Aphids
Also known as Greenfly, Blackfly, Plant lice, Aphididae
In short
Aphids are small, soft-bodied insects that suck sap from soft new growth and buds. They multiply very quickly, coat leaves in sticky honeydew and can spread plant viruses, but they seldom kill an established plant and are eaten by many natural enemies.
How it looks
Pear-shaped, soft-bodied insects with long legs and antennae. The surest field mark is a pair of small tubes (cornicles) pointing backwards from the rear of the body. Colour depends on species and host plant: green, yellow, brown, red or black. Most adults are wingless; winged adults appear when a colony becomes crowded or the season changes. Nymphs look like smaller adults. Colonies gather on shoot tips, around buds and on young leaves, often underneath, and their white cast skins collect on the leaves below. Aphids killed by parasitoid wasps turn into swollen, crusty, golden-brown "mummies".
Signs
Sticky honeydew on leaves and on surfaces below the plant, often followed by black sooty mould. Young leaves curl, yellow or grow distorted, and shoots may be stunted; a few species cause galls. Ants walking up and down the stems, and white cast skins on upper leaf surfaces, are useful clues.
Life cycle
Females give birth to live young without mating, often as many as 12 a day. In warm weather a newborn aphid can itself be reproducing within 7–8 days, so one adult can leave up to about 80 offspring in a week and colonies grow explosively. Some species also produce a sexual generation that lays overwintering eggs, sometimes on a different host plant.
What favours it
Soft, lush growth from heavy nitrogen feeding encourages them. Many species do most damage when it is warm but not hot, around 18–27 °C (65–80 °F). Ants that farm aphids for honeydew protect them from predators, and gardens sprayed with broad-spectrum insecticides lose the natural enemies that normally keep them in check.
Prevention
- Inspect new plants closely, especially shoot tips and leaf undersides, and keep them apart for a few weeks before they join your collection.
- Check growing tips at least twice a week while plants are in active growth, so colonies are caught while small.
- Avoid heavy nitrogen feeding that produces soft, lush growth.
- Keep ants off plants, for example with sticky barriers on stems or pot legs.
- Avoid broad-spectrum insecticides so ladybirds, lacewings, hoverflies and parasitoid wasps can build up.
Treatment
Start with the first step and move down only if the problem persists.
- Squash small colonies with your fingers or prune off infested shoot tips.
- On sturdy plants, knock aphids off with a firm spray of water, early in the day so leaves dry quickly.
- Leave natural enemies to work: ladybird, lacewing and hoverfly larvae eat aphids, and golden-brown mummies show parasitoid wasps are active.
- If numbers stay high, spray insecticidal soap or a horticultural, neem or plant oil, covering leaf undersides thoroughly and repeating as directed; follow the product label.
- As a last resort only, a systemic insecticide (such as the neonicotinoid class) can be used on serious infestations; it can harm bees, so never apply to plants in flower, and follow the product label.
Pros and cons
Cons
- Honeydew makes leaves sticky and feeds black sooty mould.
- Some species transmit plant viruses, notably to cucurbits, beans, potato and lettuce.
- Colonies build up very quickly, so a few aphids can become many within a week.
Look-alikes
- Mealybugs
- Covered in white powdery wax, move slowly and have no cornicles.
- Psyllids
- Adults jump readily and hold their wings roof-like; nymphs are flat and scale-like.
- Whiteflies
- Adults have white waxy wings and fly up in a cloud when the plant is disturbed.
Affected plants
Almost any plant with soft new growth, indoors and out. Viruses spread by aphids are a particular problem in squash, cucumber, melon, bean, potato, lettuce, beet, chard and bok choy.
Related
Sources
Written in our own words from these sources. Check a product label before using any treatment.