What’s damaging these leaves? A guide to leaf pests by the damage they leave

Holes in leaves

Holes in the middle of a leaf often mean something chewed it, and the size and shape of the holes help narrow down what. Some diseases leave holes too, so look for the pest itself or other clues.

Tiny, round “shot holes,” about the size of a pinhead, are the mark of flea beetles, small shiny beetles that jump when disturbed. They mostly go after vegetable seedlings in spring (University of Maryland Extension). Larger, irregular holes, or leaves eaten back from the edge, are often caterpillars, though beetles, grasshoppers, and other chewing pests can leave similar damage. On shade trees, cankerworms start with small holes between the veins and can eat everything but the midrib (University of Minnesota Extension).

Ragged holes plus a silvery, slimy trail mean slugs or snails. They feed at night and on damp, cloudy days, and tender spring growth suffers most (NC State Extension).

Not every hole is an insect. Fungal and bacterial leaf spots can leave holes when the dead tissue falls out. On cherries, plums, and their relatives, a disease called shot hole works this way, starting as small spots (Purdue Landscape Report).

How serious is it? Usually minor on established trees and shrubs. Seedlings and young vegetable plants are the most vulnerable.

Neat circles cut from the edges

Smooth, nearly perfect half-circles or ovals, mostly half an inch or less across and always taken from the edge of the leaf, are the work of leafcutter bees. They carry the pieces off to line their nests. Redbud, rose, lilac, and serviceberry are favorites (Iowa State University Extension; University of Maryland Extension).

Rough, irregular notches chewed into the edges of azalea, rhododendron, or euonymus leaves are more likely adult black vine weevils, which hide by day and feed at night. The notching itself is minor; the weevil’s grubs, which feed on roots, do the real damage and can kill azaleas and rhododendrons (UC IPM).

How serious is it? Leafcutter bees are harmless to the plant and are valuable pollinators, so there’s no need to stop them. Weevil notching is worth a closer look at the roots of shrubs that are declining.

Skeletonized, lace-like leaves

When the soft tissue between the veins is eaten away and the veins are left behind, the leaf looks like lace. One common cause is the Japanese beetle. Adults feed in groups, often starting at the top of a plant in full sun, on more than 300 kinds of plants, including linden, elm, birch, crabapple, sassafras, roses, grapes, and Virginia creeper (Missouri Department of Conservation). Depending on the region, adults appear from June into early July and feed through the summer, and some stay active into September (University of Minnesota Extension).

Elm leaf beetle larvae skeletonize elm leaves from underneath, while the adults chew small round holes (Penn State Extension). On pear, cherry, and hawthorn, look for pearslugs: slimy, olive-black sawfly larvae that scrape the upper surface and turn leaves brown (Iowa State University Extension).

How serious is it? Unsightly, but healthy, established trees usually withstand heavy Japanese beetle feeding. Repeated years of heavy damage can weaken trees such as elms.

Winding trails and blotches inside the leaf

Pale, winding lines or blotches that you can see through are leaf mines. They’re made by leafminers, the larvae of certain moths, sawflies, flies, and beetles that are small enough to live and feed between the upper and lower surfaces of a leaf (Connecticut Agricultural Experiment Station). Most do little harm and need no control (University of Maryland Extension).

Holly leafminer mines start as narrow, winding trails and widen into yellow to reddish blotches (Washington State University). On gray and paper birch, birch leafminer blotches merge into brown blisters that can brown the top of the tree first. Healthy birches tolerate it, though repeated loss of leaves can weaken them and invite the bronze birch borer (Penn State Extension). On shrubs, heavier damage is more of a concern: boxwood leafminer’s puffy blisters can lead to leaf drop and dieback (UMass Extension), and badly mined azalea leaves turn yellow and drop (NC State Extension).

How serious is it? Most leafminer damage is cosmetic, but heavy infestations can cause leaf drop and weaken some shrubs, particularly boxwood and azalea.

Tiny pale specks, stippling, and a bronzed look

A leaf covered in tiny pale dots, as if it had been dusted, has been fed on by something sucking out cells one by one. Spider mites, too small to see easily, cause fine yellow or bronze speckling on the upper surface, sometimes with fine webbing. Hot, dry weather and drought-stressed plants favor them (Washington State University). Most do their damage in mid to late summer, but the spruce spider mite attacks conifers in the cool weather of spring and fall (UConn Extension).

Lace bugs cause white stippling on the top of the leaf, and the giveaway is underneath: tiny, shiny black spots that look like varnish. Each kind sticks mostly to one group of plants, such as azalea, sycamore, oak, or hawthorn (University of Maryland Extension; NC State Extension).

How serious is it? Heavy mite or lace bug feeding can make leaves look faded by late summer, but large trees generally don’t need treatment.

Brown edges and tips

Leaves that brown along their edges and tips, often with a yellow band between the brown and the green, can be hard to read, because insects and weather both cause it.

Potato leafhoppers, small wedge-shaped insects that blow north from the Gulf States each spring, cause “hopperburn”: leaves turn mottled with brown margins, and shoot tips can curl. Red maples are a common target, and leafhopper injury can even trigger early red fall color (University of Illinois Extension; University of Maryland Extension).

