As the weather warms up in spring, it’s important to keep a close eye out for sap-sucking aphids. Mild weather will result in greenfly, blackfly and other members of this troublesome family of bugs visiting gardens. They need to be dealt with as soon as they are spotted - aphids breed at a ferocious rate and one or two can quickly become a colony.
Problems caused by aphids
Apart from gorging on the sap of plants, leading to loss of vigour, curling leaves and distorted stems, aphids are behind other plant problems.
Aphids can help to spread viruses from one plant to another and their sticky excrement or honeydew, makes the ideal surface for sooty mould fungus to develop. This black fungal growth can coat leaves and stems, checking growth by preventing light and air reaching the plant.
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Symptoms of aphid infestation
You will usually see aphids on the stems and underside of leaves of infested plants, and you might also spot the white cast skins.
Plant leaves may become curled or distorted and the plant can become weaker. Look out for honeydew - a sticky excretion that attracts sooty moulds.
Aphids are small insects (usually green) that can sometimes be seen on plant stems and leaves.
Plants that attract aphids
Aphids are not particularly discerning and love many plants. This includes vegetables, such as broad beans and lettuce, many perennials and lots of shrubs – they seem to be particularly fond of roses and philadelphus. Look under leaves at the top of plants, flower buds and on young, developing shoots.
How to get rid of aphids
Aphids are always more difficult to deal with under glass or inside garden rooms and conservatories because many of their natural predators (blue tits, ladybirds, ladybird and hoverfly larvae and parasitic wasps) may not be able to reach them to clean up the problem.
Controlling aphids by hand
Squash any aphids with your fingers now – using rubber gloves if you're squeamish. This simple act will kill many of these soft-bodied creatures meaning less honeydew on your leaves. This sticky substance is a waste aphid product usually eaten by ants in the garden setting. Though harmless it can attract sooty mould and this disfigures the leaves.
Lessen the problem by spraying any sticky leaves with tap water using an inexpensive hand held spray bottle.
Non-chemical control of aphids
Spray your aphids with an insecticide or a mustard-seed based organic mixture risks killing over-wintering predators too. If you kill these now the effects will be long lasting as many predatory insects only breed once or twice a year. If these effective aphid munchers die now, you may be without them for two years or more and that will make the problem worse - not better.
Some aphids inevitably overwinter and once the weather gets warmer the problem begins again. So once spring arrives, ventilate your greenhouse every day. This will allow ladybirds, hoverflies, lacewings and parasitic wasps free access and they will seek out any aphid colonies and lay eggs close by.
To kill large populations quickly and effectively you may want to spray. Although there are many chemical pesticides available in garden centres and DIY stores, try using an insecticidal soap, such as Savona, or a spray derived from plant extracts, but be aware you might also be killing the predators of aphids at the same time.
Wasps and ladybirds – natural predators
You may not notice their presence because tiny parasitic wasps look just like flying greenfly themselves. But these tiny creatures will lay one egg in each aphid and the new adult wasp will eat its way out leaving a neat hole in a brown rounded mummified aphid. You can spot these 'mummies' easily on the back of leaves.
Ladybirds are also excellent predators. One female ladybird will lay 1,500 eggs in batches of 30 in one year alone. These oval, mustard-yellow eggs are hard to spot but the emerging larvae feed voraciously, eating at least 50 a day. They will pupate in your greenhouse before emerging as adults. A ladybird can eat about 5,000 aphids during its lifespan. You can order native ladybird larvae and parasitic wasps from ladybirdplantcare.com if you feel you need more help.
You can also attract hoverflies and lacewings to your greenhouse by sowing nectar-rich annuals close to the door. A tub of African marigolds, calendulas and nasturtiums will help them find their way. The adult insects of both feed on nectar and pollen - although their larvae eat aphids. A single lacewing larvae is able to eat over 300 aphids before it changes into an adult.
If your plants can be given a summer holiday outside, this will help too. It will toughen up the soft growth and make your plants less susceptible to aphid attack. Finally avoid water-based, quick-release nitrogen-rich feeds - they produce soft, sappy growth. Opt for slow-release, granular fertilisers instead - like Osmacote.
Find out more about attracting beneficial insects into your garden