What's Eating Your Garden Dahlia?

Dahlia pinnata

Reviewed by Kiersten Rankel M.S.

Quick Answer

For Garden Dahlia, the most likely culprits are slugs (overnight chewing on young plants and flower petals) and Japanese beetles (skeletonized leaves and ragged petal edges in midsummer). Spider mites explode in hot dry weather. Earwigs hide inside open blooms by day and pinch petals at night. Aphids cluster densely on bud stalks and new growth.

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What does the damage look like?

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Holes in petals and leaves overnight

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Skeletonized leaves and chewed petals

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Tiny pale dots and fine webbing

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Curled, sticky bud stalks

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Pinched petals and chewed flower edges

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Pests, ranked by impact

Large red-brown slug (Arion rufus) crawling on a rhubarb leaf

Slugs

Damage

High

Removal

Moderate

What it looks like

Soft brown or grey legless mollusks, 1 to 4 inches long. Hide under mulch, pot rims, dahlia foliage, and the dense base of bushy stems during the day. Feed at night and in damp weather. Silvery slime trails across leaves and the soil are the giveaway.

What the damage looks like

Young dahlia shoots chewed off near the soil line in spring, sometimes back to the tuber. On open plants, ragged holes through leaves and chewed petal edges on the cut-flower blooms you wanted to harvest. Damage shows up overnight, paired with silvery slime trails.

How to get rid of them

Option 1

Iron phosphate bait scattered around the bushy stem base

Scatter iron phosphate slug bait (Sluggo or Garden Safe Slug & Snail Bait, ~$15) in a ring at the base of each clump every 2 weeks through spring and after rain. Safe for pets, kids, and the bees that pollinate dahlias. Slugs eat the pellets and stop feeding within a day.

Option 2

Beer trap sunk to soil level

Sink a tuna can or shallow yogurt cup into the soil so the rim sits flush with ground level.

Fill with cheap beer to within an inch of the rim. Place 1 trap per 4 feet of dahlia bed.

Empty and refill every 2 to 3 days. Slugs crawl in and drown.

Option 3

Hand-pick after dark with a flashlight

Walk the dahlia bed an hour after sunset on a damp night with a headlamp.

Pick slugs off leaves, stems, and the soil with gloves or chopsticks.

Drop into a jar of soapy water. Repeat 2 to 3 nights in a row to break the local population.

Metallic green and copper Japanese beetle (Popillia japonica) on a lantana flower

Japanese beetles

Damage

High

Removal

Moderate

What it looks like

Half-inch metallic green beetles with copper wing covers and small white tufts along each side. Feed in groups on the upper leaves and the open dinnerplate, decorative, and pompom flower heads. Active from late June through August across most of the eastern and midwestern US.

What the damage looks like

Leaves stripped down to a lacy skeleton, with only the veins remaining. Beetles cluster directly on open blooms, chewing irregular holes through petals and leaving the flower head ragged for cutting. Heavy feeding can defoliate a clump in a week and shut down further bloom production.

How to get rid of them

Option 1

Hand-pick into soapy water at 7 am

Walk the dahlia bed in the cool of early morning when beetles are slow.

Hold a wide jar of soapy water under each cluster and tap the stem or flower head. Beetles drop straight in.

Repeat daily through July and August. Removing visible beetles also cuts the pheromone scent that attracts more.

Option 2

Neem oil on foliage and buds, every 7 days

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

Spray foliage and bud stalks at dusk, when bees and other pollinators have left the blooms. Avoid open flowers.

Repeat every 7 days through peak beetle season.

Option 3

Skip the pheromone traps

Bag-style Japanese beetle traps (Spectracide and similar, ~$10) attract more beetles than they catch. Hung in your yard, they pull beetles from neighboring properties straight onto your dahlias. If a neighbor wants to use them, ask them to hang traps at the far end of their yard, not yours.

Spider mites

Damage

High

Removal

Moderate

What it looks like

Almost invisible without a hand lens. Yellow-green or red-orange specks running along the underside of pinnately compound leaves and where leaflets meet the central stem. Hot dry summer weather and dusty foliage trigger population booms.

What the damage looks like

Tiny pale yellow dots stipple the upper leaf surface, then leaves bronze and dry from the bottom of the plant up. Fine webbing strung between leaflets and along the stem in heavy infestations. Stressed plants stop producing new buds and the cut-flower window closes early.

How to get rid of them

Option 1

Strong water blast under the leaves, every 3 days for 2 weeks

Hold a hose nozzle 12 inches from the lower foliage and spray hard at the underside of each leaf. Most mites and eggs dislodge and don't make it back. Repeat every 3 days for 2 weeks. Water in the morning so leaves dry before evening to avoid mildew on dense dahlia foliage.

Option 2

Insecticidal soap rotated with neem oil, weekly for 3 weeks

Spray ready-to-use insecticidal soap (Safer Brand, ~$10) on the underside of every leaf at dusk.

Wait 1 week. Switch to a neem oil mix (2 tablespoons neem and 1 teaspoon dish soap per gallon of water).

