Q What are greenfly?
A Greenfly are one of the most common garden pests. They are found on all types of plants, and thrive outside or under cover. They feed by sucking sap through fine tubular mouthparts, which they insert into the plants' cells.
Greenfly are part of the larger group known as aphids, which also includes blackfly, root aphids and woolly aphids. There are many kinds, each with different host plants and life cycles. Most occur in small numbers and do little harm; only very few behave as pests. They spread on the breeze and tend to settle in sheltered sites, so gardens suit them very well.
Caption: Squash any greenfly you find on your plants
Q How do I recognise greenfly?
A Greenfly are teardrop-shaped, 1-3mm long, they have six legs and may or may not have wings. They usually occur in large clusters on soft plant tissues, such as new shoots and the undersides of leaves.
Q Are greenfly always green?
A Not really. There are many types of greenfly, with seven species on roses alone, and they can be yellow, blue-green or pinky-brown in colour, as well as green. Other aphids may be grey, brown or black. Whitefly are also sapsuckers, but aren't closely related. They are smaller than greenfly and have white wings as well as white bodies, whereas the greenfly's wings are transparent.
Q What damage do greenfly do?
A Small numbers of greenfly do very little damage, but in larger concentrations they can take so much sap from shoots that the young growth becomes distorted, flowers and fruit may be damaged or even abort, and conifers shed their needles.
Greenfly also excrete a surplus sugar solution, known as honeydew. This creates sticky patches on plants and is frequently colonised by a fungus called sooty mould that turns the foliage black. While sooty mould does not directly damage plants, it is unsightly. It can also block out light from the leaves and interfere with the plant’s ability to make sugars by photosynthesis.
Q Do greenfly spread disease?
A Yes, greenfly and other aphids are important carriers or vectors for virus diseases of plants. As virus infections can’t be cured, the only way to protect plants is to prevent them getting infected.
Q How do I control greenfly?
A There are a range of ways to prevent greenfly from damaging your plants.
- Physical removal Squashing greenfly may be messy, but it's quick and effective. Alternatively, you can dislodge them with a jet of water or prune off the worst-affected shoots.
- Barriers Fleece or insect-proof mesh will exclude them and may be worth using on veg and other low-growing crops, such as strawberries or herbs.
- Biological controls These can be used to control greenfly in confined spaces, such as in greenhouses or under cloches. There are two types, both midges: aphidolytes is a predator and best at dealing with large numbers of greenfly; aphidius is a parasite and good at hunting down smaller numbers.
- Insecticides Contact types will only kill greenfly that they come into contact with, so it’s important to spray thoroughly. Try our Best Buys Bayer Natria Bug Control or Westland Resolva .
- Natural predators Many birds, especially blue tits, will devour greenfly, so are worth encouraging. Even seed-eaters like sparrows take greenfly to feed their chicks. Predatory insects, such as ladybirds and their larvae, lacewings, hoverfly larvae and earwigs, hunt and eat greenfly, too. Biological controls and even some fungal diseases also help to control greenfly numbers.
Caption: Spraying isn't the only way to control greenfly
Q I have seen ants running around over greenfly, do they eat them?
A No, ants protect greenfly from predators. This is because the ants milk the greenfly for the sweet honeydew they produce, which they feed on. Ants may also move greenfly to new feeding grounds. There's not much you can do about this. In fact, ant activity can often be a useful early warning that greenfly numbers are building up.
Q What is the life cycle of greenfly?
A Greenfly spend the winter as eggs on host plants or as adults, in sheltered spots outside or under glass. In spring and early summer, adult females give birth to live young without mating. The young are all female, so numbers can build up very quickly. In fact, newly born greenfly already carry more young inside them.
When the population is high, if there are predators about, or the greenfly are ready to move to different plants, then winged greenfly develop. Once settled on their new hosts, these lose their wings and give rise to wingless greenfly again. They continue to produce more young, including some males, then, as day length shortens and temperatures fall in autumn, they mate and produce eggs that will overwinter.
Some greenfly, such as the rose aphid, spend their whole lives on one type of plant. Others, such as the willow-carrot aphid, have two hosts, spending winter and spring on one, then moving off elsewhere for the summer. This is one reason why large infestations of greenfly can suddenly disappear – or you may see serious damage, but no sign of the insects
Suppliers of biological controls for greenfly and insect-proof mesh
Defenders 01233 813121 defenders.co.uk
Gardening Naturally 0845 680 0296 gardening-naturally.com
Green Gardener 01493 750061 greengardener.co.uk
Harrod Horticultural 0845 402 5300 harrodhorticultural.com
The Organic Gardening Catalogue 01932 253666 organiccatalogue.com
Enviromesh 01285 860015 enviromesh.co.uk
Wondermesh 01561 377946 wondermesh.co.uk