Common Insect Pests

In this article, you will find practical methods for identifying, preventing, and treating the following: Aphid, Gall, Lacebug, Leaf Miner, Scale, Spider Mite, Tent Caterpillar, and Wood Borer. This information is a continuation of our article on Common Tree and Plant Diseases, and is directly sourced from The Homeowner’s Complete Tree & Shrub Handbook. The second section of the article describes practical earth-friendly strategies to manage insects, while keeping your trees and shrubs healthy.

Being able to identify various insects can be instrumental in helping you prevent stress and disease to your trees and shrubs. For that reason, we recommend keeping these two articles handy as a reference!

Aphid

Photo by Egor Kamelev on Pexels.com

Description: Aphids are sapsucking insects that come in many colors and textures from red, yellow, green, purple, brown, and black to whitish because of a light all-over secretion. Some have wings and some don’t, and many prefer congregating on the lower surface of leaves. What they have in common is their tiny size, pear-shaped bodies,
long legs, and long antennae. They tend to be group feeders on leaves and stems, but you’ll occasionally see a loner. Aphids probably won’t destroy your trees and shrubs, but sooty mold can turn the sticky honeydew they release black. You may also notice the foliage wilt, yellow, or distort.

Cause: Regular infestations.

Preventive Measures: You cant escape them: an aphid species is linked with almost every tree and shrub.

Treatment: Tree and shrub damage is usually more visual than life threatening. Live and let live, unless they really bother you or the infestation is severe. Hose foliage and stems with a powerful stream of water to displace them. If you want something more, spray your plant with insecticidal soap or horticultural oil. Many beneficial insects such as ants, lady beetles, and parasitic wasps will destroy aphids if you don’t destroy them first. Skip heavy-duty insecticides because they kill not just the bad guys but also the good.

Gall

Description: Galls are abnormal growths of plant tissue on leaves, twigs, and bark of trees. Oaks host most galls, but they also appear on other trees and shrubs.

Cause: Mites, wasps, aphids, and midge flies can all bring about galls. The insects that make galls are most active when trees leaf out in spring.

Preventive Measures: Keep trees healthy. Galls are not pretty but they don’t do permanent damage to your woody plants. You don’t know your trees have galls until
you see them, and spraying them won’t help because the gall shields the larvae.

Treatment: Hold off on chemical sprays. Sometimes tolerating a pest is the best thing to do. If the infestation is severe, you can prune out ugly twigs.

Lacebug

Description: Lacebugs get their name from their decorative wings and hood adorned with a lacy pattern of veins. These sapsucking pests about 1/8 inch long come in many species that affect a multitude of woody plants. They live on leaf bottoms, where they excrete spots of dark crud as they eat. Minor lacebug damage looks like yellowy dots on the upper leaf surface. Repeated severe infestations can kill a plant. Different species attack evergreen and deciduous plants, but damage is most common on evergreens.

Cause: Incorrect planting sites encourage some infestation of lace bugs.

Preventive Measures: Lace bugs thrive in sun. Set vulnerable plants such as azalea and andromeda in shade.

Treatment: In early spring. start checking under the leaves for eggs, newly hatched nymphs, and adults. Keep checking every couple of weeks, since several generations may hatch in a year. Dealing promptly with infestation prevents ugly damage from occurring. As with aphids, you can set upon lacebugs with a hose and give them a hard spray to displace and kill nymphs (immature insects) in spring. Encourage beneficial insects that prey on lace bugs by avoiding the use of chemical insecticides. Instead, drench leaves (especially the bottoms) with insecticidal soaps or horticultural oils to control the nymphs when they hatch.

Leaf Miner

Description: Leaf miners are similar to borers except that the latter go deeper into the plant. The larvae of leaf miners live inside leaves and their damage is visible as an irregular narrow whitish trail on the leaf surface. Most attacks are visible at the beginning and end of summer in mature foliage. Most woody plant families are susceptible to their raids.

Cause: Adult beetles and moths looking for suitable host for their larvae to develop.

Preventive Measures: Avoid stressing trees and shrubs with too much or too little water, and provide desirable conditions for their growth.

Treatment: Chemicals don’t work because the larvae are inside the plant. Anyway, leaf miners usually don’t cause permanent damage to most trees and shrubs.

Scale

Description: Scale insects feed on the sap of both evergreen and deciduous trees and
shrubs with mouth parts up to 8 times longer than their bodies. Scales hug their bodies tight to their food source and can be found on leaves, twigs, branches, and trunks. They can weaken and eventually kill the plant they infest, though that is not typical. Two kinds of scales exist, hard and soft. The latter makes honeydew while the former does not.

Cause: Scales occur on trees and shrubs that are stressed.

Preventive Measures: Maintain healthy trees and shrubs or drought. Avoid chemicals that harm lady beetles and that can survive an attack. Give your plants adequate
water and nutrition, especially when stressed by injury or drought. Avoid chemicals that harm lady beetles and parasitic wasps, their natural predators.

