Aphids have many natural enemies, including ladybirds, hoverfly larvae, lacewing larvae and parasitic wasps. Some of these are available for biological control of aphids in greenhouses (aphid predators). Unfortunately, out of doors, damaging aphid infestations often build up before the natural enemies are active in sufficient numbers to achieve control. Where practical infestations can be squashed by hand. Pest control products based on natural compounds or with a physical mode of action are less likely to have serious effect on natural predators.
During the growing season there are many insecticides that can be used. It is only feasible to control aphids on plants that are small enough to be sprayed thoroughly. Aphid infestations on tall trees have to be tolerated. Always read the label use pesticides safely. Plants in flower should not be sprayed due to the danger to pollinating insects.
Pesticides based on natural compounds and/or with a physical mode of action
These pesticides are contact in action and have short persistence, so thorough spray coverage, especially to the underside of leaves, is necessary. They can be used on ornamentals and edibles up to one day before harvest. Whilst good control can usually be gained of aphids feeding exposed on stems and leaves those protected by curled leaves are unlikely to be controlled. Products include: Pyrethrum (organic e.g. Bug Clear Gun for Fruit & Veg, Py Insect Killer Powder and Pyrol Bug & Larvae Killer); Fatty acids (organic e.g. Bayer Organic Bug Free, Doff Greenfly and Blackfly Killer, Doff Universal Bug Killer); Plant/fish oils (organic e.g. Vitax Organic Pest and Disease Control, Bug Clear for Fruit & Veg Gun!), Plant oil winter wash can be used to treat overwintering eggs on dormant deciduous fruit trees and bushes (e.g. Growing Success Winter Tree Wash); blend of surfactants and nutrients (e.g. SB Plant Invigorator).
Synthetic pesticides contact action
These usually have more persistence than those based on natural materials and so can give longer lasting control but will have limited effects on aphids within distorted leaves. Products include: Deltamethrin (e.g. Baby Bio Houseplant Bug Killer, Bayer Provado Ultimate Bug Killer, Bayer Sprayday Greenfly Killer, Bayer Provado Ultimate Fruit & Vegetable Bug Killer), lambda-cyhalothrin (e.g. Westland Resolva Bug Killer) and cypermethrin (e.g. Py Bug Killer) are contact insecticides that can be used on ornamental plants and listed edibles; which in some products includes apple, pear, plum, raspberry, strawberry, aubergine, beans, brassicas, lettuce, peas, peppers, cucumber, courgette and tomato. Check the product instructions as there are restrictions specific to the crop on how many applications can be made and the length of time that needs to be left between spraying and harvesting (harvest interval).
Synthetic insecticides systemic action
In addition to some contact action these are absorbed into plant tissues and may have an effect on aphids hidden within distorted leaves. Acetamiprid (e.g. Bug Clear Ultra and Rose Clear Ultra). In addition ornamental plants Bug Clear Ultra products can be used on apple, pear, cherry, plum, potato, lettuce, and glasshouse tomato, pepper and aubergine. Read the manufacturer's instructions regarding restrictions on the use of these products and harvest intervals.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)