Dahlias attract a predictable lineup of pests, from aphids and thrips to slugs, spider mites, and below-ground threats like wireworms and root-knot nematodes. Most dahlia pest problems are manageable once you know what to look for and when to act. Here’s what’s likely eating your dahlias and how to fight back.
Slugs and Snails
Slugs and snails are the most common culprits when young dahlia shoots disappear overnight. They feed after dark and in wet conditions, leaving ragged holes in leaves and sometimes mowing down new sprouts to the soil line. Dahlias are among their favorite targets, alongside hostas and delphiniums.
Water management is your first line of defense. Slugs thrive under high moisture, so stretching out irrigation intervals and switching to drip irrigation reduces the conditions they love. Clear away debris, old pots, and boards near your dahlias since these provide the damp hiding spots slugs need during the day.
For baiting, iron phosphate granules are the go-to choice. They’re safe around pets and children, approved for organic gardens, and perform as well as or better than older chemical baits. Scatter them around the base of plants and refresh after rain. Metaldehyde baits are another option but break down quickly in sunlight and water, and overuse can breed resistant slug populations. Copper strips placed around containers or raised beds also repel slugs on contact. Handpicking at dusk, while tedious, works well for small gardens.
Aphids
Aphids cluster on new growth, stems, and the undersides of leaves, sucking sap and leaving behind sticky honeydew that often turns black with sooty mold. They come in green, black, and peach varieties and reproduce fast enough to colonize a plant in days. But the real danger from aphids goes beyond cosmetic damage: at least 16 aphid species can transmit dahlia mosaic virus, which causes yellowing along leaf veins, stunted growth, and deformed new foliage.
The transmission happens so quickly that spraying insecticides is largely ineffective as a virus prevention strategy. An aphid can pick up the virus from one plant and deliver it to the next in seconds, long before any chemical kills it. The better approach is prevention: buy certified virus-free tubers, destroy any plants showing mosaic symptoms (vein yellowing, chlorotic spots, stunting), and sanitize your cutting tools with 70% ethanol or disinfectant wipes between plants to prevent mechanical spread.
For controlling the aphids themselves, a strong blast from a garden hose knocks most of them off. Encouraging natural predators like ladybugs, lacewings, and hoverflies handles the rest in a balanced garden. Planting sweet alyssum nearby attracts hoverflies, whose larvae are voracious aphid eaters. Nasturtiums work as a trap crop since aphids prefer them over dahlias. When you spot aphids on the nasturtiums, pull the affected stems or hose them off.
Thrips
Thrips are tiny, slender insects that rasp the surface of petals and leaves, leaving silvery streaks and brown, stippled patches. They’re especially drawn to dahlia blooms, where they hide between petals and are easy to miss until damage is obvious. Beyond the cosmetic harm, thrips are the vector for tomato spotted wilt virus and impatiens necrotic spot virus, both of which can devastate dahlias with necrotic patches and marginal leaf death.
Predatory mites are one of the most effective biological controls. Sachets of predatory mites can be hung on dahlia plants, each containing around 1,000 mites that feed on thrips larvae. These perform best below 90°F. In midsummer heat, a heat-tolerant species can be swapped in. Another strong ally is the minute pirate bug, a tiny predator that hunts thrips in flowers. Growers have had success establishing minute pirate bugs on “banker plants” like ornamental peppers planted nearby, which provide pollen and nectar to sustain the predator population. The bugs then move into dahlia blooms to feed on thrips throughout the season.
If you let cilantro or dill bolt and flower, their flat, umbrella-shaped blooms attract predatory wasps and lacewings that also prey on thrips. These wasps are tiny and don’t sting humans.
Spider Mites
Two-spotted spider mites are about 0.5 mm long, yellowish-green with two dark spots on their backs, and they do all their damage on the undersides of leaves. They pierce individual plant cells with straw-like mouthparts and suck out the contents, leaving a tiny bleached dot at each feeding site. As the population grows, these dots merge into a mottled, stippled pattern across the leaf surface. Severe infestations turn leaves pale, brittle, and papery, and fine silk webbing appears on shoot tips. Even low-level feeding can reduce a plant’s ability to photosynthesize by forcing the leaf pores to close.
Spider mites explode in hot, dry weather. Regular overhead watering or misting the undersides of leaves disrupts their preferred conditions. For biological control, a predatory mite called the “spider mite destroyer” (a tiny black ladybug beetle only 1 mm long) is highly effective. Other predatory mites work well in garden settings, especially when humidity stays above 70%.
If you need to spray, insecticidal soap and horticultural oil both work on contact. Thorough coverage of leaf undersides is essential since that’s where mites live and lay eggs. Most sprays don’t kill eggs, so two applications 7 to 14 days apart are usually necessary. Avoid using the same product more than twice in a row to prevent resistance. Adding a spreader-sticker to your spray mix improves coverage on waxy dahlia leaves.
Earwigs
Earwigs chew irregular holes in dahlia petals and leaves, often working at night and hiding in tight spaces during the day. They’re particularly fond of dahlia blooms and can ruin flowers meant for cutting or display. The classic trap is a rolled-up newspaper or a small pot stuffed with straw and placed upside down on a stake near your dahlias. Earwigs crawl in overnight and can be shaken out into soapy water each morning. Reducing mulch depth and clearing debris near plants removes their daytime hiding spots.
Tuber Pests and Storage Problems
Below ground, dahlia tubers face threats from wireworms (the larvae of click beetles), which bore round holes into tubers, and root-knot nematodes, which cause knotty galls on roots and decline in plant vigor. Rotating where you plant dahlias each year helps break nematode cycles, and working the soil in late fall exposes wireworms to birds and cold.
In storage, the main enemy is fungal rot caused by soil-dwelling fungi that enter through wounds. Tubers break down and develop a white-to-pink fuzzy coating. To prevent this, dig tubers carefully in early fall to avoid nicks and cuts, let them cure for a day or two, and store them in a cool spot with good airflow, just above freezing. Inspect stored tubers periodically and discard any that show soft spots, visible mold, or that came from plants showing wilt or dieback during the growing season.
Companion Planting for Pest Prevention
Strategic companion planting won’t eliminate pests, but it tilts the odds in your favor. Artemisia and lavender planted as a border around dahlias act as a natural slug deterrent because slugs dislike their scent and texture. Nasturtiums draw aphids away from dahlias as a trap crop. Sweet alyssum attracts hoverflies whose larvae eat aphids. Bolting cilantro and dill bring in predatory wasps and lacewings. Lettuce or marigolds planted at bed edges can intercept slugs before they reach your dahlias.
Four-o’clocks serve a unique role against Japanese beetles. The beetles are attracted to them, but the plant is actually toxic to the beetles, making it a lethal trap crop. Placing a few four-o’clock plants near your dahlia patch can pull beetles away from your blooms permanently.