Tree Pest Guide: How to Identify and Treat Common Tree Insects
There’s a moment every homeowner has where you walk out to the yard, look at a tree you planted years ago, and notice something is off. The leaves are sticky, or there’s webbing in the branches, or whole sections look chewed. Your first question is which bug is doing this and what to do about it.
Some pests need immediate action because they’ll kill the tree if you wait. Others look terrifying but are mostly cosmetic. The rest fall somewhere in between, where the right move depends on the species, the time of year, and how stressed your tree already is.
Here is how to figure out which pest you’re looking at and what your options are. If you already know the pest, skip to the species-specific guide at the bottom. If you don’t, work through the symptom triage first.
How to figure out which pest you have
Start with what the damage looks like. Most tree pests fall into one of three groups based on how they feed: sucking, chewing, or boring. The damage signature is different for each.
Sticky leaves, sooty black coating, ants running up the trunk. You have a sucking insect. They pierce plant tissue and drink sap. Their excrement (honeydew) coats the leaves below and grows a black sooty mold. Ants farm them for the honeydew. Suspect aphids, scale, whitefly, mealybugs, or adelgids depending on the species and the tree.
Holes chewed in leaves, skeletonized foliage, defoliated branches. You have a chewing insect. Caterpillars, beetles, sawfly larvae, and weevils all eat foliage. Damage ranges from cosmetic to total defoliation. Identify by looking at what’s left of the leaf and what’s on the ground beneath the tree.
Fine stippling on leaves, bronze or rust discoloration, faint webbing. Spider mites. Not technically insects (they’re arachnids), but they feed by piercing leaf cells like sucking insects do. Damage shows up as tiny white or yellow dots that look like the leaf surface has been sandblasted.
Holes in the trunk or branches, sawdust at the base, pitch oozing from the bark. You have a boring insect. Wood-boring beetles, bark beetles, and clearwing moth larvae tunnel inside the tree. These are the most dangerous category because by the time you see signs, the tunneling is well underway. Some species kill a tree within a season.
Tunnels visible inside the leaf itself (squiggly trails or blotches). Leafminers. Their damage is unmistakable once you see it. Treatment is different from other chewing pests because the larvae are inside the leaf tissue.
Nothing visible on the tree, but the lawn under it has dead patches. You might be looking at the larval (grub) stage of a beetle whose adults feed on the tree. Japanese beetles are the classic example. The adults skeletonize leaves; the grubs eat grass roots. Treating the grubs in the lawn reduces next year’s adult population.
Once you’ve narrowed the category, jump to the right section below for the deep-dive.
Sucking insects
These pierce leaves, stems, or branches and drink sap. Damage builds slowly. The first sign is usually honeydew on whatever is parked under the tree. Eventually you’ll see yellowing leaves, leaf drop, or sooty mold.
Aphids. Small soft-bodied insects in green, black, yellow, or pink depending on the species. They cluster on new growth and the underside of leaves. Maples, crepe myrtles, and citrus get hit hard. A strong blast from the hose knocks most of them off; persistent infestations call for insecticidal soap or neem oil. Full guide: How to Get Rid of Aphids on Trees.
Scale insects. Look like tiny bumps on bark or leaves. Soft scale is squishy when you press it; armored scale has a hard waxy cover that protects the insect underneath. Citrus, magnolia, camellia, maple, oak, and fig are favorites. Treatment depends on which life stage you catch. Full guide: Scale Insects on Trees.
Spider mites. Tiny enough that you’ll miss them unless you tap a branch over a white sheet of paper. Spruce, juniper, arborvitae, and indoor trees are common targets. Drought-stressed trees in summer get hit hardest. Full guide: Spider Mites on Trees.
Whiteflies. Tiny white moth-like insects that puff up in a cloud when you brush a leaf. Citrus, fig, and avocado are the main outdoor hosts; indoor and greenhouse trees get hit too. Yellow sticky traps help confirm an infestation. Full guide: Whiteflies on Trees.
Adelgids. Closely related to aphids but specific to conifers. Hemlock woolly adelgid is reshaping eastern forests right now. White woolly tufts on the underside of hemlock branches are the diagnostic sign. Full guide: Adelgids on Trees.
Chewing insects
These eat leaves, buds, or stems. Damage is visible: missing tissue, ragged edges, frass (caterpillar droppings) on the ground.
Tree caterpillars. Tent caterpillars build silken webs in tree forks in spring. Fall webworms build nests at branch tips in late summer. Bagworms drag cone-shaped silk bags around evergreens and can defoliate an arborvitae completely. Bacillus thuringiensis (BT) is the canonical organic treatment. Full guide: Tree Caterpillars.
Japanese beetles. Shiny copper-and-green half-inch beetles that show up in early summer and skeletonize leaves on linden, Japanese maple, birch, crabapple, cherry, and roses. The grub stage feeds on grass roots in your lawn. Beetle traps make the problem worse by attracting beetles from neighboring properties. Full guide: Japanese Beetles on Trees.
Sawflies. Larvae look like caterpillars but they’re not. They cluster in groups on conifer needles or rose foliage and strip a branch bare fast. The most important thing to know about sawflies: Bacillus thuringiensis (BT) does not work on them because they aren’t lepidoptera. Use insecticidal soap or a water blast instead. Full guide: Tree Sawflies.
Leafminers. Larvae feed inside the leaf itself, creating squiggly trails or blotchy mines. Birch, holly, boxwood, and oak each have their own species. Contact sprays don’t work because the larvae are protected by the leaf surface; treatment is systemic or biological. Full guide: Leafminers in Trees.
