Garden pests — how to identify and control them

You almost never see the pest itself first. What appears first is a symptom — a hole in a leaf, a sticky film on a shoot, sudden wilting despite moist soil — while the culprit is either feeding at night, sitting on the underside of the leaf blade, or down in the soil altogether. That is why we have built this guide the other way round from a classic atlas: it starts with what you can actually see on the plant, and only then leads to the name of the species and the way to control it.

The second thing worth saying straight away: the presence of a pest is not in itself a reason to spray. In a living garden something is always feeding, and that is a normal state of affairs — the decision to intervene should depend on how many there are and whether they genuinely threaten the crop or the life of the plant. The section on methods is therefore arranged as a hierarchy: mechanical first, then biological, chemicals last and only when everything else has failed.

Start with the symptom, not the name

Symptom	Likely culprit	When it is most common
Sticky, shiny leaves, curled young shoots, colonies of 1–3 mm insects	Aphids	May–June
Fine pale stippling, greying and drying leaves, delicate webbing	Spider mites	July–August
Silvery streaks on leaves and petals, black specks of droppings, distorted buds	Thrips	June–August
White insects 1–2 mm long that fly up in a cloud when the plant is disturbed	Glasshouse whitefly	all year
White, cotton-wool-like clumps in leaf axils and at the base of shoots	Mealybugs	all year
Brown scales 1–4 mm across stuck to the shoots, with a sticky film beneath	Scale insects	all year
Irregular holes in leaves, seedlings eaten off, silvery slime trails	Slugs and snails	May–September
Semicircular notches eaten out of the leaf margins	Vine weevils	May–August
Brassica leaves stripped to the veins, groups of yellow-green caterpillars	Large white butterfly	May–August
Potato foliage stripped bare, yellow beetles with black lengthwise stripes	Colorado potato beetle	May–August
Fruit with an entry hole and brown, sawdust-like frass, dropping early	Codling moth	June and August
Rusty tunnels in the carrot root, discoloured foliage	Carrot fly	May–August
Narrow, hard tunnels bored through tubers and roots	Wireworms	spring, late summer
Plant wilts suddenly and lifts out of the ground with no roots	White grubs	May–September
Small dark flies circling around houseplant pots	Fungus gnats	all year
Bark gnawed at the base of trees, shallow runs just under the turf	Voles	mainly winter

If the symptom does not match any of the rows, look at the undersides of the leaves with a hand lens and sieve a handful of soil right next to the plant — those are the two places where most of the pests that are not obvious at first glance are hiding. Full descriptions of every species are in the pest encyclopedia.

Leaves — sap-sucking and chewing pests

Sap-sucking: discolouration, stickiness, distortion

This group does not make holes. The insects pierce the tissue and suck out the sap, so the leaf loses colour, turns silvery or becomes distorted, but stays intact. Many of them excrete sticky honeydew, on which black sooty mould develops — it is often taken for a disease, although it is only a consequence of the pest being there.

• Aphids — 1–3 mm, green, black or pink, feeding in colonies on young shoots and the undersides of leaves. A strong jet of water and potassium soap are usually enough. We have covered the full control routine separately: how to get rid of aphids and spider mites.
• Spider mites — 0.3–0.5 mm, right at the limit of what the naked eye can see. Air humidity is the key factor: they thrive in warm, dry air, so misting and moving plants away from a radiator work better than yet another spray.
• Thrips — 1–2 mm, slender, with fringed wings. You will recognise them by the silvery streaks dotted with black droppings and by the distorted buds. Blue sticky traps are the ones to use for monitoring (yellow ones are for whitefly and fungus gnats). Some species transmit plant viruses, so it pays to act early.
• Glasshouse whitefly — white, 1–2 mm, flying up in a cloud when the plant is disturbed. Most common on tomatoes, peppers, cucumbers and pelargoniums under cover. Yellow sticky traps, potassium soap, ventilating the tunnel.

