Greenfly and blackfly (and indeed all their close relatives which range in colour from white-yellow through pink and brown to black!) can do an immense amount of damage. There are numerous different species of these pests , collectively known as aphids, and just about every plant in your garden will be likely to be attacked by one or sometimes more aphid species during its life.
Aphids feed by sucking the plant’s sap. Most feed above ground , being especially fond of the softer, more tender newer growth of foliage and stems. Some , the root aphids, feed below ground sucking the sap from roots and doing potentially fatal damage in the process. The above-ground feeding commonly causes symptoms including leaf yellowing and distortion but when some aphids, like the currant blister aphid feed, the toxin they produce as they feed causes the leaves to develop really bright red and purple colours too.
Plant sap has an extremely high sugar content, so high that aphids cannot digest it all. As a result they produce a very sugary (and extremely sticky) excreta or ‘honeydew’. This is then flicked out on to plants, paving, cars, garden chairs indeed anything and everything beneath the feeding aphid. Not too appetising and, when the sticky honeydew attracts a black fungus known as sooty mould it can certainly soil the appearance of plants in your garden.
A heavy and prolonged aphid infestation can cause so much weakening that, although it is unlikely to be killed, the infested plant becomes seriously stressed. But plants are often indirectly killed by aphids : As they suck sap and move from plant to plant they may carry and spread viruses . If your courgettes become infected with a virus such as Cucumber Mosaic Virus and then infested with aphids the aphids will be able to carry the virus particles to other perfectly healthy plants, spreading the infective particles as they feed.
So aphids can directly and indirectly cause a lot of problems. Unfortunately they can often produce a plentiful supply of offspring, even without a mate. I like to describe an aphid as being like a Russian doll – great great granny on the outside, then great granny within, granny in side her, mother within and then ‘baby’ right in the centre. In a very short space of time the population can rocket!!
So what can you do? Hand squidging can be very effective, if you’ve the stomach to do it and if you start early and continue regularly! You can also do everything you can to encourage natural aphid enemies – including many garden birds, lacewings, hoverflies (the larvae of both of these eat large numbers of aphids) and ladybirds.
In recent years people have succeeded in breeding the native ladybird Adelia punctata in captivity so now you can purchase ladybirds or ladybird larvae to release into your garden to increase the local population of this much-loved insect whilst also reducing aphid numbers significantly. If the aphid infestation is on plants in a greenhouse or conservatory then there is a very effective aphid parasite you can introduce, called Aphidius, this soon deals with aphids…swiftly and silently! You can find out more and buy these by clicking here
Organic aphid sprays may be necessary from time to time, but whatever you do, take action promptly because aphids soon get totally out of control!