Lady beetle larva feeding on oleander aphids on milkweed. Extension Photo: Ken Johnson.

CHAMPAIGN, Ill. -  While it may seem like every insect out there is trying to eat your plants, not all the insects in the garden are pests. Fewer than 1% of all insects are considered pests. The vast majority are beneficial or benign.

"While most people are aware of the benefits of bees and other pollinators, a lot of other types of insects are beneficial in our gardens by helping to control pest insect populations," says Ken Johnson, University of Illinois Extension horticulture educator.

Gardeners who find a mystery insect in their garden should identify it before trying to kill it. That insect may end up killing the "bad" bugs.

"If you have beneficial insects, you may not need to do anything else to manage your pests," Johnson says. 

Beneficial insects all into two different groups: predators and parasitoids.

Predatory insects

Predatory insects, such as lady beetles, praying mantids, assassin bugs, lacewings, wasps, and syrphid flies, feed on many different types of garden pests. 

Lady beetles are one of the most widely known predatory insect. Both the adults and larvae prey on small, soft-bodied insects, such as aphids. The adult dome-shaped beetles range in color from yellow, pink, red, orange, or black, usually with distinct spots. The larvae are dark-colored with orange or yellow markings and flattened and tapered, kind of like an alligator. The larvae they are commonly misidentified as a pest.

Ground beetles can also be important predators in gardens. Most are shiny and black and are typically found on the ground under leaves, logs, stones, loose bark, and grassy areas. They are most active at night and will feed on a variety of insects such as caterpillars, beetle larvae, snails, and other soft-bodied insects. Larvae also feed on soil-dwelling insects.

Praying mantids are another predatory insect. These large insects will eat whatever they can catch, whether it’s a pest or another beneficial insect, such as bees. There are several species of preying mantids native to Illinois, but the most commonly encountered is the non-native Chinese mantis. 

Assassin bugs and other predatory bugs use their straw-like mouthparts to impale their prey and then suck out the body fluids. These insects come in various sizes and colors, but one of the best known is the wheel bug.

"Even though we typically think of them as pests, some species of stink bugs are predatory," Johnson says. "These predatory stink bugs have thicker mouthparts compared to their plant-feeding cousins and will feed primarily on caterpillar and beetle larvae."

Other predators: 

  • Green lacewings can be important predators of aphids and other small, soft-bodied insects. The larvae are alligator-like and have large sickle-shaped mouthparts. They are referred to as "aphid lions."  
  • Syrphid flies range in size from ¼ to ¾ of an inch long and resemble bees and wasps with yellow, orange, and black markings. The predaceous larvae are legless with a tapered body and range in color from creamy-white to green or brown. They feed on aphids and other small soft-bodied insects.
  • Wasps such as yellowjackets, baldfaced hornets, and paper wasps can have a significant impact on the number of pests in a garden. The adults hunt insects such as caterpillars, flies, crickets, and grasshoppers to feed their young.
  • Spiders can also have a significant impact on pest species in gardens. Not all spiders We catch prey in webs like orb weavers. Many, sucyh as wolf spiders, crab spiders, and jumping spiders hunt or ambush their prey.

Parasitoids insects

Parasitoids insects tend to be much more specialized in the insects they feed on.

"They may only attack one type of insect, such as caterpillars, or in some cases, one species," Johnson says. "The adults will look for hosts to lay their eggs on or in. Once the egg hatches, it will feed on its host, eventually killing it."

A parasitoid wasp laying an egg inside an aphid. Extension photo: Ken Johnson.

Most parasitoid wasps are small and easily confused with gnats. These small wasps will attack aphids, whiteflies, mealybugs, caterpillars, and insect eggs. 

Insects that have been parasitized often look different. Parasitized aphids will eventually turn brown or black and paper-like. In some caterpillars, such as hornworms, the larvae will spin white cocoons on the outside of its host, which are commonly confused for eggs.

Some species of flies are also parasitoids. Adult tachinid flies will lay eggs on or in their hosts, such as caterpillars and beetles. The larvae of these flies will tunnel into their host to feed, eventually killing it.

Gardeners can take steps to encourage good insects. While many natural enemy insects can be purchased, making some changes to how you manage your landscape can make it much more attractive to these insects and help keep them around long term.

"Cut back on or cut out insecticides," Johnson says. "Most insecticides will kill beneficial insects as well as pests."

If you are going to use insecticides, use a selective pesticide that is toxic to specific insects and won’t directly harm beneficial insects. For example, Bacillus thuringiensis kurstaki (Btk) only affects caterpillars. When using pesticides, make sure to read and follow all label directions.

Provide flowers for beneficial insects to feed on. Many beneficial insects will feed on nectar and pollen. This is especially important if the adults aren’t predators. Plants with small flowers such as sweet alyssum, dill, fennel, garlic chives, coriander, and Queen Anne’s Lace are good choices. Other common garden plants such as blanket flower, coneflower, cosmos, and sunflowers will be visited by beneficial insects, too.

SOURCE: Ken Johnson, Horticulture Educator, University of Illinois Extension

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