Scale insects

Leaves and fruit turn sticky and shiny with sugary droppings (honeydew — the clear syrup the scales dribble out), then go black as if dusted with soot (sooty mould, a harmless mould that grows on the honeydew). Watch too for ants streaming up and down the trunk, yellowing leaves that drop early, bare dying twig-ends (dieback), and dull, blackened fruit that fetches a poor price.

, not like an insect at all — round, oval, or shaped like a little oyster or limpet shell — from the size of a pinhead up to a match-head, packed tight along twigs, on the fruit, and on the undersides of leaves beside the veins. They may be brown, black, grey, white or waxy. The giveaway: they don't move and won't wipe off like dust — you must scrape them loose with a thumbnail. Armoured scales pop off as a hard little shield and leave the insect behind; soft scales are dome-shaped, stuck fast to their own body, and are the ones oozing the sticky honeydew.

Newly hatched, before they grow a shell, they look like grains of moving dust or pale pollen, yellow to orange, wandering over the old scales and out onto the soft new growth. They matter twice over: crawlers are the only stage that spreads, and the only stage a spray can reliably kill.

dried sap or lichen and bark scab (these are dry and part of the bark — a true scale lifts off to show a soft body or a damp smear underneath); mealybugs, close cousins wrapped in loose white cottony fluff that can still crawl slowly (scales are fixed and either hard or domed); or aphids and whitefly, which also make honeydew and sooty mould but crawl or fly away and gather on the soft shoot tips. And remember the sooty mould is not a leaf disease — it wipes off with a wet cloth and simply tells you sap-suckers are feeding above it.

— through the development, mid- and late-season stages — because that tender growth is exactly where crawlers settle and build their crust. Step up your checks in hot, dry, dusty spells (around 20–32°C), the weather scale loves best.

Walk a big "W" across the plot so you cover all four corners and the middle, not just the trees along the path. Stop at about 10 trees (or 10 plants on coffee and cassava). Give extra attention to dusty roadside trees and any tree with ants running on it — that is where an outbreak starts.

At each stop, check a few new shoots, the undersides of leaves along the veins, and several fruit for the fixed bumps, sticky honeydew, black sooty mould and ants. Scrape a few scales off with a thumbnail: soft, moist and plump underneath means alive; dry, hollow and papery means dead — and a neat round hole in a dead scale means a helpful wasp has already killed it, so leave that tree unsprayed.

To time a spray, hunt for the crawlers (the mobile young): wrap a strip of sticky tape, sticky side outward, around an infested twig, or tap a branch over a dark cloth and watch for tiny moving specks. When crawlers are out and walking, that is the short window when sprays actually work — before they grow their armour.

, OR the first black sooty mould shows up, OR you spot any fresh twig dieback. Don't wait for the dieback to spread — by then the colony is already large and far harder to clear.

Plant only clean, scale-free seedlings, cuttings and budwood — scale barely travels on its own, so it mostly arrives on infested plants, tools and clothes. Keep trees vigorous and unstressed: water them through dry spells, rinse road dust off the leaves, and go easy on nitrogen fertiliser (soft, lush growth pulls scale in). Prune out the worst-encrusted twigs and dead wood, then burn or bury the prunings — never leave them lying under the tree.

Ants farm scale for its honeydew and chase off the very insects that would eat it, so break the partnership: smear a sticky grease band around the trunk, clear the weeds and touching branches they climb up, and dig out their nests. Just as vital, keep away from harsh broad-spectrum poisons (sprays that kill many kinds of insect at once) — they knock out scale's natural enemies worse than the scale itself and cause a bigger flare-up.

Tiny parasitic wasps (they lay an egg inside a scale so the grub eats it from within) and ladybird beetles will quietly clear scale for free once the ants and harsh sprays are out of their way — bring in or release them where they are available locally. On light patches, simply scrub the scales off the bark and fruit with a cloth, brush or gloved thumbnail (a little soapy water helps) and cut out the worst-crusted shoots.

Horticultural oil is a light mineral or plant oil that smothers scales (it blocks the tiny pores they breathe through) rather than poisoning them, so it is gentle on people and on the wasps and ladybirds. It is one of the few sprays that even touches the armoured adults, but it works far better on the moving crawlers, so time it to your tape-or-tap check. Wet every surface — twigs, both sides of every leaf, and the fruit, because oil kills only what it actually coats — and spray again after 2–3 weeks to catch the next hatch of crawlers.

Cover your skin, eyes, nose and mouth; spray in the cool of the morning or evening with the wind behind you; never spray in the heat of the day or on drought-stressed trees (oil can scorch hot, dry leaves). Keep the spray off food, drinking water, children, animals and bees, respect the pre-harvest interval (the set number of days you must wait between your last spray and picking the fruit, printed on the label), and wash yourself and your clothes afterwards. If you ever step up to a stronger selective insecticide (one chosen to spare helpful insects), use it spot-only and sparingly, and rotate to a different chemical group each time so the scale never builds up resistance.

When scale builds up despite your efforts, ask why: are ants still bridging in (re-band the trunk), is a neighbour's tree the source, or did you miss the crawler window (re-time your spray to it)? And when a tree is too far gone — crusted over, black with sooty mould and dying back — it has become a factory pumping crawlers onto your healthy trees: cut it down and burn it to save the rest of the block, then replant with clean stock.