Alternate weekly for 3 weeks. The rotation covers eggs and adults across the life cycle.

Option 3

Mulch and water the bed deeply during heat waves

Spider mites thrive on drought-stressed plants. Mulch dahlia clumps 2 inches deep and water deeply once or twice a week through summer rather than light daily sips. Healthy turgid leaves resist mites better than dry stressed ones.

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 new shoot tips and along the bud stalks just below developing flower heads. The soft tissue of an opening bud is their favorite spot.

What the damage looks like

Bud stalks coated in dense colonies, with new growth curling and twisting as aphids drain sap. A sticky shiny film coats the leaves below, and black sooty mold grows on the residue over a few weeks. Heavy clusters on bud stalks distort or abort flowers before they open.

How to get rid of them

Option 1

Strong water blast on bud stalks every 2 to 3 days

Hold a hose nozzle 12 inches from the affected bud stalks and spray at high pressure. Most aphids dislodge and don't make it back. Repeat every 2 to 3 days for 2 weeks. The fastest fix and protects pollinators visiting open blooms.

Option 2

Insecticidal soap on bud stalks at dusk

Spray ready-to-use insecticidal soap directly on aphid-coated bud stalks and shoot tips at dusk, when bees have left the blooms. Avoid spraying open flowers. Repeat every 5 to 7 days until colonies disappear.

Option 3

Companion plant alyssum or yarrow nearby

Plant sweet alyssum, dill, or yarrow within 3 feet of the dahlia row. These attract ladybugs and lacewings, which feed heavily on aphid colonies. Established plantings keep aphid pressure low without sprays and last all season.

Shield-shaped stink bug (Halyomorpha sp., Pentatomidae) on a plant

Earwigs

Damage

Medium

Removal

Moderate

What it looks like

Slender reddish-brown insects 12 to 15 mm long with a pair of curved pincers at the tail end. Hide inside dahlia blooms during the day, especially in the dense rosette of cactus, decorative, and pompom forms. Feed at night. The iconic dahlia complaint among cut-flower growers.

What the damage looks like

Petal edges chewed in irregular pinched bites. Flower centers look ragged and clusters of black droppings collect in the bloom base. Cut blooms brought indoors often release several earwigs that crawl out across the table. Mostly cosmetic on the plant, but ruins the cut-flower harvest.

How to get rid of them

Option 1

Rolled newspaper traps placed near plants overnight

Roll up a few sheets of damp newspaper into tight tubes about 1 inch across.

Lay one tube at the base of each dahlia clump at dusk. Earwigs crawl in to hide before sunrise.

In the morning, drop each tube into a bucket of soapy water. Replace nightly through bloom season. The classic earwig home control.

Option 2

Shake-and-soak harvested cut blooms

Cut dahlia blooms in the cool of early morning and shake each flower head sharply over a bucket of soapy water. Then submerge the whole flower head for 10 seconds. Earwigs hiding inside the bloom drop out or drown. Lift, shake off, and arrange in a vase as normal.

Option 3

Tidy mulch and pot rims through the season

Earwigs need damp daytime hideouts. Pull mulch back an inch from the base of each clump and clear plant debris, fallen petals, and pot saucers from around the dahlia row weekly. Reducing daytime shelter cuts the local population over a few weeks.

Stay ahead of all of them

Four habits that keep Garden Dahlia pests rare and the cut-flower harvest clean.

Bloom and bud-stalk check, every other morning

Earwigs, Japanese beetles, and aphids all converge on open blooms and the soft bud stalks below them. A 30-second walk down the row every other morning catches damage while it's still on one or two flower heads.

Inspect and dust tubers before storage

In zones colder than 8, dahlia tubers come up each fall for winter storage. Brush soil off and inspect each tuber for slug holes, mealybug clusters, and rotted patches before packing in peat or vermiculite. Clean tubers in storage mean a clean spring start.

Mulch 2 inches deep and pull it back from the stem base

Mulch holds soil moisture and suppresses weeds, but slugs and earwigs both shelter in mulch right against the stem. Mulch 2 inches deep across the bed, then pull it back an inch from the bushy stem base of each clump.

Cut and bring blooms in early morning

Pollinators are still asleep, beetles are sluggish, and earwigs are deepest inside the bloom (where the shake-and-soak rinse catches them). Cool blooms also last longer in the vase. Build the cut-flower harvest into the same window every day.

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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 Dahlia pinnata field reports from Greg's botanical database, cross-referenced with university extension sources and published horticultural research.

2,419+ Greg users growing this plant

USDA hardiness zones 8a–11b

Citations:
NC State Extension
Spider mite photo: Clemson University - USDA Cooperative Extension, Bugwood.org (CC BY 3.0 US)
Aphid photo: Scot Nelson (CC0)
Slug photo: Soebe (CC BY-SA 3.0)
Japanese beetle photo: Kmtnewsman0 (CC BY-SA 4.0)
True bug placeholder: Vengolis (CC BY-SA 4.0)