Chewing: missing tissue

• Slugs and snails — irregular holes and seedlings eaten to the ground, plus silvery slime trails. Collect them at dusk, use barriers, or ferric phosphate pellets (safe around pets).
• Vine weevils — the adult weevils are 8–12 mm long, flightless and feed at night. They eat characteristic semicircular notches out of the leaf margins that are impossible to confuse with anything else. Common on box, rhododendron, bergenia and strawberry.
• Large white butterfly — the caterpillars feed in groups and can reduce a cabbage leaf to nothing but veins. Products based on Bacillus thuringiensis var. kurstaki only work on young caterpillars, so the timing of the treatment decides whether it succeeds.
• Colorado potato beetle — a 6–11 mm beetle with ten black stripes; the brick-red larvae are greedier than the adults. On a small plot, picking them off by hand and crushing the orange egg clusters under the leaves is usually enough.

When it is not a pest but a disease

The three most common diagnostic mix-ups:

• White coating. Powdery, on the upper surface of the leaf, and rubbing off on your finger — that is powdery mildew, a fungus, not an insect. White, cotton-wool-like clumps gathered in the leaf axils that can be nudged with a cocktail stick are mealybugs.
• Rusty spots. Dusty, rust-orange masses of spores on the underside of the leaf are rust. Fine pale stippling with no dust but with webbing is spider mites.
• Rotting strawberries. A grey-brown, fluffy coating means grey mould. Hollows eaten into the fruit with slime beside them mean slugs.

Fruit and shoots

• Codling moth — the cause of “maggoty” apples and pears. The caterpillar bores into the fruit and tunnels through to the core, leaving an entry hole with brown droppings that look like sawdust. Pheromone traps are used to monitor moth flight and pin down the date of treatment — the working threshold is roughly 5 moths caught in a trap over the course of a week. Corrugated cardboard bands fitted around the trunks also work well (the caterpillars look for a place to pupate in them, after which the bands are destroyed), as does picking up fallen fruit regularly.
• Scale insects — immobile brown scales on the shoots, easily taken for a natural growth. They suck sap and excrete honeydew. A practical note: potassium soap on its own does not get through, because the waxy scale acts like armour — you need to scrape them off, wipe them with a cotton bud dipped in alcohol, or use an oil-based product that blocks their breathing.
• Slugs on fruit near the ground — strawberries are best mulched so the fruit does not lie on damp soil. That limits both slugs and grey mould at once.

Roots and soil — the symptoms show up too late

This is the hardest group, because the symptom is non-specific: the plant wilts, yellows and dies despite being watered correctly. Diagnosis means lifting the plant, examining the roots and sieving the soil around them.

• White grubs — the fleshy larvae of chafer beetles, 4–6 cm long, creamy white, curled into a C shape, with three pairs of legs. They spend 3–4 years developing in the soil.
• Vine weevil larvae — also white and curled into a C, but smaller and completely legless. In practice that is the simplest way to tell the two larvae apart. Very common in newly bought houseplants.
• Wireworms — narrow, tough, glossy yellow-brown click beetle larvae up to 2–3 cm long that live in the soil for as much as 5 years. They bore narrow tunnels through potato tubers and carrot roots. They are especially numerous in a vegetable bed made on freshly turned-over lawn.
• Carrot fly — the fly larvae tunnel rusty channels through the roots of carrot, parsley and celeriac; damaged roots turn bitter and rot in storage. The adults are drawn by the smell of damaged foliage, so thin your rows in the evening and clear away the thinnings straight away.
• Voles — rodents 9–13 cm long that dig shallow runs just below the surface. In winter they gnaw the bark at the base of trees under cover of snow, which can ring-bark and kill a young tree. Plant bulbs and young trees in baskets of fine mesh, and wrap the trunks with guards for the winter.

Against soil-dwelling larvae (white grubs, vine weevils) the insect-parasitic nematode Heterorhabditis bacteriophora and the fungus Metarhizium genuinely do work, but only if the soil is warm and moist — in dry or cold ground the treatment simply will not work and the money is wasted.

