Avocado

A minor pest on avocados compared with the overwhelming threat of root rot; most serious on stressed and container trees.

Rows of small brown, tan, black, white, or waxy bumps along stems, twigs, leaf midribs, and leaf undersides that do not brush off but can be scraped loose with a fingernail. A sticky shine on the leaves - and on the floor, table, or path beneath the plant - that later turns black with sooty mold. Ants running steadily up and down the stems, since they farm and defend scale for that honeydew; often the first sign anything is wrong. Yellowing leaves, weak thin growth, and dieback of individual branches or canes that are heavily encrusted. On fruit crops, scale covers and sooty mold directly on the fruit. Check the leaf undersides and the shaded inner wood, which is where colonies build up unseen.

Horticultural oil is the main control: it works by smothering, so it has to coat the insects thoroughly - stems, twigs, and leaf undersides - and it usually needs repeating, because the waxy covering shields them and no single pass reaches everything. On deciduous woody plants a dormant-season oil smothers the overwintering stage while the leaves are out of the way, and timing a growing-season spray to the crawler stage, when the young are mobile and uncovered, is far more effective than spraying settled adults. On a small plant or a light infestation, scrub or wipe the scales off by hand, or dab them with a cotton swab of rubbing alcohol. Prune out and destroy badly encrusted wood rather than trying to save it. Wash off the sooty mold once the scale is gone - it lives on the honeydew, not on the plant, and stops once the supply does. Anything you spray must be labeled for the plant, and oils can burn delicate foliage, so test first.

Inspect. Look over leaf undersides and stems regularly - on a crop you pick, every harvest is a scouting chance - and check every new plant thoroughly before you buy it, plant it, or set it beside anything else, because scale almost always arrives on a plant rather than flying in. Quarantine new houseplants for a few weeks. Keep plants unstressed and not over-fertilized, since soft, forced growth is what populations build on. Prune for airflow and light to open up the shaded inner growth where scale hides, and prune out heavily infested wood as routine maintenance - on anything that gets a hard annual pruning, that alone removes most of any buildup. Leave the lady beetles, lacewings, and parasitic wasps alone, since they normally keep scale down to nothing, and keep ants off the plant where you can, because ants protect the colonies from those predators. Indoors, avoid hot, dry, still winter air.

Scale insects are sap-suckers that spend most of their lives immobile, fixed to a stem, twig, or leaf underside beneath a waxy or shell-like cover. They look far more like bumps, blisters, or crusty encrustations than like insects, which is why an infestation is usually well established before anyone notices it. Soft scales excrete large amounts of sticky honeydew that drips onto everything below and grows a black sooty mold - and those blackened leaves are often the first thing a gardener actually sees. Armored scales produce no honeydew but sit under a hard shield that protects them from most sprays. They spread as tiny mobile crawlers in the weeks after hatching, which is the only stage that moves and the only stage that is easy to kill. Populations build on stressed plants, on crowded shaded growth, and above all on plants kept indoors in dry, still air over winter.

A minor pest overall, most troublesome on container and indoor avocados - the far bigger concern remains root rot. The mealybug destroyer ladybug and parasitic wasps control it outdoors if you leave them alone and keep the ants down.

White cottony masses tucked into leaf axils, crevices and stem joints, which smear rather than brush off. A sticky shine on the leaves and on the surface beneath the plant, later blackened with sooty mold. Ants running steadily up and down the stems - often the first thing anyone notices. Yellowing, stunted or distorted growth with no other obvious cause. On a plant that has been treated repeatedly and keeps declining, check the roots, because root-dwelling mealybugs are easy to miss entirely.

Deal with small colonies mechanically: dab them with a cotton bud dipped in rubbing alcohol, wipe them off with a damp cloth, or wash the plant down with a firm jet of water aimed into the crevices where they hide. For anything larger, spray insecticidal soap or horticultural oil directly onto the insects, which means getting into the axils and undersides where they actually live, and repeat every seven to ten days for at least three rounds, because the waxy coat protects them and contact sprays do not kill the eggs. Cut out badly infested growth rather than nursing it, deal with the ants at the same time or the colony rebuilds, and test any oil first, since it can burn foliage in heat.

