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Aphids are small, soft-bodied, pear-shaped sap-sucking insects, usually 2 – 4 mm long. Nymphs are typically miniature versions of the adults but without wings. They can cause damage to plants directly through feeding or indirectly as vectors of plant viruses.
Aphids often cluster on young shoots and flower buds or underneath older leaves. There are many different species of aphids, which vary in colour; they can be green, black, yellow or white.
Aphids attack fruit trees, roses, camellias, chrysanthemums other ornamentals and a wide range of vegetables. They also transmit virus diseases, such as broad bean wilt and mosaic viruses.
Small colonies multiply rapidly in warm weather, and large infestations can develop in a matter of days. Check your plants a couple of times a week at the beginning of warm weather, so that aphids can be controlled while populations are relatively small.
Keep an eye out for natural predators, like ladybirds or parasitic wasps. These tiny wasps lay eggs inside the aphid, giving it a hard light brown shell like appearance. Ladybird larvae (which look like tiny bird droppings) are voracious eaters of aphids. Both of these predators will reduce aphid numbers rapidly once they arrive.
Aphids typically develop through a life cycle of rapid metamorphosis. Many aphid species are capable of reproducing without males (this is called parthenogenesis). This explains why aphid colonies can increase in size so rapidly; during warm weather females mostly birth live young, without needing to mate. In warmer climates, they can do this all year round. In cooler climates, aphids will overwinter by mating and laying eggs that hatch in spring.
Aphids typically pass through 4 nymph stages (instars) before the adult stage. As they moult, they leave behind their old skin casts, which are visible in aphid colonies as white objects. Aphid life cycles can be completed in 10 to 14 days in warm weather, and there can be more than 20 generations per year, depending on the climate and the species.
Adult aphids without wings are the norm when the colony isn’t overcrowded. When a colony needs to migrate from an over-wintering site, or from drying plants, or when conditions become too crowded, winged adult aphids develop; allowing them to move to new host plants.
Leaves can be twisted and distorted. New growth may be stunted, and is often covered in dense clusters of feeding aphids. Buds may fail to open. You may notice that fruit fails to set, or experience fruit drop.
Leaves may be covered in unsightly white aphid skin casts.
Because aphids excrete a substance called honeydew, this provides an ideal environment for Sooty Mould fungus to grow. Removing the source of the honeydew will usually solve the sooty mould problem as well. Honeydew is also very attractive to ants, so they're often visible around aphid infestations. Fascinatingly, ants are known to protect aphids from predators, so they can collect the honeydew!