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The curled leaves on plums, cherries, snowballs, roses and many other plants are caused by aphids, which are small sucking insects.
Aphids are one of the most numerous and certainly the most talked-about pests of the year. More than 600 different species exist in Utah. Numerous small, soft-bodied insects are frequently found sucking the juices from the stems, leaves and roots of various plants.
Aphids are generally less than 3/16 of an inch long and usually have a characteristic pearlike shape. Virtually every plant we grow is attacked by one or more species of aphids. They are sometimes known as plant lice and come in all colors, including white, black, gray, yellow, pink, red, green, blue or brown or combinations of these colors.
They may be spotted or patterned or covered with bumps of various sizes and shapes. Some have wings and others do not. Some secrete a waxy white material called wool that they hide under as protection.
All aphids produce honeydew, or sugary droplets of waste material. This is excess sap ingested by insects and contains sugars and other wastes. The secretions turn the leaves, twigs and fruit into a sticky mess and give them a glossy, varnished appearance.
Occasionally a black, sooty mold grows on this sugary material. This damage is mainly cosmetic but can restrict photosynthesis.
Ants, flies and other insects are attracted to this honeydew material. Ants herd the aphids, protect them from predators and parasites, and use the excess honeydew as food. The presence of ants in trees usually means aphids are also present. Get rid of the aphids and the ants also leave.
Parasites and predators, including ladybird beetles, lacewings, syrphid flies and other insects, are our greatest allies. Check the numbers of aphids on the leaves. If aphid populations are continuing to grow and develop, and there are no predators present, spraying may be justified. Otherwise, allow the ladybugs, green lacewings and other natural controls to take care of the problem.
The natural populations of these predators are increasing and in many cases have eliminated the aphid infestations, even though the sticky leaves remain. Aphids seem to be worse this year because the cool weather prevented predators from developing.
Without the lady bird beetles and other creatures that feed on aphids, they reproduce rapidly and feed on the undersides of the leaves. This causes them to curl, and the dripping sap is from the excess sugar the aphids excrete from the plant sap.
If the predatory insects are feeding, leave the plants alone, and they will clean up the problem. Otherwise, wash the plants off with a strong stream of water or spray with insecticidal soap. If this is a persistent problem, use a dormant oil spray before the leaves unfold in the spring. Stronger measures are seldom needed.
Written by: Larry A. Sagers Extension Horticulture Specialist Utah Stated University Thanksgiving Point Office
