The damage done by aphids is due to a number of causes, including loss of sap, clogging of leaf surfaces with honeydew, and growth of molds and fungi on the honeydew. Leaf curl, a common symptom of aphid infestation, occurs when a colony attacks the underside of a leaf, causing its desiccation. The downward curl provides protection for the colony, but the leaf becomes useless to the plant.
Some species also transmit viral diseases of plants.
Among the aphids causing serious damage to food crops are the grain, cabbage, cornroot, apple, woolly apple, and hickory aphids and the alder and beech tree blights. The phylloxera , notorious for its damage to vineyards, is closely related to the aphids. Many larger insects that feed on aphids, such as ladybird beetles and lacewings, are used as biological controls of aphid infestations. Fungal infection and damp weather also help limit the number of aphids. Listing from Electric Library , a pay service.