugs that prey on these pests. Removing the pests by hand may be a solution in small gardens.

One of our favorite insects and most well-known of beetles are the lady beetles or ladybugs. In addition to the familiar red or orange with black spots, other species may be solid red, solid black, black with red spots, pink with black spots plus a variety of other color combinations. Adult lady beetles are great friends of the gardener by eating aphids, mealybugs, whiteflies and other small insect pests. Larval lady beetles resemble tiny alligators and are voracious predators of the same insect pests as the adults.

Ground beetles are a group of mainly nocturnal predators which feed on many insects and slugs in the garden at night. During the day they remain hidden in nearby vegetation, beneath rocks or in soil crevices. Their larvae are also beneficial by preying on pests at or below the soil surface. Some species may live for two to three years and require sheltered overwintering sites such as perennial plants or clumps of grasses.

Green Lacewings

Green lacewings are beneficial and interesting insects with attractive green wings for which the adults are named. Larval lacewings prey on many garden pests such as aphids, mites, mealybugs and scales. In fact the larvae are so voracious that they are known as aphid lions.

Adult green lacewings are nocturnal insects with some species feeding primarily on nectar and pollen while other species are predators of insects. Each female lacewing lays her eggs in small groups with each individual egg placed at the tip of a delicate stalk. This elevated placement helps to protect the eggs from their hungry siblings as they hatch.

Assassin Bugs

Assassin bugs are true to their name as both nymphs and adults possess a long, sharp beak-like mouth with which they impale their prey. Through this beak they inject enzymes into the insect victim, liquefying its body contents, which are then sucked back up through the beak. Included on their menu are garden pests such as aphids, leafhoppers, caterpillars and Japanese beetles.

One of the largest and most familiar assassin bugs is the wheel bug, named for the cogwheel shaped structure on the upper back of the adult's body. Observe but do not handle this fierce predator. Its beak can pierce the hard shell of a beetle and can give a painful bite to a human.

Written by York County Master Gardener Debra Carman, February 2017

References:

  • Pollinators of Native Plants by Heather Holm, Publisher: Pollination Press LLC; 1st edition (January 1, 2014)
  • Insects Their Natural History and Diversity by Stephen A. Marshall, Publisher: Firefly Books; Updated edition (June 2006)
  • Natural Enemies in Your Garden: A Homeowner’s Guide to Biological Control, Michigan State University Extension
  • Beetles of Eastern North America by Arthur V. Evans, Publisher: Princeton University Press (June 8, 2014)
  • http://www.scorpionfly.ca/plants/wildlifegarden/Beneficial%20wasps.html

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