Arugula

Can erase a seedling patch overnight in damp, cool weather. Arugula is fast enough that getting the stand past the seedling weeks is the whole battle.

Irregular ragged holes chewed out of the middle of leaves and along the edges - not the clean-edged cuts a caterpillar leaves. Seedlings that vanish overnight, or that are cut off at soil level with nothing left behind. Silvery dried slime trails across leaves, soil, pots, and paving in the morning: this is the giveaway that tells you it was mollusks rather than an insect. Chewed pits in ripe fruit, especially where it rests on the ground. Damage that shows up on the lowest, dampest leaves first and gets worse after rain or evening watering. Turn over a board, an upturned pot, or a clump of dense mulch during the day and look for the animals themselves sheltering in the cool underneath.

Hand-picking works and costs nothing: go out an hour after dark with a headlamp, drop what you find into soapy water, and repeat nightly for a week or two - persistence is what wins, not any single product. Shallow beer traps sunk to soil level catch them too, but need emptying and refilling daily to be worth anything. Iron-phosphate bait is the standard control and is far lower-risk around pets and wildlife than the old metaldehyde products, but it is not harmless: many formulations include EDTA, and a dog that eats a quantity of pellets can develop iron toxicosis. Scatter it thinly per the label, never in piles, and keep dogs off a baited bed. Clear fallen foliage and debris in autumn, where the eggs are laid. In a wet climate this is a population you manage each season rather than one you eradicate - and on most established plants light grazing is cosmetic and needs nothing at all.

Take away the daytime shelter first: lift boards and upturned pots, clear piles of debris, keep dense mulch pulled back from crowns and stems, and give crowded plantings enough airflow to dry out. Water in the morning rather than the evening so the surface is dry by the time they come out. Copper tape around containers and raised beds gives them a shock they will not cross, and it is the one barrier that reliably works. Get plants past the vulnerable stage quickly - start seedlings in pots and set them out at a decent size rather than direct-sowing into a cold wet bed, since most plants stop being worth defending once the leaves toughen. Leave the ground beetles, frogs, toads, and ground-feeding birds alone, because they eat mollusks all night for free and broad-spectrum slug pellets kill them too. Where thick-leaved or resistant varieties exist for what you are growing, they take far less damage.

Slugs and snails are nocturnal mollusks that rasp ragged holes in soft leaves, shear off seedlings at ground level, and chew into ripe fruit that touches the soil. They feed at night and in the early morning and spend the day hidden under boards, pots, dense mulch, leaf litter, and low foliage, becoming active in exactly the cool, damp, shaded conditions that suit young plants. Damage is heaviest in wet spells and on tender spring growth: a single wet week can erase a row of seedlings while an established, tough-leaved plant of the same species is barely marked. They lay clusters of small pearly eggs in damp soil and under debris, so a garden with plenty of cover carries a resident population year after year rather than being invaded from outside.

Arugula is one of the crucifers this turns up on most readily. Catch it early, strip the marked leaves, and the cutting stays usable.

Shiny white raised blister-pustules on the undersides of the leaves, yellow spots or blotches on the upper surface above them, and thickened, swollen or distorted shoots and stems where infection is heavy.

Strip and destroy affected leaves promptly, and harvest ahead of a prolonged wet spell - the crop is usually still perfectly usable if you catch it early. Copper can protect new growth where pressure is high through a long wet season.

Space and thin for airflow, water at the base and in the morning so the foliage dries, rotate crucifers, and clear infected leaves and debris rather than leaving them standing in the bed.

A fungal-like disease (Albugo) of brassicas and other crucifers, common in cool, wet weather. It produces raised, shiny, chalky-white blister-like pustules on the undersides of the leaves, with yellow blotches showing through on the upper surface, and in bad cases it swells and distorts stems, shoots and flowering growth.

Aphids hide down in the rosette and are easy to miss until you wash the leaves. Row cover from sowing keeps them off, and arugula's speed usually beats them anyway.

Dense clusters of tiny insects packed onto shoot tips, flower buds, and the undersides of young leaves. Curled, cupped, or distorted new growth that never opens properly. A sticky shine on the leaves - and on whatever sits under the plant - that later goes black with sooty mold. Tiny white cast skins left behind where a colony has been feeding. Ants running steadily up and down the stems, which is often the first thing you notice. Mottled, streaked, or misshapen leaves that do not grow out can mean a virus the aphids brought with them rather than the feeding itself.

Knock the colonies off with a hard jet of water and repeat every few days - on most plants that alone ends it. Pinch out or prune off the worst-infested tips rather than treating the whole plant. If colonies keep rebuilding, spray insecticidal soap or horticultural oil directly onto the insects, covering leaf undersides, and repeat weekly until they are gone; treat early or late in the day, and never spray a wilted or heat-stressed plant. Avoid broad-spectrum insecticides - they kill the ladybugs, lacewings, and parasitic wasps that were about to solve the problem for you, and aphid numbers usually come back worse afterward. Anything you spray must be labeled for the plant you are treating; follow that label and observe its pre-harvest interval. On anything you eat, wash leaves, heads, and buds well before use, since aphids hide down in crowns, folds, and tight growth.

Go easy on nitrogen: the soft, lush growth that heavy feeding produces is exactly what aphids breed on. Check new growth once a week through spring and early summer so you catch colonies while they are still small, and rub the first few out with your fingers. Leave flowering plants nearby and skip broad-spectrum sprays - ladybugs, lacewings, hoverflies, and parasitic wasps do most of this work for free, and they only stay if there is something for them to eat. Keep ants off the plant where you can, since they protect aphids from those predators. Water steadily so plants are not stressed, and check any new plant over before you bring it home or move it indoors, since aphids usually arrive on something else.

Aphids are small soft-bodied sap-suckers - green, black, gray, pink, or waxy white, and mostly wingless - that gather in dense colonies on the tenderest new growth: shoot tips, buds, and the undersides of young leaves. They breed extremely fast in mild weather, so a few can become hundreds inside a week. Their feeding curls and distorts soft growth, and the sticky honeydew they excrete coats everything below and turns black with sooty mold. Ants often farm and defend the colonies for that honeydew. A few aphids are normal on almost any plant and a healthy plant shrugs them off; only dense, persistent colonies do real damage, and some aphids also spread plant viruses as they feed.

Cosmetic damage, but it ruins the look of salad leaves. Cutting arugula young usually stays ahead of it.

Squiggly white or tan tunnels and translucent blister patches inside the leaves, sometimes with a tiny maggot visible when a leaf is held up to the light, and small clusters of white eggs lined up on the leaf undersides.

Pick off and destroy mined leaves before the larvae mature, and do not compost them - the larva is inside the mine, so removing the leaf removes the insect. Sprays rarely reach it. On a plant that regrows fast, a hard cutback forces clean new growth, and row cover then protects the fresh flush.

Float row cover or fine mesh over the crop from sowing so the adult flies cannot reach the leaves to lay, crush egg clusters on the undersides when you find them, and clear the weed hosts nearby - lambsquarters, chickweed and wild dock are the usual reservoirs. Avoid broad-spectrum sprays, which kill the parasitic wasps that do most of the real control.

The larvae of small flies that tunnel between the upper and lower surfaces of a leaf, leaving winding pale trails and translucent blister-like blotches. Because the larva lives inside the leaf, sprays rarely reach it. On an ornamental or an established plant the damage is cosmetic; on a leafy crop it is a direct hit on the part you eat.