Houseplants — a different set of problems

Indoors there is no pressure from the weather, but dry winter air and permanently damp compost take its place.

• Fungus gnats are in practice a symptom of overwatering. Water less and let the top layer dry out — the larvae die in a dry substrate. A layer of sand or perlite on the surface makes egg-laying harder.
• Mealybugs should be removed spot by spot with a cotton bud soaked in surgical spirit, after which you wash the plant down with potassium soap.
• Scale insects — scraping them off and an oil-based product; checking the undersides of the leaves regularly finds them before they spread through the whole collection.
• Spider mites appear in force in winter next to radiators — changing the conditions works better here than another spray.
• Quarantining new plants for 2–3 weeks away from the rest of the collection saves the most work. A newly bought specimen is the most common source of mealybugs, scale insects and vine weevil larvae.

When to reach for a spray and when not to

1. Set a threshold. A few aphids on a rose shoot or a single caterpillar on a mature cabbage need nothing more than keeping an eye on them. For Colorado potato beetle the working threshold is around 10 larvae per plant, and for codling moth 5 moths per pheromone trap per week.
2. Mechanical. A jet of water, collecting at dusk, crushing egg clusters, trunk bands, fleece, wire baskets. Dull methods, but if you catch the problem early they are usually enough and completely safe.
3. Biological. Bacillus thuringiensis (var. kurstaki for large white caterpillars, var. tenebrionis for Colorado potato beetle larvae), insect-parasitic nematodes for soil larvae, predatory mites and other predatory beneficials under cover, ferric phosphate for slugs, granulosis virus for codling moth.
4. Chemical — as a last resort. Rotate the active ingredients, because Colorado potato beetle and thrips develop resistance exceptionally fast. Do not spray flowering plants during the day while the bees are working, and always observe the harvest interval before picking.

What does not work, or does harm

• Garlic and onion brews are at best a deterrent, and a short-lived one. They will not wipe out an established aphid colony.
• Barriers of coffee grounds, ash and eggshells stop slugs only while they stay dry — after the first shower of rain they are pointless.
• Beer traps really do catch slugs, but they also lure them in from neighbouring plots. Set them away from the plants you are protecting, not among them.
• Yellow sticky traps for fungus gnats catch the adult flies, but the larvae stay in the compost. Without cutting back on watering the problem comes back.
• Sprays for white grubs, wireworms and voles — for the amateur gardener, effective products of this kind essentially do not exist. Only mechanical measures, nematodes and barriers work here.
• Rodent poisons risk secondary poisoning of cats, buzzards and weasels — precisely the animals that keep voles in check for free.
• Broad-spectrum insecticides kill ladybirds, lacewings and predatory mites along with the pest. After a treatment like that the pest population often bounces back stronger than before.

Prevention that genuinely reduces the problem

• Crop rotation. Do not plant vegetables from the same family in the same spot two years running. And do not put a root-vegetable bed on freshly turned-over lawn — that is the direct route to wireworms and white grubs.
• Physical barriers. Fleece laid down from the day of sowing blocks egg-laying by carrot fly and large white. Bulbs and young trees planted in wire baskets are safe from voles.
• Allies. Ladybirds, lacewings, earwigs, parasitic wasps (Cotesia glomerata, which cuts down large white caterpillars), insect-eating birds and hedgehogs. They stay in a garden that has something in flower and somewhere to hide.
• A weekly check through the season, especially of the undersides of leaves. Catching things early decides whether a jet of water will do or whether you will have to reach for something stronger.
• Moderate nitrogen feeding. The lush, soft growth that follows overfeeding is a delicacy for aphids.
• Water in the morning and at the base of the plant. Evening damp favours slugs and fungal diseases at the same time.

Look up the specific species

Every card in the pest encyclopedia includes an identification description, the feeding symptoms and the control methods, along with a list of the plants at risk. The plant cards in the catalogue in turn have their own pests and diseases section — worth checking before you buy, especially for species grown for a crop.