Helpful bugs vs. harmful bugs

Ron Johnson (left) and Candice Miller, both Master Gardeners with the University of 
Illinois Extension, inspect the kitchen garden at the Sycamore History Museum for 
insects and pests. Some insects are bad news for gardeners, but others are beneficial 
and should be encouraged.
Ron Johnson (left) and Candice Miller, both Master Gardeners with the University of Illinois Extension, inspect the kitchen garden at the Sycamore History Museum for insects and pests. Some insects are bad news for gardeners, but others are beneficial and should be encouraged.

Not all bugs are bad bugs.

Some bugs, such as spiders, are natural predators that eat other insects and pests. These “good” bugs serve as a biological control, controlling pests that can hurt people or damage the landscape.

Some of the most pervasive pests are invasive species that have no natural local biological controls. B. J. Miller, a technician at the Kishwaukee College greenhouse and gardens, said Japanese beetles were a large problem last year.

“Pests develop the same time as the plants, and that time is now,” he said. “Japanese beetles eat anything. They particularly love roses and grape vines. ...But with the harsh winter and the depth of the frost, there should be less Japanese beetles this year.”

Another insect that is on the rise is the emerald ash borer. According to the Illinois Department of Agriculture’s website, the emerald ash borer has killed tens of millions of ash trees. Since there is no known treatment to control the insect, trees that are found to be infested must be removed.

“Emerald ash borers have become a big problem in Illinois,” U of I Master Gardener Ron Johnson said. “On a bicycle ride the other day, I saw 30 to 50 trees infested. It takes several thousand dollars to take a mature tree down, which is a problem for newer communities, since over 70 percent of their tree population is made up of ash trees.”

Candice Miller, a Master Gardener with the University of Illinois Extension, said there are four ways to control insects in the garden: cultural control like weeding and mulching, mechanical control like hand-picking bugs or using row covers for prevention, biological control such as spiders and other predators, and chemical control.

“The earlier you catch the insects, the easier it is to control them,” she said.

A few friends


What to look for: Webs, but not all spiders create webs

Benefit to the landscape: As predators, spiders feed on their prey. Some eat insects caught in webs. Others, such as jumping spiders and wolf spiders, hunt relying on their excellent vision, and crab spiders ambush their prey.

Asian ladybird beetles (a.k.a. ladybugs)

What to look for: Adults and larvae on plant leaves

Benefit to the landscape: Both the larvae and adult ladybird beetles are predators that can eat hundreds of aphids in their lifetime. They also eat insect eggs, mealybugs, and other soft-bodied insects and mites. Some species of ladybird beetles have favorite prey, as indicated by the names given to the “mealybug destroyer” and the “spider mite destroyer.” Flowering, pollen-producing plants in the landscape attract ladybird beetles.

Fireflies (a.k.a. lightning bugs)

What to look for: Fireflies' flashing lights are an integral part of their reproduction process and are used by adults for courtship.

Benefit to the landscape: Larvae of most species are specialized predators and feed on other insect larvae, earthworms, snails, and slugs. Adults of some species do not feed, but some are also predatory.

...and some foes


Damage: Insert their mouth part into leaves and remove sap from plants

What to look for: Most will be underneath leaves or at the growing point of the plant and they create residue known as "honeydew" – a sticky, shiny substance below where they are feeding.

Prevention: Maintain vigorous plant growth, remove weeds, monitor nitrogen (high nitrogen makes plants more susceptible)

Treatment: The best is to encourage natural predators like ladybird beetles. Chemical options are available.

Japanese beetles

Damage: Chew “lacework” holes into leaves

What to look for: Chewed leaves indicate the presence of adults; grubs may be located at the base of plants and in the soil

Prevention: Weeding and maintaining around crops where grubs may be present, reduce watering of lawns

Treatment: Removing them by hand and dropping into soapy water is recommended; chemical options are available.

Emerald ash borers

Damage: Larvae damage the vascular system underneath the bark of ash trees, cutting off water movement through the tree.

What to look for: 1/8-inch D-shaped emergence holes in the trunks of ash trees. Adults typically emerge in May and June. The first symptoms of infestation in a tree will be parts of the canopy dying. Unusual shoots of new growth at the base of the tree or up in the canopy may be noticable.

Prevention: Preventative chemical treatments can be used once an infestation is confirmed within 15 miles of your tree. Trees can be difficult to treat once infested. White ash trees seem to resist damage longer than green ash trees.

Treatment: Chemical treatments are available.

Striped and spotted cucumber beetles

Damage: Can transmit bacterial wilt as they feed on foliage and stems

What to look for: Large populations on the leaves, inside flowers, and at the plant's base

Prevention: Yellow sticky traps can trap some adults. Rotating crops and using floating row covers pre-bloom can prevent the pest from getting to the plant.

Treatment: Yellow sticky traps and hand picking are recommended. Chemical options are available.

Buffalo gnats (a.k.a. black flies)

Damage: They bite exposed skin, typically leaving a small, red welt. When the gnats are numerous, the toxins from their bites can kill poultry and other birds. They also feed on the thinly-haired areas of dogs, cats, and horses, such as ears and undersides.

What to look for: Small, 1/16- to 1/8-inch-long, humpbacked black flies

Prevention: Wear head nets or hats with insect protective netting that covers the head down to the shoulders. Unlike mosquitoes, buffalo gnats do not bite through clothing; only exposed skin is susceptible to attack. They also do not enter buildings.

Treatment: DEET repellents may provide the most effective protection.


Damage: Bagworm insects feed on a wide range of plants, with arborvitae and junipers being the most commonly attacked evergreens. Feeding damage can be severe enough to kill plants.

What to look for: Small grayish-black worms just emerged from last year’s bags and have begun to feed on leaves. They are so small they usually go unnoticed until damage becomes significant. Young larvae migrate to the tops of trees and shrubs; look in these areas for early infestations. Most bagworms are actively feeding in July.

Prevention: Hand pick the bags.

Treatment: Apply bacillus thuringensis kurstaki while the bagworm is still larval. The treatment is available at stores that sell insecticides.

Viburnum leaf beetles

Damage: Heavy infestations can defoliate viburnum shrubs, cause dieback, and eventually kill plants.

What to look for: Larvae are greenish-yellow and develop dark spots as they age. They are usually found feeding in groups. Between early and mid-June, larvae drop to the ground and pupate. They remain in the ground for about 10 days. Adults generally emerge in mid- to late July. Adults are 1/4 to 3/8 of an inch long and a golden brown color with a sheen in sunlight. Adults will remain active until the first frost. 

Prevention: Plant resistant varieties, including Koreanspice viburnum, Burkwood viburnum, doublefile viburnum, Judd viburnum, lanatanaphyllum viburnum, and leatherleaf viburnum.

Treatment: If plants are experiencing defoliation, the best option is to use pesticides. Apply directly to the larvae, since spraying adults or eggs is less effective.

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