I’m a fan of beech (Fagus spp., family Fagaceae) from American beech (F. grandifolia) to European beech (F. sylvatica) to beechwood-aged potations (F. beerlignumia). Members of the genus have long been considered relatively free of serious insect pest and disease problems. Unfortunately, beech bark disease and the enigmatic beech leaf disease are changing that perception.
The Woolly Leaf Beech Aphid (Phyllaphis fagi) is representative of the pest category, “obvious, but not serious.” They may become obvious, and they may present collateral issues like sticky, sugary honeydew; however, they cause no appreciable harm to the overall health of their beech tree hosts.
The aphids have no common name that’s been approved by the Entomological Society of America. However, entomologists generally refer to them as woolly beech aphids or woolly beech leaf aphids owing to where we most commonly find them on their European beech host, although they sometimes cluster on leaf petioles and on the fruit and fruit stems.
Woolly beech leaf aphids only infest European beech including cultivated varieties. They do not feed on American beech.
The aphids overwinter as eggs that are tucked into bark crevices and around buds. The eggs hatch at around the same time as when the leaves fully expand. Multiple generations may occur during the growing season with as many as 10 generations observed in two Danish nurseries in one season. This means outbreaks may occur at any time during the growing season.
Nymphs of this woolly aphid are beginning to appear on the underside of the leaves of European beech in southwest Ohio. The nymphs cover themselves in white filaments which is responsible for the “woolly” in their common name.
The woolly nymphs also exude copious quantities of honeydew and droplets of this sugary, sticky substance create a gummy mess on underlying beech leaves, sidewalks, building decking, slow-moving gardeners, etc. The deposited honeydew may become colonized with black sooty molds adding to the unsightly appearance of the goo.
However, one of the greatest impacts of this woolly aphid is the reduction in aesthetic appeal caused by the high contrast between the brilliant white aphid colonies and the lustrous dark green or dark purple beech leaves. The aphids are very obvious if you stand beneath a large, heavily-infested European beech. Of course, raining honeydew may cause you not to stand too long beneath the tree.
Your Mama Don't Dance
As noted above, woolly beech leaf aphids do not infest American beech. However, our native beech tree may become infested later in the growing season by another woolly aphid: the uniquely entertaining Beech Blight Aphid (Grylloprociphilus imbricator).
When disturbed, the entire colony will pulse their woolly rear ends in unison in what appears to be a synchronous samba. This peculiar behavior has earned the aphid the alternate common name of the "boogie-woogie aphid." I believe no other insect upstages beech blight aphids in entertainment value.
You can see what I mean by clicking on this hotlink to a YouTube video I uploaded featuring the boogie-woogie aphids: https://youtu.be/a7p8UAF_CuU
Beech blight aphids only infest American beech; they do not feed on European beech. As noted above, woolly beech aphids only infest European beech, not American beech. Also, woolly beech leaf aphids don’t dance.
However, beech blight aphids are prolific producers of honeydew, and the sooty mold fungus, Scorias spongiosa (Ascomycete), has an obligate relationship with honeydew exuded out of the rear end of the aphids. It’s why the fungus is sometimes called the “beech blight aphid-poo fungus.” However, the sooty mold may also colonize honeydew produced by a few other woolly aphids such as the woolly alder aphid (Prociphilus tessellatus).
Despite the beech blight aphid’s destructive-sounding name, they cause little to no damage to the overall health of infested American beech trees. However, they may have an indirect effect on forest regeneration.