Woolly aphids
Woolly aphids (family Aphididae) are species that cover themselves and plant parts with white waxy excrement.
Identification
The aphids themselves may not be apparent because they are hidden under white wax. Woolly aphids can be confused with other insects with similar excretions including certain adelgids, mealybugs, psyllids, and whiteflies such as woolly whitefly, Aleurothrixus floccosus. Gently brush away wax to observe the insects themselves to identify the cause of waxiness.
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. In contrast, woolly aphids generally have relatively short antennae (commonly less than one-third the body length). Their cornicles are generally undeveloped and not obvious; for example, they can appear as two, slightly elevated rings or pores on top the rear of the abdomen. Because many species resemble each other and can be difficult to discriminate, their host plants and biology help to identify the species.
Woolly apple aphid. Eriosoma lanigerum feeds in groups on bark, green stems, leaves, and roots of apple, cotoneaster, and pyracantha and less frequently on hawthorn, pear, and certain other Rosaceae in the Pyroideae subfamily. This feeding commonly causes infested bark to develop globular, warty galls on branches, trunks, and roots. The aphids are blackish, brown, reddish, or purple and as adults are 1/20 to 1/10 inch (1.2 to 2.5 mm) long.
Woolly ash aphids. Prociphilus americanus is a black, dark brown, or dark green species that is 1/16 to 1/10 inch (1.5 to 2.5 mm) long as adults. During spring it feeds on ash leaves and green stems, causing them to become curled and distorted. During the summer P. americanus migrates to feed in wax-covered colonies on the roots of true firs. In the fall the aphids migrate back to ash and occur on the bark.
Prociphilus fraxinifolii, also called ash leafcurl aphid or leafcurl ash aphid, is pale green to yellowish green. As adults they are 1/16 to 1/9 inch (1.5 to 2.8 mm) long. It spends its entire life on ash and during the growing season its feeding causes leaves to curl and grow in tight clumps. It overwinters on ash roots and can occur on ash roots throughout the year. In spring some of the aphids move from roots to colonize growing shoots. The shoot-feeding aphids then move to leaves as they flush. In the fall some of the aphids aboveground move to roots.
Woolly beach aphid. Phyllaphis fagi occurs on the underside of beech leaves and spends its entire life cycle on beech. The aphid's feeding causes leaves to curl downwards on both sides of the mid-rib, forming a gall-like distortion. The aphids are pale yellowish green and elongate oval. Adults are 1/25 to 1/8 inch (1 to 3 mm) long. Winged individuals can have dark, cross-wise bands on the abdomen. Older individuals are covered with tufts of white wax. Younger nymphs and recently molted individuals lack wax because it is secreted new after each molt (development to the next instar). In the fall some individuals develop wings, mate, and the adult females lay overwintering eggs on young shoots.
Woolly elm aphids. Eriosoma americanum and E. crataegi are bluish black to dark green. As adults they are 1/12 to 1/8 inch (2 to 3 mm) long. They cause globular, warty swellings (galls) on elm bark, stems, or roots and curling and galling of elm foliage. Eriosoma americanum migrates seasonally between elms and the roots of serviceberry (Amelanchier species). Eriosoma crataegi migrates between elm and the stems and twigs of hawthorn (Crataegus species).
Woolly hackberry aphid. Shivaphis celti, sometimes called Asian woolly hackberry aphid or hackberry woolly aphid, infests the widely planted Chinese hackberry and other Celtis species. The aphids are dark green to pale green or sometimes pinkish. Adults are 1/12 to 1/10 inch (2 to 2.5 mm) long. Winged individuals have extensive black areas bordering the wing veins, but this can be mostly obscured by wax. Shivaphis celti feeds on the leaves, stems, and petioles of hackberry fruit and on the underside of leaves. It spends its entire life cycle on hackberry and overwinters as eggs laid on dormant buds in the fall.
