The weeping willow is a member of the Salicaceae family and is also known as the Babylon weeping willow. This tree is often found growing near the edge of water and has graceful, pendulous branches. The weeping willow is susceptible to insect infestations, which damage the health and appearance of the tree.
Caterpillars are common pests of the weeping willow tree. The larval form of the mourning cloak butterfly is a black, hairy caterpillar that can defoliate a weeping willow. At maturity, the larvae of this pest reaches 5 cm (2 inches) in length, and its body is black with tiny white dots. Adult forms are butterflies that do not feed on weeping willow leaves. Another common weeping willow pest are aphids, which are black, brown or white. Some species of aphids are brightly coloured in shades of pink, yellow or red. Aphids inject sharp mouth parts into weeping willow leaves and suck the sap from it. Aphids are very tiny but are usually found on the lower portions of leaves and may live in colonies.
Caterpillars feed on the leaves of the weeping willow, causing holes to appear in the leaves or the edges of leaves to appear ragged. Heavy infestations of caterpillars can defoliate a large portion of the tree and cause it to be unsightly. Aphids feed on the leaves of willows, causing leaf stippling, leaf yellowing and premature leaf drop. Some trees have adverse reactions to aphid feeding and develop distorted shoots and buds. After feeding on plant sap, aphids excrete a substance known as honeydew, which is a very sticky substance that causes sooty mould fungi to stick to the weeping willow leaves. Sooty mould is easily recognised by the black coating of fungus that looks similar to chimney soot. This condition is not dangerous to the health of the willow but creates an unattractive, sticky mess on tree leaves.
Hand-picking caterpillars from your weeping willow can keep light infestations under control. Drop caterpillars into a bucket of soapy water after removing them from the tree. Aphids are often eliminated and kept at bay by spraying a forceful stream of water from the water hose twice each week, which removes aphids from the tree and keeps sooty mould from becoming unsightly.
Heavy infestations of caterpillars often require insecticides for control and elimination of these pests. Choose an insecticide with the active ingredients Bacillus thuringiensis for best results. Aphids may be controlled with the use of horticultural oils. These products work by coating the insect and disrupting their breathing. Applying horticultural oils to your weeping willow may also help loosen sooty mould, making it easier to remove. Washing your weeping willow tree with soapy water may also reduce aphid populations. Mix 1 tbsp of washing up liquid with 1.14 litres (1 quart) of water, and spray it on the tree. Reapply as necessary for aphid control.