In Managing Woolly Aphids on Trees, Consider the Alternate Hosts

Over the summer, woolly aphids may be key pests of some plants. These flying fuzz-balls resemble cotton swabs in their meandering travel. As their piercing-sucking mouth parts remove plant fluids, they may transmit diseases during feeding. Toxins in their saliva may stunt plant growth, deform leaves and fruit, and may cause abnormal galls to form on the plant. Excess plant sap passes through the feeding aphid as a sugar-rich "honeydew" that attracts mold, ants, and wasps.

Depending on their species woolly aphids infest particular pairs of plant species throughout the year. Last year, I received a call in July from a friend whose silver maples were covered with the white fluffy insects. I discovered his visitor was the woolly alder aphid, and populations were spilling over his tolerance threshold. As part of the name implies, the two hosts of this aphid species are silver maple and alder.

Each year, a generation of wingless female aphid nymphs called stem mothers hatch from eggs on the host plant early in spring. Newly born aphids mature in about ten days, and populations can balloon rapidly. They feed, grow to maturity in about ten days, and bear live, self-reproductive females without mating until populations become overcrowded. A generation of winged females called alatae fly to other spring host plants to colonize them.

As woolly aphids mature through nymphal stages, they grow waxy filaments that appear as wool. These woolly adults and can move passively to new hosts via wind, as well as by animals, such as birds, deer, squirrels, and humans. In our travels, we may carry the insects unknowingly as they rest, or get stuck on vehicles or tools. In my friend’s case, they traveled to his house siding, where honeydew and subsequent sooty mold were turning his tan house black.

Aphids are attracted to stressed plants of their host species. Cultural practices such as proper pruning, fertilizing, and watering maintain plant vitality to prevent or suppress an aphid infestation.

Naturally occurring beneficial insects play an important role in aphid control. Lady beetles and tiny parasitic wasps will use aphids as a source of nourishment for the development of their offspring. These natural controls often control aphid infestations over the course of a season.

In some years, other controls may be needed. Clusters of aphids can sometimes be removed by pruning infested branches, dislodging with water, or reducing in number sufficiently with horticultural oil or insecticidal soap. Woolly alder aphids arrived at my own alder tree early last September, and I was able to control them by applying horticultural oil.

If more stringent chemical use becomes necessary, try least toxic measures first, timing controls to overwintering eggs, the most vulnerable early spring crawler stage, or late summer alternate host stage. Remember that certain insecticide applications will destroy beneficial insects as well as targeted pest species. This practice could leave trees and shrubs unprotected if pest populations should increase in the future.

To learn more, read Aphids on Ornamentals from Penn State Extension.

Resources

Frank, Steven. Aphids on Ornamental Landscape Plants. NC State Extension Publications. Aug. 29, 2019.

Hoover, Greg. Aphids on Ornamentals. Penn State University. June 19, 2023.  

Woolly Aphids on Trees. University of Maryland Extension. February 28, 2023.