Thecabius populimonilis

Dasineura p-trichocarpa-big-bud-gall

Aceria parapopuli

Saperda populnea

Thecabius populi-monilis

Host: Populus trichocarpha [tristis]

This is a common gall on black cottonwoods. In spring a single black aphid stem mother produces a red pocket, 5 to 15 mm long and 2 mm wide, that bulges up from the upper leaf surface. The gall lies between the midvein and leaf edge and runs parallel to the midvein. The underside of the leaf shows only the gall's slit opening. Usually only a single gall occurs on a leaf. The galls have a single chamber and, depending on the time of spring, contain one to several aphids. 

As the stem mother produces young in midsummer, they move out of the spring galls and travel to the new flush of summer leaves, where they produce many of their own summer galls (photo 25). The summer galls look very much like the spring galls, except that they are usually wider (5 to 7 mm) and are arranged end-to-end for the length of the leaf. They look like sausage links and have a single chamber each. Several rows of the summer galls occur on a single leaf, with the result that the leaf is often curled. The summer foliage is often very heavily galled by these aphids.

The galls are fairly simple in construction and design. The wall of both spring and summer galls contains no layer of thick-walled cells and no food cells line the aphid chamber. Unlike most free-living aphids that feed on phloem sap, these and other gall-forming aphids feed for part of their lives on the contents of non-vascular cells. Unlike other species of gall-forming aphids on cottonwood, T. populi-monilis probably completes its entire life cycle on black cottonwood. 

Cecidomyiid. Undescribed.
Big Bud

Hosts: Populus trichocarpa [tristis]

These galls (photo 27) are most easily spotted during the fall and winter when the leaves are off the trees. The gall is an enlarged terminal or side bud and will stay attached to the plant for some time, even after the gall has died and turned brown and brittle. In fact, a dead big bud from the previous year can frequently be found on the same twig with a current year's big bud. 

Attacked buds begin to swell in early May and are often covered with a yellow sticky resin. The bud is enlarged because the bud scales and stipules are thicker than normal. Nestled between the thick scales, you should be able to find several very small orange larvae that are often bathed in yellow plant resin. These are the midge larvae. They use the spaces between scales as their larval chambers. 

In August, the fully formed galled buds are green and about 2.5 cm long and 1.5 cm wide. All bud parts are enlarged. The outer scales are green, and the inner ones are yellow to cream-colored. By mid-October, the 5 to 15 larvae in each bud are white (because of stored fat) and have a prominent sternal spatula. Most larvae occur near the center of the big bud and are often awash in the sweet-smelling resin. The bud tissue near a larva is sometimes brown. The larvae probably feed by scraping the cells on the surface of the bud parts. Feeding may cause the browning. 

Eriophyes parapopuli Keifer

A fairly common gall on black cottonwood caused by eriophyoid mites, Eriophyes parapopuli Keifer, occurs at the base of buds along the stem. When branches are bare in the autumn, the galls are woody, brown, cauliflower-like, and rarely more than 2.5cm in diameter. Several occur along a single stem. The mites live in the cracks and fissures of the gall.

Saperda populnea

Long-horned beetles, (Saperda populnea L. or a closely related species) cause spherical or spindle-shaped stem galls (2 cm in diameter) on black cottonwood and on willows. Infestations are often heavy; the shape of the tree may be somewhat disfigured as a result. Adult females lay eggs in late spring in holes that they chew in the bark. When the egg hatches, the larva mines the cambium and then turns into the wood where it excavates a gall chamber. Growth from the wounded cambium causes the swelling. One or two adults emerge from each gall after one to two years of larval development. The adults feed on poplar leaves.

Common insect and mite galls of the Pacific Northwest