Woolly Alder Aphids

As the last of the leaves part company with the branches that produced them, other aspects of the forest are revealed. The forest denizens once obscured by the leafy green blanket now lie exposed to prying eyes and curious minds.

            One such concealed animal, or, rather, millions of them, are aphids. In particular, woolly alder aphids appear as a white, cottony covering on the bare branches of alder shrubs. They are not only easier to see because the leaves have fallen, but because they are at the most visible stage of their life cycle.

            Alder is a shrub that grows along stream banks, pond margins, and other wet areas. It has roundish leaves, mostly gone by now, and small cones that may persist through winter. The shrubs rarely reach 15 feet in height and form dense, brushy thickets where they grow.

            Alder aphids belong to a group of aphids that makes white, waxy fuzz to cover and protect their soft bodies. Look closely at the white patches and you will see the many gray, soft, fat bodies of the insects. The white fuzz is just an external covering. Although the fuzz makes them more visible, it also makes them distasteful. Instead of getting a mouthful of nice, juicy insect, a predator gets a mouthful of waxy, cottony fibers first.

            Many types of aphids are protected by ant guardians, and woolly alder aphids are no exception. These ants protect the aphids from predators and competitors. Whether a human finger poking the branches or a caterpillar looking for a leaf to chew, the ants rush out to bite and otherwise discourage anything that touches the aphids’ alder. In return, the ants milk the aphids for honeydew, which is a sweet concentration of plant sap that the aphids excrete. When the ant guardian strokes an aphid with its antennae, the aphid, like the cow being milked by the farmer, releases a drop of honeydew.

            Aphids feed on plant sap by piercing the bark of the plant and sucking out the juice. Once in place, they remain on the host until fully grown, although they can detach and move around to more favorable areas. Like ticks on a dog, they parasitize the plant for its sap. Rarely do they move beyond the tree on which their mother placed them unless they are members of the last generation, born just before winter sets in. Only these aphids have wings and can disperse to other plants.

            Almost every aphid you see is female. They reproduce parthenogenetically, without the need for males to fertilize them. Only under stressful conditions, such as the approach of winter, are males produced and reproduction takes place by mating between the sexes. The aphids born with wings are both males and females, and this last generation can reproduce sexually. The advantage to sexual reproduction is that it leads to genetic mixing, producing new combinations of genes that might be naturally selected for by the changing environmental conditions. This is the stuff of evolution. Under stable conditions, the parthenogenetic clones are already best adapted to their environment and need not bother with genetic mixing, for a large percentage of the offspring produced by mating will be poorly adapted to the environment and will die.

            Two other woolly aphids are noteworthy, but less directly visible. Their effects, however, are extremely obvious. The Balsam Woolly Adelgid and Hemlock Woolly Adelgid are both introduced species that attack firs and hemlocks, respectively. They so heavily infest a tree and suck its sap that they can eventually kill those trees that are already weakened by other environmental stressors such as air pollution and acid rain. The dead firs on the Blue Ridge Parkway, perhaps most evident at Richland Balsam, have been killed by the combination of these factors. Christmas tree farmers who raise fraser firs must also be ever vigilant of these pests.

PHOTO: The woolly alder aphid, Prociphilus tessellatus, with ant guardians.


copyright 2002 by Jennifer E. Frick

Text and photos may not be used without the author's permission