Woolly apple aphid—Eriosoma lanigerum
Woolly apple aphid (family Aphididae) feeds in groups on bark, green stems, leaf axils, and roots of apple, cotoneaster, and pyracantha. It is especially a problem on apples growing in coastal areas. Less frequently it infests hawthorn, pear, and certain other Rosaceae in the Pyroideae subfamily.
The aphids themselves may not be apparent because they occur hidden under white wax. Woolly aphids can be confused with other insects with similar excretions including certain mealybugs and whiteflies.
Aphids that feed openly on plants have slender antennae that are long (commonly about the length of the body) and generally held backwards over the body. Two, slender tubes (cornicles) on top the rear of the abdomen reliably discriminate openly feeding aphid species, such as the green apple aphid (Aphis pomi) and rosy apple aphid (Dysaphis plantaginea). Woolly apple aphid has relatively short antennae (about one-fourth the body length). The cornicles are undeveloped and not obvious; they are commonly covered with wax and when not wax covered, the cornicles appear as two, slightly elevated rings or pores on top the rear of the abdomen.
Woolly apple aphids are brown, reddish, or purple and as adults are 1/20 to 1/10 inch (1.2 to 2.5 mm) long. When parasitized by the Aphelinus mali wasp, the aphids turn blackish, but this can be obscured by a covering of pale wax. An exception is aphids overwintering aboveground. They lack wax, but their small, dark bodies can be difficult to observe on bark. Aside from their wax, the obvious indication that woolly apple aphid is or was present is that their feeding causes globular, warty galls on branches, trunks, and roots.
Woolly apple aphid sucks and feeds on phloem sap of limbs, roots, shoots, trunks, and the axils of leaves on first- and second-year wood. Aphid colonies are commonly found in bark cracks and wounds, such as the site of pruning cuts. The feeding aboveground is generally of minor significance to tree health despite the development of obvious globular, swollen, warty galls on bark caused by the aphids’ feeding.
The main injury is the stunting (slowing) of plant growth due to the formation of root galls. Heavily infested young apple trees may die from the root galling. In some apple varieties woolly apple aphids can infest the calyx end of fruit. Aphid-infested and wax-fouled apples can be unappetizing.
On aboveground plant parts, woolly apple aphid can be completely controlled by a parasitoid (parasitic) wasp, Aphelinus mali, if broad-spectrum insecticide is not applied for any pests on its hosts. Woolly apple aphids parasitized by A. mali become blackish, puffy mummies. After the adult wasp emerges, parasitism can be recognized by a rounded hole in the mummified aphids. Larvae of aphid flies, larvae of green lacewings, adults and larvae of lady beetles (ladybugs) such as multicolored Asian lady beetle, larvae of syrphid flies, and various other predators also feed on woolly aphids infesting aboveground plant parts.
Where woolly apple aphid is a serious problem on apples, consider replacing trees with those on rootstocks resistant to this pest. These include the Malling series of rootstock, M106 and M111.
During the growing season horticultural oil or insecticidal soap can be sprayed to thoroughly cover infested plant parts. These sprays can provide partial control.
Where spray drift may be a problem or more effective control is desired acetamiprid or imidacloprid can be applied after petal fall. Wait until blossoms are no longer present before making the application because these neonicotinoids move systemically and contaminate nectar and pollen where they can poison bees and the adults of various beneficial parasitic and predatory insects. To avoid mechanical injury to plants and prevent the potential spread of plant pathogens on contaminated equipment and tools, avoid injecting or implanting trees when applying systemic pesticides. Instead, spray bark or drench or inject soil whenever possible as directed on product labels.
Adapted from Aphids on the World's Plants: An Online Identification and Information Guide and Integrated Pest Management for Apples and Pears, Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Galls of woolly apple aphid on an apple trunk.
Galls of woolly apple aphid on apple roots.
Excreted wax covering colonies of woolly apple aphid.
Woolly apple aphids exposed.