Overwintering female aphids seek a crevice or bark flap on elm in which to lay a single egg to survive winter’s perils.
Years ago a summer microburst shredded several mature red maple trees in my backyard. Seeking a replacement for these carbon sequestering shade givers, I selected a young American elm of the Princeton clan. During the past two decades, this once spindly sapling has expanded to dominate my modest landscape. In recent weeks in the golden sunbeams of late afternoon, the airspace around my elm has been filled with a flurry of hundreds of tiny flying insects. Upon closer examination, I discovered these aerial acrobats to be none other than the sexual life stage of the elm cockscomb gall aphid, making their way from their grassy summer feeding grounds back to the elm tree that serves as their overwintering site.
On bright autumn afternoons the air near my elm is filled with a flurry of elm cockscomb gall aphids returning to their winter home, which is a Princeton elm. Watch as a female alights on an elm branch briefly before taking off, perhaps in search of another place to lay eggs or to escape my camera lens. For a much closer look, check out the aphid through the lens of a microscope. These are pretty cool insects.
This strange gall resembling the comb of a rooster is the early summer home for hundreds of aphids. Early in formation, these galls are green.
Many species of aphids, including the elm cockscomb gall aphid, live complex lives. To this you might respond, well, don’t we all. Yes, but not like these tiny sap suckers. For much of the summer these aphids have been living underground sucking sap from the roots of grasses in my lawn. With the onset of cooler temperatures and shorter days, these grass feeders produce a generation of vagabonds that take to the air in search of their winter refuge, which is my elm tree. Having consummated a relationship with a male, females of the sexual generation move to the tree and deposit a single large egg in their winter hideout beneath a bark flap. In this protected location, the egg passes the wicked winter. When the warmth of spring causes leaf buds to burst, tiny nymphs hatch from the surviving eggs and trek to rapidly expanding leaves. As nymphs feed on undifferentiated leaf tissue, they secrete compounds that co-opt the leaf’s genetic machinery and cause it to produce a strange looking structure called a gall. This gall bears a striking resemblance to the comb of a rooster, hence the name elm cockscomb gall.
Later in the season galls assume the brilliant red color of a cockscomb. This elm tree might hold the record for the most galls ever. Image credit: Bob Rabaglia
After reaching maturation in the hollow space within the cockscomb gall, the nymph completes her development and begins the task of reproducing asexually, meaning without the usual interlude with a male aphid. This parthenogenetic female is called a stem mother and she will give birth to hundreds of young within the confines of the gall. During spring and early summer, the inside of the gall is a riotous mass of aphids large and small sucking sap and producing honeydew. These nymphs develop into winged adults that exit the cockscomb gall through a slit on the undersurface of the leaf. By now the exhausted stem mother has perished, as her progeny depart for their summer feeding grounds. If you have a moment on a bright autumn afternoon, visit an elm tree and you may have a chance to witness an unusual migration by one of the interesting members of the aphid clan.
Acknowledgements
The fascinating article “Gall aphids on elm” by Edith Patch was used to prepare this episode. Special thanks to Bob Rabaglia for sharing his amazing image of elm cockscomb galls on what must be the most sought-after elm tree for these aphids.