Woolly Alder Aphids (Prociphilus tessellatus, family Aphididae) produce large, white fluffy colonies on the branches of their namesake host (Alnus spp.). Their appearance is variously described as looking like white pom-poms, cotton candy, or white hair covering alder branches.
A close look will reveal that woolly aphid nymphs exude strands of white waxy filaments from block-like structures on their backs (tessellatus means "mosaic pattern"). I recently encountered these unusual woolly aphids in a landscape in southwest Ohio and Jim Chatfield has been observing them in the northeast part of the state.
Woolly alder aphids closely resemble Beech Blight Aphids (Grylloprociphilus imbricator) which are found exclusively on the twigs and branches of American beech (Fagus grandifolia). Indeed, both aphids belong to the same subfamily, Eriosomatinae (woolly and gall-making aphids), and the same tribe, Pemphigini.
Beech blight aphids practice an unusual defense behavior. When disturbed, they pulse their posterior ends in unison which is responsible for the alternate common name, "boogie-woogie aphids." They have great entertainment value!
Unlike their beech-sucking cousins, woolly alder aphids don't pulse their posterior ends when disturbed. They just remain clustered tightly together like a flock of sheep presumably relying on safety in numbers. Of course, this means a few will end up on someone's plate every now and then, but more about that later.
Woolly alder and beech blight aphids both practice the same phloem-sucking behavior and both produce copious quantities of honeydew which becomes colonized by a specific sooty mold fungus, Scorias spongiosa (Ascomycete). The fungus has an obligate relationship with these and a few other woolly aphids.
The sooty mold fungus looks like other black sooty molds at the beginning. The mycelia form a dense, black, "fuzzy" mat on top of the honeydew.
Over time, the fungus progresses into a growth phase that is totally unlike most sooty molds; it produces a spongy, golden-yellow heap that may rise 1 - 2" or more above the leaf or twig surface. The odd-looking fungal growths look like nothing else that would commonly be associated with aphids or honeydew.
The spongy growths eventually harden and turn black in the fall. They remain evident throughout the winter and are sometimes mistaken for other tree maladies.
Woolly Alder Aphids or Maple Blight Aphids?
Woolly alder aphids have two trees hosts: alders and silver maple (Acer saccharinum). In fact, the alternate common name for this aphid is Maple Blight Aphid.
Toward the end of this season, some of the aphids on alder will mature into males and females with wings and fly to silver maple. Others remain on alder for the winter. Indeed, a portion of the colony will always stay on alder year-after-year. Given the time of the year, size of the colonies, and accumulation of sooty mold, I believe Jim and I are seeing colonies that did not migrate from silver maples.
If the aphids do migrate to silver maples, the females lay eggs in bark crevices on silver maples; this is the overwintering stage. The eggs hatch in the spring and the resulting nymphs migrate to newly expanding maple leaves where they line-up on midveins and use their piercing-sucking mouthparts to tap into phloem vessels.
These aphids are all females and they reproduce asexually to form large, fluffy colonies. Heavy infestations can cause noticeable leaf curling and the colonies produce copious quantities of honeydew that rains down on leaves, branches, etc. to become colonized by black sooty molds
By mid-summer, the colonies of "maple blight aphids" produce adults with wings that fly to alders. These relatively large aphids are covered in waxy filaments and look like flying puffballs. They are not particularly good flyers and large numbers can make them a serious nuisance pest as they drift around landscapes with heavily infested maples.
Woolly alder aphids/maple blight aphids mainly affect the aesthetics of their tree hosts. Heavy honeydew production can make them a nuisance, but they cause little to no harm to the overall health of infested trees so insecticide applications are not warranted.
The aphids attract a wide range of predators and parasitoids that play a significant role in naturally regulating populations. It's common to find lady beetles (family Coccinellidae) feasting on the bouquets of aphids. The characteristic eggs of predaceous lacewings (family Chrysopidae) are harbingers of doom for the aphids.
A Harvester of Woolly Aphids
Woolly alder aphids may also attract one of the most unusual caterpillars found in Ohio. Late last season, I came across the slug-like caterpillars of the Harvester Butterfly (Feniseca tarquinius, family Lycaenidae) chowing down on alder aphids. This is the only strictly meat-eating butterfly caterpillar found in the U.S. The butterfly species is also the only member of the monotypic genus Feniseca.
I was amazed to see the carnivorous caterpillars slinking among their woolly prey without producing any reaction from the aphids. The sheep-like aphids seemed totally unaware of the marauding wolves converting their sisters into watery, yellowish smears.
Harvester butterfly caterpillars commonly feed in groups (wolfpacks?) and apparently confine their feeding to woolly aphids. You may also find the caterpillars creeping among colonies of other woolly aphids such as the Woolly Elm Aphids (Eriosoma americanum). The adult butterflies may be spotted fluttering in close proximity to their woolly caterpillar food.
However, the harvester butterfly does not seem to hunt beech blight aphids. Research has shown that the nymphs are highly aggressive against predators. If the organized boogie-woogie doesn't work, the nymphs will mass-attack using their piercing-sucking mouthparts to inflict serious damage to predators; possibly including harvester caterpillars. Perhaps the aphid's gyrations are actually a war dance!