Inspect new plants thoroughly before you buy, and quarantine anything you bring indoors for a few weeks - mealybugs almost always arrive on another plant rather than flying in. Check leaf axils and crevices every few weeks on anything grown under cover, since a colony found early is trivially dealt with. Control the ants that protect and spread them; a sticky barrier on a pot rim or a stem does a lot of work. Keep plants unstressed, avoid heavy nitrogen feeding, and do not leave them standing in hot, dry, stagnant air.

Soft, slow-moving sap-suckers that coat themselves and their egg masses in a white waxy fluff, so a colony looks like scattered specks of cotton wool rather than like insects. They wedge into the places hardest to see and hardest to spray - leaf axils, crevices, stem joints, leaf undersides, the base of the plant, and sometimes the roots - and feed there for weeks, weakening growth and excreting sticky honeydew that blackens with sooty mold. Ants farm them for that honeydew, carry them from plant to plant, and drive off the predators that would otherwise control them. Populations explode in warm, dry, still air, which is why indoor and greenhouse plants suffer worst.

The persea mite is an avocado-specific pest that leaves brown necrotic spots rather than plain stippling, but it is usually a manageable, secondary problem next to root rot. Predatory mites are the main biological control and healthy trees generally recover.

Fine pale stippling or speckling across the upper leaf surface, as though the color had been sandblasted out of it, and later a dull bronze or rusty cast over the whole leaf. Delicate webbing across leaf joints, shoot tips, and undersides - that comes with a heavy infestation and means you caught it late. Leaves that dry, curl, and drop early while the plant looks thirsty no matter how much you water it. Hold a leaf over a sheet of white paper and tap it: the specks that fall and then start walking are mites. With a hand lens, look on the undersides for the mites themselves, pale or reddish, and their round translucent eggs. Damage that shows up first on the hottest, dustiest, most sheltered side of a plant, or on a pot sitting near a radiator indoors.

Start with water. A strong jet aimed at the leaf undersides, repeated every few days for a couple of weeks, knocks populations down and breaks the cycle - on many plants that alone ends it. On anything that regrows quickly, shearing back badly infested growth resets the plant to clean foliage. If it keeps building, spray insecticidal soap or horticultural oil directly onto the undersides where the mites actually live, and repeat every five to seven days for three rounds: contact sprays do not kill the eggs, so a single application always fails. Anything you spray must be labeled for the plant you are treating - follow the label and observe its pre-harvest interval on anything you eat. Predatory mites are an effective release in a greenhouse or on an indoor plant. Above all, correct the heat, dust, and drought stress that caused it, or the mites simply come back.

Do not let plants get drought-stressed - that is the single strongest trigger, and a plant watered steadily through heat is far less likely to have a mite problem at all. Hose the foliage down, undersides included, during hot dry spells: it knocks mites off, washes away the dust they thrive in, and raises the humidity around the plant for a while. Indoors and under cover, keep the air from going hot, dry, and stagnant, wipe or rinse the leaves occasionally, and keep pots away from radiators and heating vents. Inspect any plant carefully before you bring it indoors for winter or set it among others, since mites almost always arrive on something else. Avoid broad-spectrum insecticides: they wipe out the predatory mites, lacewings, and lady beetles that do most of this work for you, and reliably leave you with a worse outbreak than you started with.

Spider mites are not insects but tiny sap-feeding arachnids, smaller than a grain of salt, and you almost always notice the damage before you notice them. They live on the undersides of leaves and drain the cells one at a time, which is why the leaf goes finely stippled with pale dots before it bronzes and dries. They breed explosively in hot, dry, dusty, still air - a generation can pass in a week at high temperature - so a barely visible population becomes a webbed, bronzed plant within a fortnight. Drought-stressed plants and plants wintered indoors in dry heated air are the classic hosts. Broad-spectrum insecticides make matters worse rather than better, because they kill the predatory mites and lady beetles that normally hold mites in check.