Treatment: Rub them off by hand or prune off severely infested branches. Because adults have a waxy coating that shields them from insecticides, you have to control them when they are overwintering or immature crawlers. Dormant oils work in early spring before trees and shrubs leaf out.

Spider Mite

Description: Mites are in the spider family. These teeny red, brown, or spotted sapsuckers damage leaf tissues. Fine webbing will appear on deciduous trees with large infestations. Leaves become spotty, yellow, and then brown before dropping. Common
hosts include spruces, arborvitaes, raspberries, roses, crabapples, and shrubby cinquefoils. To see if you have mites, tap a branch while holding a sheet of white paper
under it. If you see moving dots, you have spider mites.

Cause: Hot, dry, and dusty conditions grow their populations, while wet or humid weather lowers them.

Preventive Measures: Keep trees and shrubs healthy because spider mites thrive on stressed plants.

Treatment: While infestations are light, you can control them by spraying plants with hard jets of water. Do this whenever mite damage is apparent and repeat it weekly for at least 3 weeks. Use repeated applications of insecticidal soap or horticultural oil to further lower mite populations.

Tent Caterpillar

Description: You know them-those ugly, white, larvae-holding, silken webs or tents hung up in tree branches. The caterpillars eat the leaves of deciduous trees. They attack many kinds of trees but like rose, alder, birch, willow, ash, and apple more than most. Although they don’t kill trees, they can weaken them and make them vulnerable to other problems. Vigorous trees withstand an attack and releaf quickly.

Cause: Periodic infestations.

Preventive Measures: Keeping trees healthy helps them survive an infestation.

Treatment: Eliminate egg cases from trees. They are made of a foamy-looking gray to brown hard substance and are about 1% inches long. Remove the cases with pruners or
by hand. Get rid of hatched caterpillars by eliminating their nests from the limbs.

Wood Borer

Description: These insects, typically moths and beetles, grow under the bark of trees and shrubs, mining the inner bark in an immature, larval state. Most borers are drawn to dead, stressed, or dying trees. The beetles are dark brown, black, or red with tiny hard bodies. Many species attack conifers but the European elm bark beetle is a transmitting agent for the Dutch elm disease fungus that decimated the American elm (Ulmus americana) population in North America. Other examples of borers are longhorned beetles and carpenterworms, which become moths. Sawdust on the ground and sap mixed with sawdust oozing from little holes are signs that bark beetles have emerged from the trunk of conifers.

Cause: Trees stressed by drought, disease, and physical damage are more prone to wood borers than robust trees and shrubs.

Preventive measures: Keep shrubs and trees in tiptop health. Plant pest-resistant species and take good care of landscape shrubs and trees. Don’t over- or underwater
them, and make sure their growing conditions are conducive to maximum vigor. Baby new transplants.

Treatment: Cut off and get rid of infested branches to prevent the spread of the beetles. If the trunk shows lots of beetle damage, you may have to dispense with whole trees to save nearby trees. A certified arborist will know the right time to prune diseased branches from different types of trees.

Earth Friendly Remedies

Bordeaux mix can kill some disease-causing bacteria and fungi that affect both woody ornamentals and trees and shrubs grown for fruits or nuts. Bordeaux is good to use full strength as a long-lasting fungicide in fall, winter, and spring before a plant breaks dormancy; you can also use it at reduced strength after spring growth begins. You need proper safety gear (goggles and protective clothing) to apply this mixture, which you can buy prepackaged or make fresh. One gallon of Bordeaux mix requires blending 3 1/3 tablespoons of copper sulfate and 10 tablespoons of hydrated lime with 1 gallon of water. It should be used right after mixing. Bordeaux mix can control some mildews, fireblight, and apple scab.

Fixed copper fungicides, which are different copper compounds mixed with water, are safer to apply and safer to use on tender plants. You can also buy these ready made.

Insecticidal soaps kill vulnerable soft-bodied insects on contact. For efficacy, you have to ensure thorough coverage of affected plants, including stems and upper and lower leaf surfaces. The application may work better if you spray early in the morning or in the evening to prevent quick evaporation. These soaps are people-safe when you follow the package directions. As with any insecticide, use only when absolutely necessary.

Horticultural oils, which are made of refined petroleum or plant oils mixed with water, control many pesky insects on plants but tend to have limited effects on beneficial insects. Oils can kill aphids, mites, caterpillars, and scales on woody ornamentals as well as help prevent powdery mildew and rust. Dormant oil is horticultural oil used on trees and shrubs during their dormant season. Summer oil is safe to use on mature leaves during the growing season. Sometimes oils can injure sensitive species, such as juniper, hickory, black walnut, redbud, smoketree, some azaleas, spruce, Douglas fir, and Japanese cedar or cryptomeria, and Japanese, sugar, and red maples. Always follow package directions when spraying these products.

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