Wooly Aphids

White squiggles and cupped leaves may mean your plants have wooly aphids.

Ants tending wooly aphids (Katya Schulz) CC BY 2.0

Living on a rather busy residential street, my boxwood hedges take a beating. Dogs marking their territory, car exhaust, dust… And sometimes that beating is more literal that figurative. July 2016, our house was hit by a car. The only thing that prevented the car from shearing off our gas main (and blowing up our home?) was the boxwood hedges that lifted the car above the main. The hedges have been replaced, along with the fence, but my peace of mind may never recover completely.] Checking on my hedges yesterday, I noticed patches of little white squiggles all over the place. The wooly aphids have arrived.

Wooly aphid identification

​Wooly aphids (Eriosomatinae) protect themselves by creating white waxy filaments that look like cotton or wool around themselves. To their many predators, these waxy filaments are believed to look like nasty tasting fungal growths, so they are left alone. [Green lacewing larva and Harvester caterpillars do the same thing.] Adult wooly aphids fly from place to place, laying egg masses. The eggs hatch and the nymphs that emerge cluster together in large cottony groups.

Wooly aphid host plants
There are several different types of wooly aphid and most of them are host specific. This means they have a strong preference for one or two specific types of plant. Their names generally indicate their hosts of choice. Some of the more common wooly aphids include:

• ash leaf curl aphid (Prociphilus) 
• cabbage aphid (Brevicoryne brassicae)
• hackberry wooly aphid (Shivaphis celti)
• mealy plum aphid (Hyalopterus pruni)
• rosy apple aphid (Dysaphis plantaginea)
• woolly apple aphid (Eriosoma lanigerum) - also feeds on pear, pyracantha, hawthorn, ash, elm, oak

Wooly aphid damage

Like other aphids, the woolies are sap suckers. They have piercing mouthparts which they insert into leaves, buds, bark, and even roots to feed. As they feed, they leave behind a trail of honeydew, which feeds ants and creates habitat for sooty mold. Sooty mold covers leaves and prevents photosynthesis. Wooly aphids can also act as vectors for diseases such as powdery mildew. Wooly aphids will not kill a mature plant, but they can cause deformed leaves, chlorosis, twig dieback, and overall poor health. Since a single female aphids can produce 600 billion offspring in a single season, the damage can add up.

Wooly aphid controls

Aphids have many natural predators. These include ladybugs and their larva, hoverfly larva, parasitic wasps, green lacewings and their larva, and crab spiders. You can make your garden more attractive to these co-workers by avoiding the use of chemical pesticides and by planting a wide variety of nectar sources. Aphids are also susceptible to many fungal, bacterial, and viral infections of their own. Since aphids are soft-bodied insects, a strong spray from the hose is often all that is needed to dislodge these pests from their hosts.