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.
Adopt a two-way approach to solve your problem. 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 honey dew 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.
Don't spray your aphids with an insecticide or a mustard-seed based organic mixture because you could have 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.
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. You can order ladybird larvae and parasitic wasps from www.just-green.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.
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.