Woolly aphids on Minnesota trees and shrubs
Woolly aphids feed on hardwood and coniferous tree and shrub species throughout Minnesota. In residential landscapes woolly aphids are common on elm, silver maple, ash, alder, apple, pear, hawthorn, and serviceberry (Amelanchier). Unlike their "true" aphid relatives, these aphids produce waxy secretions that they use to protect themselves, making them appear "woolly".
Woolly aphids are small (2-4 mm [1/8 inch] in length), pear shaped insects covered with white waxy strands. The wax filaments give this pest a fluffy, cottony appearance, as though they are covered with wool. Woolly aphids are similar to true aphids, which are very common in Minnesota (see Aphids on Minnesota Trees and Shrubs). The waxy filaments on the woolly aphids serve as a deterrent to predators and also reduce friction helping the aphids move easily around plant hairs.
Pennsylvania Department of Conservation and Natural Resources, Bugwood.org
Waxy filaments give some wooly aphids a fuzzy texture. Wooly alder aphid produces tufts of waxy strands.
Whitney Cranshaw, Colorado
State University, Bugwood.org
Waxy filaments on Wooly apple aphid are not as obvious.
Woolly aphids usually alternate feeding between two host plants. The primary host plant is the plant on which they lay eggs to overwinter. Female aphids hatch from the eggs on the primary host during spring and begin producing live offspring without mating. The aphids are clones of the original overwintering aphids. After one to two generations on the primary host, depending on the species of aphids, the new aphids will develop wings when they reach adulthood. Winged female aphids will fly to a secondary host plant and begin feeding and producing additional clonal generations.
Most of the growing season is spent by woolly aphids on their secondary host. Each female produces hundreds of clonal offspring over several generations. The average lifespan of an aphid from birth to adult is approximately one month. They reach sexual maturity in four to ten days and then are able to produce their own offspring.
In late summer or early fall, another generation of winged females is produced on the secondary hosts that then fly back to the primary host. After feeding again on their primary host, the winged aphids begin producing clones that are both male and female. These aphids mate with each other. The eggs from this generation will overwinter and start the cycle again in the spring. While most woolly aphids have two hosts, there are some species that can sustain themselves entirely on one host.
Woolly aphids inject saliva into their host plant which helps digest the sap. The pre-digested sap is sucked up by fine needle-like mouthparts of the aphid. They feed on leaves, buds, twigs, bark, and even the roots. Symptoms of feeding include twisted and curled leaves, yellowed foliage, poor plant growth, and low plant vigor. In addition to the physical distortions to the plant, woolly aphids also leave accumulations of wax and shed skins on the leaves, twigs, and bark. Fortunately, severe woolly aphid infestations only occur periodically.
As aphids feed, they secrete a sticky, shiny waste product called honeydew. Honeydew will coat objects beneath infested trees including other plants, structures and cars parked along tree covered boulevards or parking lots. An unsightly black fungus called sooty mold sometimes grows on honeydew on infested plants and on any plant surfaces coated with honeydew. Sooty mold does not damage the plant and no control measures for the fungus are required.
Common wooly aphids in Minnesota
Wooly Elm Aphid (Eriosoma americanum):
Feeding by woolly elm aphids produce curled leaves with white cotton-like masses. They have two generations each spring on American elm. In early summer, winged females are produced that fly to serviceberry. All aphids produced on serviceberry migrate to the roots where they feed for approximately two months (two generations). In the early fall, winged females are produced from the root colonies. They climb up the plant and fly back to American elm. On the elm, they produce both male and female aphids, which then mate. The mated females lay a single large egg in a bark crack and then die. Woolly elm aphids seldom cause significant injury to mature hosts, although large numbers can damage serviceberry shrubs that are less than three years old.
Steven Katovich, USDA Forest Service, Bugwood.org
Woolly elm aphid feeds on the foliage of American elm in the early spring initiating leaf curling and gall formation.
Whitney Cranshaw, Colorado State University, Bugwood.org
Woolly elm aphids migrate to the roots of serviceberry (Amelanchier spp.) in the same growing season.
