Aphids as Garden Pests
Aphids are one of the most widespread and prolific pests of fruit, vegetables, ornamentals and houseplants. Small, soft-bodied creatures with long legs, they are often known as greenfly or blackfly, although they come in a wider range of colours than green and black. There are over 500 species of aphid in Europe. Some will feed on hundreds of unrelated plants; others restrict their activities to a very limited selection. Some will stay on the same plant all year round, while others migrate to different hosts at certain times of year (see Blackfly Iifecycle).
Aphids feed by sucking sap out of the plant. Their numbers can build up very quickly, weakening growth, distorting leaves and young shoots. Those that feed on roots, such as lettuce root aphids, can kill a plant.
The indirect effects of aphid feeding can be more serious. They are one of the main means of transmitting viruses in the garden and greenhouse. They also exude a sticky honeydew — the excess sugars and water from their diet of sap — which provides a site for unsightly black sooty moulds to grow. The wounds caused by aphids can allow disease entry.
Finding out more about the habits of a particular aphid — knowing when it is likely to be where and whether it is going to spread or not — can help you to devise a control strategy.
Prevention and treatment
• Encourage natural enemies such as bluetits, hoverflies, lacewings, ladybirds, spiders, earwigs, anthocorid bugs and parasitic wasp into your garden. Give them a chance to work
• In the winter, hang up pieces of fat overand in fruit trees to attract bluetits which will eat overwintering aphid eggs.
• Give plants the best growing conditions possible. Those that are struggling are always more susceptible to attack.
• Do not overdo nitrogen fertilizers. The soft growth they produce is very attractive to aphids.
• Useto prevent build-up of root aphids.
• Grow resistant varieties where available such as lettuce, raspberries and flowering cherry.
• Autumn-sown broad beans are less attractive to blackfly in the spring than the less tough spring-sown plants.
• Inspect vulnerable plants regularly and squash any aphids or pick off infested shoots and leaves.
• Scrape off woolly aphid colonies as noticed. Prune out and burn branches if damaged.
• Pick out the tops of broad bean plants once they have reached the required size.
• Break the cycle of infection by removing all susceptible plants at one time of year and burying them in a trench.
• If total control is essential, grow plants under horticultural fleece.
• Biological control agents can be used in the greenhouse.
• Spray affected area with derris, pyrethrum or an insecticidal soap.
Common Species of Aphid
Woolly aphid (Eriosoma lanigerum)
A common pest of apples, crab apple, cotoneaster,, sorbus and other ornamental plants. It may seriously disfigure young plants and allow canker into older trees.
Lettuce root aphid (Pemphigus bursarius)
This aphid feeds on the roots of lettuce, primula, carnation and pinks. Once the effects of its feeding are visible, it is too late to save the plant.
Cherry blackfly (Myzus cerasi)
These black aphids infest ornamental and fruiting cherries in the spring, distorting leaves and shoots. Quantities of sticky honeydew are produced, on which black moulds grow. Cherry blackfly do not attack broad beans.