Aphids

What you are looking at

Aphids are soft, pear-shaped insects between one and three millimetres long, gathered in a crowd on the newest growth. About five hundred species live in Britain and they come in green, black, pink, grey, yellow and woolly white, so colour tells you very little. The habit tells you a great deal. Aphids sit still in dense groups on shoot tips, flower buds and the undersides of young leaves, and they do not scatter when you disturb them.

One character settles it. At the rear of the abdomen, a pair of short tubes project backwards like exhaust pipes, and no other insect you will meet on a rose has them. They are called cornicles, and the aphid uses them to release a waxy alarm secretion when a predator grabs it. Look at a colony through a hand lens and you will see them on every individual.

Most of a summer colony has no wings. Winged aphids appear when the plant starts to decline or the colony grows crowded, and those are the ones that leave to find somewhere better. A sudden appearance of winged forms is the colony telling you it has outgrown the shoot.

Why the numbers go from nothing to everything

Through spring and summer, female aphids give birth to live young without mating. No eggs, no males, no waiting. A nymph matures in about a week and begins producing several daughters a day of her own.

Inside each of those daughters, the following generation is already forming before she is born. Three generations are therefore present in one insect at one time, which is what turns a handful of aphids in early May into a coated shoot by the end of the month. Nothing else in a garden compounds at that rate.

In autumn the pattern changes. Males are produced, mating happens, and the females lay eggs that sit out the winter on woody stems. Those eggs are what you are looking at when a rose stem carries small black specks in February.

Where they came from

Many aphids spend the year on two different plants, which explains a good deal of apparently random damage.

Blackfly on broad beans overwinter as eggs on spindle, guelder rose and mock orange, then fly to the bean rows in late spring. Cherry blackfly overwinter on the cherry itself and move to bedstraws and speedwells in the summer, which is why the tree looks ruined in June and recovers by August. The lime tree that drips honeydew onto a car all July is host to an aphid that will leave it entirely in the autumn.

The practical use of this is timing. Spraying a bean row in June deals with a population that arrived weeks earlier from a hedge you did not plant.

What the damage amounts to

Aphids feed on phloem sap, and an established plant loses very little to them. The leaf distortion is more alarming than harmful. Cupped, puckered new growth comes from the plant continuing to expand tissue the aphids have been feeding on, and it does not straighten out afterwards, but the leaves that follow are normal once the colony has gone.

Honeydew is the sugary waste they excrete, and it lands on everything below. Black sooty mould then grows on it, which blocks light rather than infecting anything. Wash it off with plain water and it will not come back if the aphids above it have gone.

Virus is the serious part. Aphids are the main insect vector of plant viruses, and the peach-potato aphid alone transmits well over a hundred of them. Cucumber mosaic virus, which infects around a thousand plant species, moves this way. Many of these viruses transfer within seconds of an aphid probing a leaf to taste it, well before any spray could kill the insect, and that is the strongest argument against treating a spray as virus protection. Growing resistant cultivars and removing infected plants does more.

What to do about them

Start by doing nothing for a fortnight. The colonies that appear in April feed the ladybirds, hoverflies and parasitoid wasps that will otherwise never settle in your garden, and by June those predators handle numbers no amount of picking over could. Gardens sprayed clean in spring are the ones with the worst aphid years, because the aphids recover in days and the predators take a season.

Where a colony has to go now, use your fingers. A rose shoot can be cleared in a few seconds by running finger and thumb up it, and a hose will knock most of a colony off a bean row. Neither harms a ladybird larva.

Then look for ants. A stem with ants running up and down it is a farmed colony, and the ants are protecting it from everything that would otherwise eat it in exchange for honeydew. Deal with their route rather than the aphids. A grease band around a fruit tree in autumn stops the ants climbing, and standing a container away from an ant run does the same job in a pot.

The predators, and how to recognise them

Ladybird larvae look nothing like the beetle they become. They are elongated, spiny and slate grey with orange markings, they move quickly, and they are squashed on sight by gardeners every summer. One eats several hundred aphids before it pupates.

Hoverfly larvae are stranger and better. They are legless, blind, greenish maggots that haul themselves over a leaf and rear up to seize an aphid, and a single larva accounts for hundreds. Adult hoverflies eat nectar and pollen and will only lay near aphids in a garden that feeds them, so let some fennel, dill or coriander flower, and grow pot marigold and poached egg plant.

Parasitoid wasps leave evidence you can read. A swollen, papery, conker-brown aphid stuck to a leaf is a mummy, and there is a wasp inside it that has not yet emerged. Leave the mummies where they are.

Lacewing larvae, earwigs and small birds all take aphids too. Earwigs in particular do more good on a fruit tree than harm.

Under glass or indoors nothing arrives by itself, so buy it in. Aphidius wasps and the aphid midge Aphidoletes aphidimyza both establish well in a greenhouse or conservatory once spring temperatures allow them to fly.

Why no spray is listed here

Broad-spectrum contact insecticides kill ladybird larvae, hoverfly larvae and parasitoid wasps along with the aphids. The aphids recover first, because telescoping generations rebuild a colony in a fortnight while the predators need the rest of the season, so a sprayed plant tends to be worse off by August than an untreated one. The peach-potato aphid has also developed resistance to several classes of insecticide, which makes a spray that damages the predators and fails on the pest a poor bargain.

Fatty acid and soap sprays are sold as the gentle option. They work by contact on soft-bodied insects, which describes a ladybird larva as accurately as it describes an aphid, and they scorch foliage in sunshine or on a plant that is short of water. If you use one, spot-treat a single colony in the evening rather than covering the plant.

What it gets mistaken for

Whitefly fly. Disturb an infested leaf and a cloud of tiny white insects rises and resettles, which no aphid colony will do. Scale insects are immobile brown or yellowish domes stuck to stems and leaf veins, and they will not move when pushed with a fingernail. Mealybugs hide in leaf joints under white waxy fluff and look like small blobs of cotton wool.

The white flecks in an aphid colony are shed skins and get reported as whitefly or as mildew several times a season. They sit among the aphids, they are the same shape as the aphids, and they blow away when you breathe on them.

Distorted young leaves without a single insect on them point somewhere else. Herbicide drift and contaminated manure both twist new growth in a similar way, and they affect the whole plant evenly rather than the shoots the aphids happened to reach.