Work out what is eating your plants before you spray anything

The instinct when something is chewing a plant is to find a bottle that lists that plant on the back. It is worth resisting for about ten minutes, partly because the diagnosis is often wrong and partly because broad-spectrum insecticides have a habit of making the problem worse a month later.

That last part is not a matter of opinion. A spray that kills aphids also kills the ladybug larvae, hoverfly larvae and parasitic wasps that were eating them. Pest populations rebound faster than predator populations do, because the predators need prey to be present before they can build back up, so the second wave arrives into a garden with nothing left to slow it down.

First question: is anything actually attacking it

A good share of what gets diagnosed as pest or disease is the growing conditions. Blossom end rot on tomatoes, the sunken leathery patch at the base of the fruit, is the standard example: a calcium delivery problem caused by irregular watering rather than a shortage of calcium in the soil, fixed by consistent watering and a mulch and by nothing you can spray. Brown crisp leaf margins usually mean wind, salt or drought. General yellowing with soil that never dries is waterlogged roots. Distorted, cupped, strappy new growth on tomatoes and beans, with the older leaves untouched, is the signature of herbicide drift.

Read the pattern, and look underneath

If it is biological, the shape of the damage is the evidence. Turn leaves over while you look, because a large fraction of the things you are hunting for live on the undersides and will not be visible from above.

Ragged holes and chewed margins	Caterpillars or sawfly larvae. Check leaf undersides and stem junctions
Neat notches cut into leaf edges	Adult vine weevil. The grubs are in the root ball doing the real damage
Irregular holes plus a dried slime trail	Slugs or snails, feeding at night
Fine pale speckling and a dull bronze cast	Spider mites. Look for webbing where leaf meets stem, worst in hot dry spells
Sticky shine, black sooty film, ants on the stems	Aphids. The ants are farming them, and defending them from predators
Silvery streaks with tiny black specks	Thrips
Leaves reduced to a lace of veins	Beetles or sawfly larvae working in groups
Seedlings severed cleanly at soil level overnight	Cutworms, just below the surface within an inch or two of the stem

Some of these can only be confirmed in the dark. If leaves are disappearing and you cannot find anything on the plant at four in the afternoon, go out at ten at night with a flashlight. Slugs, snails, vine weevils and cutworms are all nocturnal, and one late trip settles the question that three days of daytime searching will not.

The two mildews are different problems

Powdery mildew is the white dusty coating on the upper surface of leaves, common on squash, phlox, roses and monarda. It is unusual among fungal diseases in that it does not need the leaf to be wet to get started, and it is worst on plants under drought stress in dry soil with still, humid air around them. So the useful responses are spacing plants for airflow, keeping the roots evenly watered rather than the leaves dry, and buying resistant varieties next year. Late-season powdery mildew on a squash plant that has already fruited is not worth treating at all.

Downy mildew looks superficially similar and is a different organism with different requirements. It shows as yellow patches on the upper leaf surface with straight edges, boxed in by the leaf veins, and a grey or purplish fuzz on the underside directly behind them. It does need prolonged leaf wetness, so evening overhead watering feeds it and morning watering starves it. Confusing the two leads people to water differently in exactly the wrong direction.

Yellowing tells you where to look

Which leaves go yellow first is a real diagnostic. Nitrogen moves around inside a plant, so when it is short the plant pulls it out of old growth to supply new growth, and the yellowing starts on the lowest, oldest leaves and works upward. Iron and manganese do not move, so a shortage shows on the newest leaves at the tips, as yellow tissue with a network of green veins still showing through. That second pattern is usually not an absence of iron in the soil but iron that has been chemically locked up, generally by high pH or by waterlogging, which means the fix is the soil rather than a feed.

Damage from things with a backbone

Browsing is easy to tell apart once you know the trick. Rabbits have upper front teeth and cut a stem cleanly at a sharp angle, like secateurs, and they work low down. Deer have no upper incisors at all, so they grip and tear, leaving a ragged shredded end, and they reach much higher. If the damage stops at a level line about waist height on every plant in the yard, you are not looking at an insect problem and nothing you spray on the leaves is going to help.

What to do, least drastic first

1. Remove by hand. Squashing an aphid colony on new growth, or picking off six caterpillars, ends most outbreaks that get caught early.
2. Fix the conditions that invited it. Spider mites collapse when the humidity comes up and you hose the plant down; slugs thrive in permanently damp mulch pushed right against the stems.
3. Put a physical barrier in the way, timed to the pest rather than the calendar. Insect mesh over carrots and brassicas, a collar around a young stem, copper tape on pots.
4. Leave some prey. A garden with no aphids at all supports no ladybugs, so the first infestation of the year has nothing waiting for it. Tolerating a light population in spring is what buys you control in July.
5. Only then treat, and treat narrowly. Choose something specific to what you have identified, apply it in the evening when pollinators are not flying, and target the life stage it actually works on.

Running underneath all of it is the question of how much damage actually matters. A mature plant sheds a good deal of leaf area with no measurable effect on what it produces, and holes in the outer leaves of a cabbage are cosmetic. What counts is whether the plant is losing growing points, whether the damage is on the part you meant to eat, and whether it is spreading or static.

Two habits prevent more trouble than any treatment cures. Clear away infected material rather than leaving it lying, and keep it out of a compost heap that never gets hot, because a cold heap will happily carry the spores back to the beds next spring. And move crop families around the vegetable garden year to year, especially tomatoes and potatoes, brassicas, and onions, so the soil-borne problems that specialize in each one never get two seasons in a row in the same ground.