How to Tell If Your Plant Has a Pest Problem (and What's Causing It)
A plant doesn't look right. The leaves are off. Something has changed. You are standing in the garden trying to work out whether it is a pest, a watering problem, a fungal disease or just the weather — and a quick guess at the wrong cause will waste money and possibly make things worse.
Most pest articles list insects and tell you to find yours. This one works the other way round. Start with the symptom you are actually looking at, and work back to the likely cause.
The short version: Start with the symptom, not a guessed pest. Yellowing usually means overwatering, not pests. Hole shape tells you the culprit - neat notches are vine weevil, ragged holes are slugs, missing leaves with webbing are caterpillars, clean round holes are capsid bugs. Sticky, sooty leaves mean aphids, scale or mealybug feeding above. Wilting despite damp soil, or sudden collapse, means damaged roots - check for vine weevil grubs or rot before doing anything else.
Yellowing leaves
Yellowing is almost always a root or watering problem, not a pest. Lower leaves yellowing from the bottom of the plant up usually means overwatering or compacted, waterlogged compost. Yellowing scattered across the whole plant with green veins still showing is usually a nutrient deficiency, often iron or magnesium. Sudden yellowing on a single side of a plant is more likely to be wind damage or recent transplant shock.
The exception is when yellowing is combined with sticky leaves and small visible insects on the new growth — then aphids, scale or mealybug are draining sap fast enough to show in the foliage. Check stems and leaf undersides for visible pests before assuming a watering issue.
Holes and chewed edges
The pattern of the hole tells you the culprit. Look at the damage carefully.
Neat semicircular notches. Almost always vine weevil adults feeding at night. Cosmetic on the leaves themselves, but the grubs underground are the real problem.
Ragged irregular holes. Slugs and snails. Look for silvery slime trails on stems and surrounding hard surfaces in the morning.
Whole leaves missing with frass and webbing. Caterpillars — box tree caterpillar on box, sawfly larvae on roses and Solomon's seal.
Small clean round holes through the leaf. Capsid bug damage on hardy geraniums, fuchsias and hydrangeas. The bugs themselves are usually long gone by the time you see the holes.
Sticky leaves and black coating
Stickiness is honeydew — the sugary waste from a sap-sucking pest sitting somewhere overhead. The black layer is sooty mould growing on the honeydew. Both are symptoms, not the cause. To find the pest, look up from where the stickiness is collecting and check the undersides of leaves and the joints of stems for one of three culprits: aphids on soft new growth, scale insects as fixed brown bumps along stems, or mealybugs as white fluffy clusters in joints.
If you cannot find the pest, look higher. Stickiness collecting on patio paving or garden furniture often comes from a tree several metres above — limes and sycamores are famously messy for this in summer.
Wilting that doesn't recover after watering
A plant that wilts in dry soil and perks up after a good drink is just thirsty. A plant that wilts despite the soil being damp has something wrong with its roots. In a container, the most likely cause is vine weevil grubs eating the roots from below — tip the plant gently out and look for white C-shaped larvae in the compost. In the ground, root rot from waterlogging or honey fungus damage is more common.
Either way, the answer is not more water. Reduce watering, check the roots, and if grubs are present, treat with nematodes once soil temperatures allow.
Sudden collapse with no warning
A previously healthy plant that collapses overnight has almost always lost its root system. In containers, this is the classic vine weevil pattern — the foliage looks fine, then one day the whole plant lifts out of the pot with no roots holding it. In the ground, it is usually root rot from prolonged waterlogging, or honey fungus on shrubs and trees. None of these are dramatic enough to spot above ground until it is too late, which is why prevention matters more than rescue.
Quick check: When a plant goes wrong, tip it out of the pot or scrape a little soil away from the base. Healthy roots are pale, firm and smell earthy. Brown, mushy or absent roots tell you everything you need to know.
Speckling, distortion and discolouration
Fine yellow stippling on leaves with a dusty bronze tinge is classic red spider mite, especially in hot dry positions. Distorted, curled new growth without visible holes is usually aphids working unseen at the growing tip. Pale silvery patches with tiny black specks on the undersides are likely thrips. A magnifying glass is more useful here than any spray bottle — you cannot treat what you have not identified.
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Frequently Asked Questions
How do I check my plant's health?
Look at the leaves first — colour, shape, any speckling, holes or stickiness. Then check stems and leaf undersides for visible pests. Then push a finger into the soil to see whether it is dry, damp or saturated. Finally, gently lift the plant out of its pot if you can, or scrape soil from the base if in the ground, and look at the roots. Healthy roots are pale, firm and smell earthy. Five minutes of inspection beats a guessed treatment every time.
Why are my plants dying?
In UK gardens, the three most common reasons a previously healthy plant suddenly fails are overwatering (especially in containers), root pests such as vine weevil grubs, and root rot from waterlogged soil. Pests above ground rarely kill a plant quickly — they weaken it over a season. Sudden death is almost always something happening underground. Check the roots before assuming the problem is on the leaves.
When should I get professional help?
If you suspect honey fungus, Phytophthora or a notifiable disease, and especially if multiple unrelated plants are dying in the same area, it is worth consulting a horticulturalist or sending samples to a diagnostic service. These cause patterns spread through soil and root contact, and getting the diagnosis early changes which plants you can safely use in the same spot afterwards.
Prevention beats rescue every time. For a structured, season-by-season approach to keeping plants healthy, read our pest and disease management guide.
Vigorous, well-rooted plants are easier to keep healthy and easier to diagnose when something does go wrong. Browse our Architectural Collections. Delivered free to your door.
