Aphids on leaves
Most commonly thought of as the small, fat, green insects covering roses. However, aphids can also be black, yellow, pink, greyish-white and brown. They are all about 2mm long, round and full of sap.
Bad infestations can reduce the vigour of a plant and leave it vulnerable to attack by other pests and diseases. Virus diseases can be spread by aphids as they move from one infected plant to another. Aphids feed by sticking their mouth parts straight into the plants veins. Any excess sap they cannot consume simply spills out around them. This is why leaves often feel sticky. This sap is often colonised by black sooty mould which can smother the upper surfaces of the leaves and reduces the plants ability to make food.
Chemical – Spray with a suitable contact or systemic insecticide.
Organic - the best thing to do is encourage the predators of aphids, such as ladybirds, lacewings and hoverflies, into your garden. This can be done by planting nectar-rich flowering plants which attract them, such as buddleia, calendula, sedum, stocks, sweet William and wallflowers. Get a head start on the aphids by growing a patch of nettles with a small colony to help build up natural predators in your garden. Cut back the nettles when aphids appear in other parts of your garden to encourage the predators to seek them out. Also, lacewing houses can be bought to put up in the garden so they have a shelter for the winter.
Organic insecticide sprays include those based upon Pyrethrum, derris and insectisidal soap.