Grapefruit

One of the most common citrus pests; the sooty mold it feeds is often the first thing you notice.

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.

On grapefruit the main significance is that its leaf wounds open the door to citrus canker, to which grapefruit is very susceptible.

Silvery, squiggling serpentine trails inside new leaves, and curled, twisted, distorted new growth on the flushes. Mature leaves and fruit are untouched.

Largely cosmetic on an established tree - prune off heavily mined flushes if you want to, and otherwise leave it alone rather than spraying for appearance. Horticultural oil applied to new flushes deters egg-laying. Systemic controls are used mainly on young trees or where canker pressure is high.

Established trees tolerate it, so the aim is to give it less to work with: avoid over-fertilizing, which pushes out the excess soft flushes it feeds on, and stop feeding by late summer so there is no tender autumn flush to mine. Encourage the parasitic wasps that control it, and protect young trees and new grafts, whose growth actually matters.

A tiny moth whose larvae mine winding, silvery serpentine tunnels just under the surface of new citrus leaves, curling, twisting and distorting them. It attacks only the tender new-growth flushes, never mature leaves or fruit, so on an established tree it is essentially cosmetic - but it can stunt a young tree, and the wounds it leaves in the leaf provide entry points for citrus canker bacteria, which is its real significance.