Boring insects and conifer specialists
These are the most dangerous category. By the time you see exit holes or pitch tubes, the tunneling inside the tree is well underway. Some species kill within a single season.
Wood-boring beetles and clearwing moths. Bronze birch borer, emerald ash borer, Asian longhorned beetle, peach tree borer, and dogwood borer all tunnel through trunks or large branches. Different beetles target different species; the diagnostic signs are exit holes, sawdust (frass) on the ground, and dieback in the upper canopy. Full guide: Tree Boring Insects.
Pine bark beetles. Mountain pine beetle, southern pine beetle, and Ips engraver beetles attack stressed pines and can wipe out a tree in one growing season. Pitch tubes on the bark and J-shaped galleries under the bark are the diagnostic signs. Once a pine is infested, removal is usually the answer. Full guide: Pine Beetles.
When to call an arborist vs handle it yourself
Some pest problems are weekend warrior territory. Others need a licensed arborist with prescription-strength chemicals and climbing gear.
You can handle:
- Aphid infestations on small to medium trees (hose blast + insecticidal soap)
- Scale on accessible branches that can be pruned out
- Spider mites caught early (hose blast + horticultural oil)
- Tent caterpillars and webworms reachable from the ground (BT spray, pruning out nests)
- Mealybugs and whiteflies on ornamentals (soap spray, sticky traps)
- Most aphids, mites, and soft-bodied insects on container plants
Call an arborist:
- Anything on a mature tree taller than you can reach with a pole pruner
- Confirmed or suspected emerald ash borer, Asian longhorned beetle, or mountain pine beetle
- Hemlock woolly adelgid on a tree you want to save
- Any borer activity in the trunk of a tree worth saving
- Scale or mite infestations heavy enough to require trunk injection of systemics
- Pest damage on trees overhanging structures or in tight property lines
A consultation usually runs $75 to $200 and is worth it before you spend several times that on treatment.
Prevention through tree health
Healthy trees fight off pests on their own. Stressed trees are pest magnets. Most of the “what should I spray” questions homeowners ask have a simpler answer: fix the underlying stress and the pest problem either resolves or never starts.
Water deeply, not often. Drought stress is the number one reason spider mites, bark beetles, and aphids explode on landscape trees. Deep watering at the dripline once a week in summer keeps trees stress-resistant. See our guide on watering newly planted trees for the schedule that works.
Fertilize only when soil tests say so. Heavy nitrogen pushes soft new growth that attracts aphids and scale. A soil test kit tells you what your tree needs. Skip the spring fertilizer ritual if the soil is already balanced.
Mulch correctly. Two to three inches of mulch in a ring out to the dripline reduces drought stress and protects roots from mower damage. Don’t pile mulch against the trunk. Volcano mulching invites trunk-boring insects.
Prune dead and weak wood promptly. Dead branches attract bark beetles and borers. Get them out as soon as you spot them.
Don’t broadcast-spray for “prevention.” Calendar-based pesticide applications kill beneficial insects that would otherwise control pests for free. Predatory wasps, ladybugs, lacewings, and predatory mites do most of the pest control on a healthy property if you let them. Spray only what you can see and only when it threatens the tree.
Treat the disease side too. Pests and diseases often travel together. Aphid honeydew grows sooty mold. Borer entry holes open the bark to fungal infection. Our companion tree fungus identification guide covers the disease half of tree health.
FAQ
What’s the most common tree pest in residential yards?
Aphids. They show up on almost every species at some point and most homeowners notice them first through the sticky honeydew coating cars, decks, or sidewalks parked under the tree. Most aphid problems clear up on their own once predator populations catch up. Active treatment is only necessary when the infestation persists for more than a few weeks or the tree is young and stressed.
Are beetle traps a good idea?
For Japanese beetles, no. The pheromones in the trap attract beetles from up to a mile away, and only a fraction of them end up in the trap. The rest find your trees instead. For codling moth in apples and other species-specific monitoring traps, they’re useful for detection but not control. Skip the trap and address the source.
Will neem oil kill everything?
Neem oil works on most soft-bodied sucking insects (aphids, mites, whiteflies, mealybugs, soft scale) and some egg masses. It does not work well on adult armored scale, sawfly larvae, or wood-boring insects. It’s also not selective; it’ll kill beneficials if applied directly. Spray in early morning or late evening when pollinators aren’t active, and only on the plant tissue that’s infested.
My tree has white fuzzy stuff on the branches. What is it?
Three possibilities. Cottony cushion scale on citrus and ornamentals looks like white waxy cushions stuck to the bark. Woolly aphids on apple, elm, or alder leave white woolly tufts on twigs. Hemlock woolly adelgid produces white woolly material on the underside of hemlock needles. Identify the host tree first; the same white fuzz means different pests on different species.
Can pesticides hurt the tree itself?
Most homeowner products at label rates won’t hurt an established tree. Concentrate, oil, and systemic products applied incorrectly can damage foliage, kill beneficial soil life, or accumulate in fruit. Read the label. Apply at the lowest effective rate. If you’re unsure, an arborist consultation is cheaper than killing a tree by mistake.
References: UC IPM Pest Notes (ipm.ucanr.edu) for California-specific pest biology; Penn State Extension for eastern US species; USDA APHIS for regulated pests including emerald ash borer and hemlock woolly adelgid quarantines.