Elm trees had all but disappeared from US streets after Dutch elm disease swept across the continent during the 20th century. Now, many Dutch elm disease resistant or tolerant cultivars are available, having undergone years of development and evaluation. Urban foresters are always thirsty for new trees that perform reasonably well along streets and in other urban settings. So now these new elms are being planted in enormous numbers across the country.
It remains to be seen how well these elms hold up as they are susceptible to many pests and structural problems. The wooly elm aphid, Eriosoma americanum, is one of those pests. Besides basic life history, this creature it is essentially unstudied. However, I found a cool publication from 1913 and one from 1910 that describe the appearance and natural history of this and other aphids on elm. (Both of these publications are by Edith Marion Patch, who deserves a whole post or book of her own. She was a scientist and writer who organized the department of entomology at University of Maine in 1901 and was its head for 33 years).
I am in Eugene, Oregon and saw wooly elm aphids congregating on elm trunks while ants were collecting and eating them. Wooly elm aphids each lay a single egg in a nook or cranny of elm bark in late summer or early fall. In spring those eggs hatch and the aphids climb up to feed on elm leaves and mature. These mature aphids produce a second generation asexually that mature into winged females. The female aphids fly to Amelanchier trees where they produce another asexual generation on the leaves. After one generation on leaves the aphids crawl down and feed for multiple generations on Amelanchier roots forming dense colonies.
Finally, a winged generation is produced that flies back to elm to produce a sexual generation of males and females. These females oviposit and die relying on their lone egg to overwinter. The aphids I am seeing now in Eugene are probably the females trying to lay their one egg. Unfortunately for them, there are other phloem-feeding insects in the tree attracting ants, which are grabbing aphids as they head up and down the trunk.
Damage by wooly elm aphids on elm is usually not severe. Leaves become curled from the edges inward and contain hundreds of aphids along with wooly wax. However, as I mentioned, there are many other elm pests including several other aphids described in books by Edith Marion Patch and fact sheets such as this one from the Ohio State University.
Patch, E. M. (1913). Woolly Aphid of the Elm, Maine Agricultural Experiment Station.
Patch, E. M. (1910). Gall Aphids of the Elm, Maine Agricultural Experiment Station.