What's Eating Your Lilac?

Syringa vulgaris

Reviewed by Kiersten Rankel M.S.

Quick Answer

For lilac, the most likely culprits are lilac borer (sawdust at the base of trunks and wilting branches in summer) and oystershell scale (gray mussel-shaped bumps on woody bark). Aphids cluster on new growth and developing flower panicles in spring. Spider mites flare in hot dry summers and stipple the heart-shaped leaves.

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What does the damage look like?

Tap the closest match to jump straight to the fix.

Sawdust at the base of trunks

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Gray mussel-shaped bumps on bark

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Sticky clusters on flower buds

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Pale specks on heart-shaped leaves

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Pests, ranked by impact

White spittlebug foam mass on a plant stem

Lilac borer

Damage

High

Removal

Hard

What it looks like

The larva of a clearwing moth (Podosesia syringae) that mimics a wasp in flight. Cream-colored grubs up to 1 inch long tunnel inside the lower trunks and main stems. Adults emerge and lay eggs on rough bark in May. The larvae are hidden inside the wood and rarely seen until damage shows.

What the damage looks like

Coarse sawdust-like frass piled at the base of trunks or stuck in bark cracks. Round exit holes about 1/4 inch wide on lower trunks. Whole branches wilt suddenly in summer while the rest of the shrub looks fine. Old established lilacs can lose entire trunks and collapse from the base over a few seasons.

How to get rid of them

Option 1

Prune out wilting branches back to clean wood

Trace each wilting branch down until you find the entry hole or sawdust.

Cut 6 to 12 inches below the lowest sign of damage, into clean white wood with no tunnels visible in the cross section.

Bag and trash the prunings the same day. Do not compost or stack as firewood near the lilac. The larvae finish their life cycle in cut wood and emerge as adults next May.

Option 2

Spray trunks with permethrin in early May

Time a permethrin trunk spray (Bonide Eight or Hi-Yield 38 Plus, ~$15 to $20) for early May when adult clearwing moths emerge and lay eggs on rough bark. Coat the lower 4 feet of every main trunk and the base of each sucker shoot. Repeat once 3 weeks later. The spray kills eggs and newly hatched larvae before they bore into the wood, where no spray can reach them.

Option 3

Keep older trunks vigorous with renewal pruning

Borers target stressed and aging trunks with thick rough bark. Each winter, cut out 1 or 2 of the oldest trunks at ground level and let young sucker shoots from the base take over. A lilac on a 5 to 7 year renewal cycle keeps young smooth bark on most of its trunks, which the moths struggle to lay eggs on.

Brown soft scale (Coccus hesperidum) clustered on a plant stem

Scale insects

Damage

High

Removal

Hard

What it looks like

Oystershell scale (Lepidosaphes ulmi) is the iconic armored scale of lilac. Look for gray to brown mussel-shaped or oyster-shaped bumps, 2 to 3 mm long, packed tightly along the woody bark of trunks and main branches. Don't move because they're glued in place. Heavy clusters can coat the bark in a crust.

What the damage looks like

Bark looks rough and crusty, sometimes silvery gray from packed scale shells. Branches above heavy clusters fail to leaf out in spring or push small yellowed leaves and short flower panicles. Whole sections of older trunks decline and die back over 2 to 3 seasons. New sucker growth from the base usually starts clean but gets colonized as it matures.

How to get rid of them

Option 1

Dormant oil spray on bare bark in late winter

Wait for a still day in February or March when temperatures are above 40F and the lilac is fully dormant with no green buds open.

Spray dormant horticultural oil (Bonide All Seasons, ~$15) until every inch of trunk and branch bark runs wet. The oil suffocates overwintering scale under the shells.

Inspect in May. If gray bumps are still firmly attached, scrape the worst patches off with a stiff brush and respray oil at the lighter summer rate.

Option 2

Time a crawler spray for late May to mid-June

Oystershell scale crawlers (the mobile young stage) hatch in late May to mid-June in most climates. Spray insecticidal soap or summer-rate horticultural oil on the bark and any fresh growth twice, 10 days apart, during this window. Crawlers have no waxy shell yet and are easy to kill. Once they settle and form shells, sprays bounce off them.

Option 3

Cut out the worst-encrusted old trunks

If a main trunk is silver-gray with scale and barely leafing out, cut it to the ground in winter. The plant will push fresh sucker shoots from the base in spring, and they start clean. Removing the worst reservoir of breeding adults drops pressure on the rest of the shrub for years.

Aphids

Damage

Medium

Removal

Easy

What it looks like

Tiny pear-shaped insects 1 to 3 mm long, in shades of green, brown, or black. Cluster densely on the soft new growth at branch tips and on developing flower panicles in spring, just before bloom. Spring is peak aphid season as the lilac pushes its bloom.

What the damage looks like

Soft new leaves curl and twist as aphids drain sap from the underside. Developing flower clusters look wilted or distorted before they open. A sticky shiny film coats leaves and falls onto whatever is below the shrub. Black sooty mold grows on the residue over a few weeks. A vigorous lilac shrugs off the damage and blooms anyway, but heavy spring infestations can shorten the bloom display.

