Poplar gall aphids—Pemphigus and Thecabius spp.
Several Pemphigus and Thecabius spp. aphids (Aphididae) cause galls on cottonwood and poplar leaves. Their alternate hosts include lettuce, related weeds, and various Brassicaceae.
The lettuce root aphid, Pemphigus bursarius, commonly galls Lombardy poplar leaves during the spring. It also feeds on lettuce roots in clustered colonies covered with white powdery wax.
Lettuce root aphid and other Pemphigus and Thecabius spp. that infest cottonwood and poplar can be distinguished from most other aphid species by their short antennae (less than one-third body length) and the absence of obvious cornicles (tailpipelike tubes on top their rear). Their body is gray, pale green, yellow, or white, varying by the age and species of aphid and the host plant.
Aphid feeding on leaves or leaf petioles stimulates plant tissue to form a hollow gall around the aphid. The enclosed aphid gives birth to nymphs, many of which develop wings and emerge and migrate to feed on the same plant or alternate hosts, depending on the aphid species and time of year. The lettuce root aphid migrates from poplar leaves to feed on the basal stem and roots of lettuce and other alternate hosts, then in the fall flies back to poplars where females lay overwintering eggs.
Poplar gall aphids cause relatively harmless galls on cottonwood and poplar leaves. On their alternate host lettuce, plants heavily attacked by P. bursarius may wilt during the day. Developing lettuce heads remain soft and fail to develop properly and yields are reduced. Extremely heavy aphid populations over a prolonged period can cause collapse and death of lettuce.
These aphids can be significant pests on their vegetable crop hosts. They are harmless to poplar and no control has been shown to be effective or is recommended on trees.
Aphid galls on poplar leaves
Poplar stemgall aphids exposed
Adult lettuce root, or poplar gall, aphid
Waxy aphids on lettuce roots