Aphid Waste

Sooty mold fed by aphid waste can put unsightly spots on spring plants
COLUMBIA, Mo. –That black or grayish green coloration on some of your newly bought spring landscaping plants is likely a fungus called sooty mold, which feeds off the leftovers from pesky insects called aphids.
Aphids are small, soft-bodied insects, usually wingless, with long legs and antennae. The insects congregate in considerable numbers on new plant growth in the early spring.
Aphids cannot digest all the plant sugars they ingest. They excrete the excess in the form of a sticky substance called honeydew, which supports the growth of the unsightly sooty mold.
Aphids can be recognized by their cornicles, a pair of tube-like structures projecting from the rear of their bodies. Curled, distorted leaves usually are a good indicator of the aphids at work, said Mary Kroening, University of Missouri Extension horticulturalist.
"Aphids damage plants by piercing the leaves, stems and flowers, then sucking sap and other fluids, leaving the plants discolored or stunted," she said. "Sooty mold will not directly kill a plant, but if severe, will weaken the plant, making it more susceptible to other detrimental factors such as drought."
Garden plants that tend to have aphid infestations on an annual basis include butterfly milkweed, aster, dock, euonymus, viburnum and hydrangea.
There is no effective control of sooty mold except by reducing the aphid population. The best approach may be to let the natural enemies of the aphid do their work, she said.
To help aphid predators, use synthetic insecticides as little as possible. Insecticidal soaps or horticultural oils also work to control aphids. Aphids are a favorite diet of ladybird beetle (ladybug) larvae and adults. One ladybird beetle can eat up to 100 aphids in a day.
Parasitic wasps are very successful in attacking aphids. Other predators include lacewings, syrphid flies and assassin bugs.
Aphids can be knocked off the plants with a strong jet spray of water. Aphid populations also can be controlled by pruning out and disposing of aphid-infested plant material.
More information about aphids and other sap-sucking pests can be found in MU Extension guide G7274, "Aphids, Scales and Mites on Home Garden and Landscape Plants," available online at http://extension.missouri.edu/explore/agguides/pests/g07274.htm.