Woolly oak aphids. Stegophylla essigi, S. querci =S. quercicola, and S. quercifoliae as adults are 1/25 to 1/12 inch (1 to 2 mm) long. Stegophylla essigi are broadly oval and pale grayish green to olive green. Stegophylla querci adults are oval and pale greenish yellow or brownish yellow. Stegophylla quercifoliae are yellowish to yellowish green with a brown head.
These aphids overwinter as eggs on oak buds and shoots. After egg hatch, in spring through summer they reproduce without mating, and most or all individuals are wingless. Infested leaves may curl and discolor. Prolonged high populations may cause scattered dieback of shoot tips. During the fall winged aphids and mating occur and the adult females lay overwintering eggs. The aphids spend their entire life cycle on various species of oak.
Woolly Pine Needle Aphid. Schizolachnus (=Cinara) piniradiatae occurs on needles of numerous pine species. The aphids are long legged, dark green or brown, and coated with grey tufts of wax. As adults they are 1/12 to 1/10 inch (2 to 2.5 mm) long. Prolonged feeding by abundant aphids can cause foliage to turn yellowish.
Life cycle
Aphids develop through 3 life stages: egg, nymph, and adult. After hatching, the nymphs develop through four, increasingly larger instars before maturing into adults without any pupal stage. All these aphids have multiple generations per year.
Some species of woolly aphids alternate feeding between two or more different host plants, migrating between them seasonally. After overwintering on the primary host, during spring, these aphids feed and reproduce without mating. After one to several generations, some of the female aphids develop wings and fly to a secondary (different) host plant. There they feed and reproduce during the growing season. Some winged individuals migrate back to the primary host in the fall. Woolly elm aphids are examples of host-alternating species.
Other species spend their entire life cycle on a one genus of host and do not migrate between different types (genera or families) of plants. These include the woolly hackberry aphid and woolly oak aphids. Some of these species overwinter as eggs on leaf and shoot buds while others overwinter on host roots.
Damage
Aphids suck and feed on phloem sap. The excess plant fluid they consume is excreted as white wax and clear, sticky honeydew. Honeydew attracts ants and yellowjackets that can be a nuisance. Blackish sooty mold grows on the honeydew. The honeydew, sooty mold, and wax foul plant parts and surfaces underneath. Woolly aphids also distort bark, foliage, or both. Most species of woolly aphids are just a nuisance and do not threaten plant health or survival. Woolly apple aphid is an exception. Its main injury is the stunting (slowing) of plant growth due to the extensive formation of root galls.
Solutions
Many woolly aphids on aboveground plant parts can be killed by parasitoid (parasitic) wasps if broad-spectrum insecticide is not applied for any pests on its hosts. Parasitized aphids become crusty and puffy (mummies). After the adult wasp emerges, parasitism can be recognized by a rounded hole in the crusty, mummified aphid bodies. Larvae of aphid flies, larvae of green lacewings, adults and larvae of lady beetles (ladybugs) such as the multicolored Asian lady beetle, and various other predators also feed on woolly aphids infesting aboveground plant parts.
Generally, no control of woolly aphids is needed to protect the survival of otherwise-healthy trees; apples can be an exception. Where intolerable, these aphids are difficult to manage without using systemic insecticide in part because the insects are protected beneath wax, within galled tissue, or underground on roots.
On shrubs and relatively small trees, the abundance of species overwintering as eggs on buds can be reduced by thoroughly spraying terminals with horticultural oil during the dormant season. During the growing season horticultural oil or insecticidal soap can be sprayed to thoroughly cover infested plant parts; these foliar sprays can provide partial control.
On large trees where the aphids are not tolerable and spray drift is 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 parasitoid 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.
For more information see A Field Guide to Insects and Diseases of California Oaks, Aphids on the World's Plants: An Online Identification and Information Guide, and Phyllaphis fagi, Woolly Beech Aphid.
Adapted from the publications above and Integrated Pest Management for Apples and Pears and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
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