Wooly Alder Aphid, Maple Blight Aphid (Prociphilus tessellates):
Feeding by the woolly alder aphid produces dense, white, woolly masses on the leaves and twigs of silver maple in the early spring and on alder twigs and branches in the summer. Maple leaves in severe infestations may fold lengthwise and cover the aphids inside. Although not damaging to the tree, the white, waxy threads can be mistaken for a fungus. In the fall, the aphids create males and females who mate and then lay overwintering eggs on the maples. Some of the aphids also overwinter as nymphs on the branches of alder.
Mississippi State University Extension
Woolly alder aphids create fuzzy extrusions on infested silver maple leaves in the spring.
Tim Tigner, Virginia Department of Forestry, Bugwood.org
Woolly alder aphids move to alder where they feed on twigs and branches in late spring through summer.
Leafcurl ash aphid (Prociphilus fraxinifolli):
Feeding in the spring by the leaf curl ash aphid causes distortion of terminal leaves on white, black and green ash species, but is most commonly observed on green ash in urban and residential areas. These woolly ash aphids live and reproduce on the foliage of ash. Colonies last until mid-summer. Winged forms are produced which migrate to the roots of ash where they remain for the rest of the year.
Jeff Hahn, University of Minnesota
Leafcurl ash aphids feed on the terminal leaves of ash in the spring.
Steven Katovich, USDA Forest Service, Bugwood.org
Leafcurl ash aphid feeding causes leaf curling and distortion which while unsightly does not impact the health of the tree.
Other wooly aphids
Several other woolly aphid species occur on deciduous trees and shrubs in Minnesota. The woolly apple aphid, Eriosoma lanigerum and Prociphilus corrugatans which feed on serviceberry leaves are two species of commonly encountered woolly aphids in urban landscapes. The woolly apple aphid infests apples, including crabapples, American and red elm, pear, hawthorn and mountain ash. Both aphids cause damage similar to the woolly elm and alder aphids on their specific host plants.
Woolly aphid damage is aesthetic and rarely impacts the overall health of larger established trees. In most cases natural biological controls, such as lacewings, lady beetles, hover flies, and parasitic wasps keep woolly aphid populations below numbers that can damage trees, despite the appearance of distorted leaves.
Once leaf deformation begins, it is too late to prevent damage. Treatments can kill the woolly aphids but distorted leaves will remain on the tree for the entire growing season even after the aphids have moved on. Fortunately distortion of the leaves does not damage the health of the tree. The damage is primarily aesthetic despite peoples' concerns.
Removing honeydew from plants is not necessary or recommended as it does not harm plants. In cases where residents find the honeydew to be a nuisance, dish detergents and tar removers may be used to remove the substance from structures and vehicles. Both will also require significant amounts of forcible scrubbing with a wash cloth or sponge. Tar removers and heavy duty dish washing detergents may damage painted surfaces and remove the clear coating from cars. When using these products, follow the labeled directions and test it on a small portion of the object to make sure it does not cause additional damage.
Treating these aphids for the health of plants is usually unnecessary. However, in cases where the honeydew is a significant issue, treatments can be applied after the leaves have started forming (or as soon as honeydew problems are noticed) to kill the aphids and reduce the honeydew that is produced.
Woolly aphids are less susceptible to horticultural oils and soaps than true aphids, due to the protection of the distorted leaves and their waxy secretions. This also applies to contact insecticides that cannot penetrate the leaves and/or the waxes.
Insecticidal sprays are most effective when systemic products are used. Sprays using imidacloprid or acephate are effective because they will enter the leaves and become locally systemic. This means they will penetrate the foliage on which they are sprayed.
Systemic insecticides can be applied to the soil adjacent to the trunk of the tree or shrub or sprayed on the trunk of the tree. The most common systemic insecticides available to homeowners through garden retailers are imidacloprid and dinotefuron. These products are taken up by plants and transported through the sap to the foliage, stems and branches where the woolly aphids are feeding. This process can take between two and four weeks depending on the product that is used and the size of the tree. These products will also kill many other insects feeding on foliage in the canopy of the tree.
Professional landscape companies can also be hired to treat woolly aphids on trees and shrubs. They have the training and experience to successfully manage a woolly aphid problem.
*All chemicals are listed by common name, not by trade name. Please read common names and ingredients listed on bottle to make sure you are using the correct management tool. The University of Minnesota Extension does not recommend or endorse any particular commercial brand of the products listed.
CAUTION: Read all label directions very carefully before purchasing and again before using an insecticide. Information on the label should be used as the final authority.