There seems to be an aphid for every occasion! Also known as 'blackfly', 'greenfly' or 'woolly aphids', they display a wide variation in colour and appearance.
Aphids overwinter as eggs on trees and shrubs. In Spring, as the weather warms up, the eggs hatch into aphids which then produce their own live young, (missing out the egg stage). Thus vast colonies can build up very quickly.
After several generations of wingless aphids, winged aphids are produced which can migrate to new host plants. Aphids feed by sucking out the plant's sap, causing deformed or stunted plant growth, damaged fruits and flowers, and curled leaves, sometimes with galls and blisters. Aphids can transmit viruses between plants, attract ants, and promote the growth of sooty mould.
A crop of greenfly on a Honeysuckle!
There are many different species of aphids that are problems in the garden. Some are specific to certain plants, others are happy to hunt around for a wide range of plants that have the necessary lush or tender foliage!
Basically, greenfly and blackfly are the most common groups of aphids, but are sometimes other colours - pink or orange even. No matter the colour, they are all pests. The other aphids - such as mealybugs - with their white waxed finish - are difficult to control. Same goes for woolly aphids.
Greenfly and blackfly both attract ants - who are rather partial to the honeydew secretion left by the feeding aphids. It is not correct to think that the ants will control the aphids - far from it.
Like it or not, the best of the insecticide chemical sprays are those of a synthetic - non organic - nature/manufacture. These can either be of a contact nature, where they will need to be sprayed on the aphids direct. Not normally a problem. The other type of chemical spray, is the systemic spray which gets taken into the plant, and is then made available via the sap for these sap sucking insects. the systemic types are best applied to smaller plants where the chemical can easily be translocated through the plant system. This is not generally the case with large plants or shrubs. More effective control is by use of 'contact sprays.
There are several organic sprays developed from natural toxins, including pyrethrum. These contact sprays will need to be actively applied all over the plant, and in particular under the foliage. They are not long lasting in effect, so may need more regular applications than their synthetic chemical counterparts.
Insecticidal horticultural soap can be used with some plants, and is claimed to give some success. So is ordinary household soapy water. The latter simply washes the aphids off - soon to return with seemingly no ill effects, but perhaps a little cleaner!
There are a number of pesticides which are suitable for edible crops, and these will be clearly marked on the label. You MUST read the label to ensure that you adhere to the instructions. In particular the recommended times between application and harvesting. Fruit bushes, vegetables of all types, ornamental plants, herbs and even chillies can be attacked by aphids.
More or less any plant with good young foliage can fall prey to
either blackfly or greenfly. Notably roses attract greenfly and
Nasturtiums attract blackfly. Broad beans are notorious for providing a
feast for blackfly.
Some potted plants can be infected by root feeding aphids. These are best dealt with by drenching the root ball - pot and all in one of the above treatments.
Ladybirds, Lacewing larvae and Braconid parasites do attack aphids, but rarely doing sufficient damage to aphid numbers to control infestation.
There are many proprietary sprays and 'dusts' on the market; Choose carefully according to the type of plant, (e.g. edible, ornamental), to be sprayed, bearing in mind the stage of development of the plant as well as the aphid! Particular care is required when treating the Water-lily aphid if there are fish present in the pool or pond.
Ladybirds are often killed off by use of insecticide sprays. It is a shame and counter productive, for the ladybird's first culinary choice, is a dish (or plant full) of aphids. the larvae of the ladybird in particular can devour several times its bodyweight of the little pest s in a day. Morale being, that if you see ladybirds on a plant, DO NOT SPRAY! Let nature take its course.
Marigolds emit a scent which is repugnant to greenfly and blackfly aphids. Don't be shy of planting a few of the larger growing African marigolds amongst your roses to help keep the pests away. The marigolds make a good colourful display as well. A bit of inter-planting with a row or two of French marigolds in the vegetable plot works wonders - especially with the lettuce. An added bonus is that the marigolds will be first target for slugs en route to your lettuce!
How to prevent Aphid blackfly and greenfly in the first place, is not something we are able to comment upon - other than experimenting with companion planting.