How to get rid of them

Option 1

Strong water blast every 2 to 3 days for 2 weeks

Hold a hose nozzle 12 inches from the affected new growth and spray at high pressure. Most aphids dislodge and don't make it back to the lilac. Repeat every 2 to 3 days for 2 weeks. Cheap, fast, and won't harm the bees and pollinators visiting the flower panicles.

Option 2

Insecticidal soap on new growth at dusk

Wait until evening when bees and pollinators have left the open panicles. Soap kills any insect it touches while wet.

Spray ready-to-use insecticidal soap (Safer or Garden Safe, ~$10) on the underside of curled leaves and into branch tips where aphids cluster.

Repeat every 5 days until the spring flush hardens off. The new growth becomes too tough for aphids by early summer.

Option 3

Leave the ladybugs alone

Ladybug larvae (the spiky black-and-orange ones, not the round adults) eat hundreds of aphids each. They show up on lilac within days of an aphid bloom. Don't spray broad-spectrum insecticides during the aphid window. The natural predators usually crash the aphid population before the lilac bloom finishes.

Spider mites

Damage

Medium

Removal

Moderate

What it looks like

Almost invisible without a hand lens. Yellow-green to red-orange specks running along the underside of the heart-shaped leaves, especially along the central vein. Hot dry summers and drought-stressed lilacs trigger population booms. A well-watered lilac in a normal summer rarely sees enough mites to do real damage.

What the damage looks like

Tiny pale yellow dots speckled across the upper leaf surface, with the heaviest damage on leaves nearest the trunk where airflow is poorest. Whole leaves bronze and crisp by late summer. Fine webbing strung between leaves and along the leaf-petiole junction in heavy infestations. Damage is cosmetic for the year but signals the shrub is drought-stressed, which also makes it more attractive to lilac borer.

How to get rid of them

Option 1

Water deeply at the base every 7 to 10 days

A drought-stressed lilac is a spider mite magnet. Soak the root zone with a slow hose for 30 minutes every 7 to 10 days through any hot dry stretch. Mulch 2 to 3 inches deep around the base, kept a few inches off the trunks. A well-hydrated lilac fights mites off on its own.

Option 2

Hose the foliage down once a week in summer

Spray water onto the underside of the leaves with a strong hose blast for 30 seconds. Mites can't reattach quickly when knocked off, and the rinse humidity slows survivors. Repeat weekly through any heat wave. A free fix that targets the worst of the population without sprays.

Option 3

Neem oil at dusk, every 5 days for 3 rounds

Mix 2 tablespoons cold-pressed neem oil and 1 teaspoon dish soap per gallon of water.

Spray the underside of every affected leaf at dusk to avoid pollinators and leaf scorch.

Repeat every 5 days for 3 rounds. Covers the full egg-to-adult life cycle of the mites.

Stay ahead of all of them

Four habits that keep lilac pests rare and protect the spring bloom display.

Trunk and bark check, every spring and fall

Walk the base of every trunk twice a year looking for sawdust frass, exit holes, and gray mussel-shaped scale. Lilac borer and oystershell scale both attack the woody bark and both reward early detection. A 2-minute check in March and October catches problems while they're still local to one trunk.

Renewal-prune the oldest trunks each winter

Cut 1 or 2 of the thickest oldest trunks to the ground every winter. The plant pushes fresh sucker shoots from the base, which keeps the bark mix young and unattractive to lilac borer and scale. A lilac on a 5 to 7 year renewal cycle stays vigorous and pest-free for decades.

Water deeply through summer drought

A drought-stressed lilac is the prime target for both spider mites and lilac borer. Soak the root zone for 30 minutes every 7 to 10 days through any hot dry stretch, especially in the first 3 years after planting. A vigorous shrub fights pests off on its own.

Time the May moth window

Adult lilac borer moths emerge and lay eggs in May. A single permethrin trunk spray in early May catches the eggs before they hatch into the wood. This is the only pest treatment for lilac that depends on hitting an exact week, so put it on the calendar with the spring bloom.

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About This Article

Kiersten Rankel M.S.

Botanical Data Lead at Greg · Plant Scientist

About the Author

Kiersten Rankel holds an M.S. in Ecology & Evolutionary Biology from Tulane University. A certified Louisiana Master Naturalist, she has over a decade of experience in science communication, with research spanning corals, cypress trees, marsh grasses, and more. At Greg, she curates species data and verifies care recommendations against botanical research.

See Kiersten Rankel's full background on LinkedIn.

Editorial Process

Pest identification and treatment guidance verified against Syringa vulgaris field reports from Greg's botanical database, cross-referenced with university extension sources and published horticultural research.

1,011+ Greg users growing this plant

USDA hardiness zones 3a–7b

Citations:
NC State Extension
Missouri Botanical Garden
Scale photo: Whitney Cranshaw, Bugwood.org (CC BY 3.0 US)
Aphid photo: Scot Nelson (CC0)
Spider mite photo: Clemson University - USDA Cooperative Extension, Bugwood.org (CC BY 3.0 US)
Borer placeholder: Cody Logan (CC BY-SA 4.0)