More often, brown edges are leaf scorch, which isn’t caused by a pest at all. Heat, drought, wind, or root problems leave the leaf short of water, and the edges and the tissue between veins brown first, usually starting on the sunny or windy side of the tree. Maples, beech, oaks, and lindens are prone to it (University of Missouri Extension).

A disease called bacterial leaf scorch can look almost the same, with brown edges sometimes ringed by a yellow or red band. It’s caused by a bacterium that clogs the tree’s water-conducting tissue and is spread by sap-feeding insects such as leafhoppers. Oaks, maples, elms, and sycamores are among its hosts. It can be confirmed only with a lab test, and it is often fatal over time (The Morton Arboretum).

How serious is it? Brown edges are often caused by heat, drought, or other conditions that keep leaves from getting enough water, and watering during dry spells can help. If the same tree browns earlier and worse year after year, ask an arborist or your extension office about testing for bacterial leaf scorch. Leafhopper injury tends to show on new shoots from late spring into summer.

Curled, puckered, or twisted leaves

Leaves that curl, pucker, or cup, especially new ones, often have aphids hiding inside. These small, soft insects suck sap, and some species curl the leaves they feed on. Most rarely harm plants, and lady beetles, lacewings, and tiny parasitic wasps keep many in check (Colorado State University Extension).

If you don’t find insects, consider other causes. Drift from certain weed killers can cup, pucker, or twist the new leaves of trees like maple and redbud (University of Illinois Extension), and a spring frost can leave freshly opened leaves distorted overnight, on many kinds of plants at once (University of Illinois Extension).

How serious is it? Usually temporary. Leaves that grow out later in the season come in normally.

Sticky leaves and black, sooty coating

A shiny, sticky film on leaves (and on cars and patios underneath) is honeydew, the sugary waste of sap-feeding insects such as aphids, scale insects, mealybugs, and leafhoppers. Honeydew draws ants, wasps, and bees (Colorado State University Extension). A black, sooty mold then grows on it. The mold doesn’t infect the leaf, but it blocks light the leaf needs (UMass Extension).

In much of the East and Midwest, heavy honeydew and sooty mold can be a sign of spotted lanternfly. This invasive insect from Asia was first found in Pennsylvania in 2014 and has since spread to more than a dozen states. It prefers tree-of-heaven but feeds on dozens of plants, including maples, black walnut, and grapes, and its honeydew can smell like vinegar as it ferments (USDA APHIS; University of Illinois Extension).

How serious is it? Sooty mold itself is mostly cosmetic. The insects making the honeydew are what matter, especially if spotted lanternfly is the source.

Bumps, warts, and balls on leaves

Galls are growths the plant makes in response to an insect or mite feeding or laying eggs. On silver and red maples, clusters of tiny round bumps that turn from green to red to black are maple bladder galls, caused by microscopic mites as the leaves open in spring (University of Wisconsin–Madison Extension).

On red, scarlet, and black oaks, round, green “oak apples” about two inches across come from gall wasps (NC State Extension). Small nipple-shaped bumps on hackberry leaves are made by tiny psyllids, whose adults come out in late fall and can squeeze through window screens (Purdue Extension).

How serious is it? Galls on leaves rarely harm a tree, and sprays don’t help once they’ve formed. Galls on twigs and branches can be more serious (University of Missouri Extension).

Webs, tents, and rolled leaves

Silk webs wrapped around the ends of branches in mid to late summer are fall webworms, which feed inside the web. Tents built in the forks of branches in early spring, on cherry, apple, and crabapple, are eastern tent caterpillars, which leave the tent to feed (University of Kentucky Forestry Extension).

Leaves rolled or tied together with silk usually hide leafrollers or leaf tiers feeding inside (UC IPM). On arborvitae, juniper, and other evergreens, small hanging bags made of plant bits are bagworms, which can kill evergreens when there are many. Picking off the bags before late spring, when the eggs inside hatch, helps on small plants (University of Maryland Extension).

How serious is it? Fall webworms look worse than they are, because the leaves they eat are about to drop anyway (Purdue Extension). Bagworms on evergreens deserve quicker attention.

When it’s not a pest

Plenty of leaf damage has nothing to do with insects. Raised, tar-black spots on maple leaves, especially Norway maples, are tar spot, a fungus. Pale yellow spots appear in early summer and turn black by midsummer. It’s cosmetic, and raking up fallen leaves cuts down on it the next year (University of Illinois Extension).

Brown blotches along the veins of sycamore, oak, and maple in a cool, wet spring are often anthracnose, another fungal disease that rarely threatens mature trees (University of Maryland Extension). Brown, crispy edges after a hot, dry spell are usually leaf scorch.

And a tree that turns color weeks before others of its kind nearby may be stressed by drought, heat, damaged roots, or decay (Purdue Landscape Report). For how normal fall color works, see why leaves change color.

How serious is it? Leaf spots are mostly cosmetic. Early color on one tree is worth watching, since it can point to a problem below ground.