Scale insects

What you are looking at

Small domes stuck to a stem or lined up along the underside of a leaf vein. Brown, yellowish, grey or black, two to six millimetres, flattish or shaped like a limpet, and completely still. Most people take them for part of the plant, a scab, a bud scale or a fungal growth, and the sticky leaves get blamed on something else entirely.

Push one with a fingernail. It will not move. Slide the nail under the edge and it lifts off in one piece, leaving a pale mark on the bark and, if it is a live one, a soft yellowish insect underneath.

The dome is the animal. An adult female scale has no legs, no wings and barely any body plan left. She settled once as a nymph, pushed her mouthparts into the plant and stayed there for the rest of her life under a cover she secreted over herself.

The two kinds, and the difference it makes

Soft scales include the brown scale on bay and currants, the soft scale on citrus and ficus, and the hemispherical scale on ferns and orchids. Their cover is part of the body, they feed on phloem sap, and they excrete honeydew. Sticky leaves mean a soft scale.

Armoured scales, including mussel scale on apple and the oleander scale, build a separate hard shield that is not attached to them and can be flicked off with a pin. They feed differently and produce no honeydew, so the plant declines with nothing sticky to explain it.

Both respond to the same handling. The distinction is worth making because the absence of honeydew leads people to rule scale out on a plant covered in it.

The one week they can be reached

Everything about controlling this comes down to the crawler. Eggs hatch under the mother’s cover into tiny flat mobile nymphs, and those walk out, spread over the plant, and settle wherever the sap suits them. Once settled, each secretes its own cover and stops moving for good.

While they are crawling they have no armour and nothing waxy over them. That is the only stage anything applied to the plant reaches easily, and it lasts days. Outdoors on brown scale it falls in late June and July, and under glass it can be any month.

An adult under a cover is a different proposition. Whatever you spray sits on the outside of a waterproof shield with the insect sealed beneath it, which is why sprays disappoint against scale and why a thumbnail does not.

What the damage amounts to

They take phloem sap, and slowly. A mature bay tree can carry scale for years and look no worse for it. What accumulates is a general thinning, yellow blotches on the leaves directly above the feeding points, early leaf fall and a plant that never quite grows.

The honeydew is what most people notice first. It drips onto the leaves below, a windowsill or a patio, black sooty mould grows on it within weeks, and the mould shades the leaves without infecting them.

Under glass and indoors, with no natural enemies and no season off, numbers keep climbing until an orchid or a ficus is carrying hundreds. Outdoors, parasitoid wasps and predatory beetles usually hold it somewhere tolerable unless the garden has been sprayed.

What to do about them

Scrape them off. A thumbnail on a stem, an old toothbrush and soapy water on bark, a cotton bud dipped in surgical spirit for the ones tucked along a vein. An evening spent on a potted bay clears it, and nothing sold in a bottle performs better.

Go back a fortnight later and again a fortnight after that, because eggs under the covers you removed will have hatched in the meantime and settled somewhere you did not check. Wash the leaves down with plain water at the same time to take off the honeydew and the sooty mould.

Prune out any stem that is encrusted along its length. It will not come clean and it is manufacturing crawlers while you work on the rest of the plant.

Before you scrape everything, look for neat round holes in the older scales. Those are exit holes made by parasitoid wasps that developed inside, and a plant showing a good number of them has help already. Take the intact scales and leave the drilled ones.

Why no spray is listed here

The cover is waterproof and it is there to keep things out. A contact spray reaches settled adults poorly and eggs not at all, so anyone spraying a scaly bay is treating the ten per cent of the population that happens to be crawling that week and killing whatever predators are on the plant at the same time.

Fatty acid and plant oil products smother, which needs total coverage of every stem and leaf underside to achieve what a thumbnail achieves in one pass. In a garden they also remove the parasitoid wasps that keep outdoor scale from ever becoming a problem, which is why sprayed gardens have the worst scale.

What it gets mistaken for

Mealybugs are the same family of insects, but they keep their legs, they move when prodded, and they sit under white waxy fluff in leaf joints rather than under a smooth hard dome on a vein.

Whitefly rise in a cloud when the plant is disturbed. Their immature stages sit still on the leaf underside and do look like small pale scales, so check for adults before deciding.

Buds, lenticels and leaf galls are part of the plant and will not come away without tearing tissue. A scale lifts off cleanly and leaves a mark rather than a wound.

Honeydew and sooty mould with nothing visible on the plant usually means aphids higher up, or a lime, sycamore or maple overhead. Look above the sticky surface before searching the plant beneath it.