Flower pests are rarely a mystery once you know what to look for, and they are rarely an emergency either. A rose bud covered in greenfly looks alarming, but the plant is usually in no real danger, and the aphids are often gone within a few weeks, eaten by ladybirds or washed off by rain. A dahlia nibbled by slugs is a different matter — a single night's feeding can take a young plant down to the stem. The skill in flower pest control is telling the difference between a pest that needs action and one that needs patience, and acting in the right order: identification first, the gentlest control second, and sprays only as a last resort.
This guide covers the pests that actually attack garden flowers, the damage each one causes, and organic-first control that works in a real garden. It also covers the predators that do the work for you, and the habits — such as overfeeding and overcrowding — that invite pests in the first place. If a pest does not match anything described here, the pest identifier helper can help you compare, and your local extension service can confirm an identification from a photo or a sample.
Aphids: the flower garden's most common pest
Aphids are small, soft-bodied insects, 1–3 mm long, that cluster on soft shoot tips, buds, and the undersides of young leaves. On roses they are usually greenfly or blackfly, and our aphids on roses guide describes them in full: curled young leaves, distorted buds, sticky honeydew, sooty mould, and ants farming the colony on the stem. Aphids also attack dahlias, sweet peas, and lupins, and they can spread viruses between plants. Control starts with inspection — check buds weekly from spring onward. A strong jet of water knocks most of them off and they rarely climb back. Rubbing a small colony off by hand works too. Encourage ladybirds, lacewings, and hoverflies by leaving a few aphids alone and planting nectar flowers such as calendula nearby; a garden with predators needs almost no spraying. Soap sprays work on contact but must hit the insects directly and can scorch leaves in hot sun, so use them in the evening and only when the colony is heavy. Never use a broad-spectrum insecticide on flowering plants — it kills the predators as well as the pests, and the aphids return first.
Slugs and snails: the night shift
Slugs and snails are the worst pest of young dahlias, hostas, delphiniums, and any soft-leaved flower planted out in spring. They feed at night and in damp weather, leaving ragged holes in leaves, chewed stems, and silvery slime trails. A young dahlia can be eaten to the ground in one night, so protection matters most in the first weeks after planting. The most effective controls are the simplest: go out with a torch after dark and hand-pick them, protect young plants with copper rings or a collar of crushed eggshells or grit, and keep the ground around the plants clear of debris and dense ground cover where slugs hide by day. Water in the morning rather than the evening so the soil surface is dry at night. Beer traps catch some slugs but also attract them from elsewhere, so place them away from the plants you are protecting. Nematodes — microscopic worms watered into the soil — are a reliable biological control for slugs in damp soil, and they are harmless to everything else. Avoid slug pellets containing metaldehyde where pets or wildlife visit the garden.
Thrips, whitefly and other sap feeders
Thrips are tiny, thin insects, barely visible, that rasp at petals and leaves and leave silvery streaks, distorted buds, and brown-edged flowers. They are a particular problem on gladioli, dahlias, roses, and sweet peas in hot, dry weather. The damage looks like the flower was scorched or bleached, and buds may fail to open. Whitefly are small white moth-like insects that cloud up from the undersides of leaves when a plant is disturbed; they attack fuchsias, geraniums, and other flowers grown in greenhouses or on sheltered patios. Both pests are hard to control once established, so prevention matters: keep plants well watered, since drought-stressed flowers attract both, and avoid overcrowding, which gives them sheltered, humid hiding places. Blue or yellow sticky traps catch the flying adults and tell you when they arrive. A strong water jet dislodges many of them. If a plant is badly affected, remove the worst leaves and buds, bag them, and put them in the bin rather than the compost heap, so the pest does not spread.
Beetles, caterpillars and sawfly larvae
Several larger pests chew flowers outright. Rose sawfly larvae — small green caterpillars with black heads — eat rose leaves from the edge inward and can strip a bush in a week; pick them off by hand, checking the undersides of the leaves, and they rarely return in force. Lily beetles are bright scarlet, easy to spot, and their larvae cover themselves in their own black excrement; hand-pick the adults daily in spring and crush the larvae, or shake the plants over a tray. Japanese beetles, where they occur, skeletonize rose and dahlia flowers in midsummer; knock them into a bucket of soapy water in the morning when they are sluggish, and avoid traps, which draw more beetles into the garden than they catch. Caterpillars of various moths chew holes in petals and buds, usually at night; hand-pick them, and encourage birds, which feed caterpillars to their young. None of these pests needs a spray in a home garden if you check the plants weekly and pick them off early.
Soil pests: vine weevil and cutworms
Some flower pests never show themselves above ground. Vine weevil adults notch the edges of leaves at night, but the real damage is done by their creamy white, legless grubs, which eat the roots of container flowers, fuchsias, and begonias from below. A plant that wilts despite wet soil, or that lifts out of its pot with no roots attached, has vine weevil grubs. Tip the plant out, remove the grubs by hand, repot in fresh mix, and water in a biological nematode treatment, which is the most reliable control. Cutworms are fat grey-brown caterpillars that live in the soil and cut young annuals off at ground level overnight; a cardboard collar around each seedling, pushed a couple of centimetres into the soil, stops them. Both pests are worst in soil that is left undisturbed and in pots reused without cleaning, so empty and clean containers between plantings and check the rootball of any plant you buy before it goes into the garden.
Prevention: the garden that pests avoid
Most flower pest problems trace back to the same few habits. Overfeeding with nitrogen produces soft, sappy growth that aphids and thrips love; feed flowers for flowering, not for leaf, as our flowers fertilizing guide explains. Overcrowding creates the damp, still air that slugs and fungal diseases thrive in — give each plant its recommended spacing, and our flowers spacing guide gives the numbers. Drought stress makes flowers more attractive to sap feeders, so water consistently, especially in containers. Diversity helps too: a bed of one flower is a feast for its specialist pest, while a mixed bed with calendula, marigold, and nasturtium among the roses gives predators somewhere to live and confuses the pests. Finally, inspect weekly. A pest found in its first week is a ten-minute job; the same pest found a month later is a lost plant. If you are unsure about a diagnosis, a product, or a local rule, check your local extension service — its advice is tailored to your soil, climate, and the pests and diseases active in your area.
Frequently asked questions
What is eating my roses at night?
Usually slugs, snails, or sawfly larvae. Look for slime trails for slugs, and check the undersides of leaves for small green sawfly larvae. Hand-pick at night with a torch.
How do I get rid of aphids on roses without spray?
Blast them off with a strong jet of water, rub small colonies off by hand, and encourage ladybirds by leaving a few aphids and planting nectar flowers nearby.
Why are my dahlia leaves full of holes?
Slugs and snails are the usual cause, especially on young plants. Protect with collars or copper rings, water in the morning, and hand-pick after dark.
What are the tiny silver streaks on my gladioli?
Thrips. They rasp petals and leaves in hot, dry weather, leaving silvery streaks and buds that fail to open. Keep plants watered and use sticky traps to monitor them.
Are slug pellets safe around pets?
Pellets containing metaldehyde are not safe around pets and wildlife. Use hand-picking, barriers, or nematodes instead, and check the label of any product before use.