Thecabius populimonilis (Riley)

The galls on Populus spp. (angustifolia, balsamifera, fremontii, trichocarpa) are rows of ovoid, bead-like cells, pale green or with a reddish tinge, developed from upper side of leaf lamina on both sides of mid-rib (see influentialpoints.com). Each gall in spring contains one developing aptera or alata. The fundatrices inhabit similar, solitary galls. This species is apparently facultatively monoecious; Gillette (1913) suggested that the large alatae (BL 2.7-3.0mm) produced in individual galls in June-July might recolonize cottonwoods, but it seems more likely that they are emigrants and found colonies of apterous exules in wax on Salix roots such as those observed by Lange (1943) in California. Presumably alatae produced in these colonies in October are sexuparae and return to Populus. However, alate sexuparae (BL 1.7-2.0mm) are also produced in galls on Populus, up to 12 per gall, in August-October (Maxson & Knowlton 1929, Palmer 1952, Harper 1959a). A more recent study by Aoki et al. (1996b) has confirmed most of these observations; they found heteroecious and monoecious life cycles at different locations, but their data on sex ratios indicate that some gene flow is probably occurring, rather than that two separate species are involved. Pike et al. (2012) have also now confirmed by DNA barcoding that colonies on willow roots (probably Salix dodgeana) in Washington are T. populimonilis. This species is widely distributed in USA, western Canada and Mexico. (Pemphigus saliciradicis may be a synonym; see note under that species.)

Thecabius populi-monilis (Riley)

Gall. Pale green. The fundatrix gall containing a solitary fundatrix consists of a pocket like gall on upperside of a leaf at the base of the new growth, gall opening on underside, her young migrating soon after birth and locating separately near the margins on the underside of new leaves, forming a row of pocketlike or beadlike galls each 6-10 in diameter, each containing a solitary aphid, apterous or alate. The alatae migrate, probably to another tree. The apterae produce alate virgogeniae, which form new galls, and in August and September, alate sexuparae which develop in the gall with the mother. The sexuales are certainly deposited on the Populus, some were found in the galls in October; but certainly not all are deposited in the galls since the sexuparae are alate and are found leaving the galls as early as August 9.

Collections. On Populus angustifolia and P. tamacahaca, rarely on P. sargenti, in galls on leaves (apparently no alternate host existing). Throughout region. Fundatrices June 25, apterous and alate fundatrigeniae July 17 to Oct. 18, alate sexuparae Aug. 9 to Oct. 18, sexuales Oct. 18; rather common.

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.

Thecabius populimonilis (Riley)

Row of galls on either side of mid-rib of P. angustifolia or P. balsamifera

T. populimonilis is found on small trees along river banks in southern Alberta. The hosts of this species are P. angustifolia and P. balsamifera.

The galls of this species do not contain colonies but only a single fundatrix or fundatrigenia. The fundatrix causes the gall, in which the young are produced, but shortly after the young are born they leave the gall and start to feed on the same leaf or on another leaf, causing galls to be produced at the point of feeding, usually in a row on either side of the mid-rib. The galls are pale green but may have a reddish tinge (Fig. 8).

Records of the distribution of T. populimonilis in Alberta are as follows: 1948, Elk Island Park (Strickland, 1953); 1951, Lethbridge, Populus angustifolia; 1956, Monarch, Picture Butte, P. angustifolia; 1957, Spirit River, P. balsamifera.

Thecabius populimonilis (Riley, 1879) (Pemphigus) [Bead-like cottonwood gall aphid].

The life cycle of this aphid is reviewed by Gillette (1913) and by Maxson and Knowlton (1929). The stem mother feeds on the underside of the leaf blade, causing formation of a blister-like pseudogall that projects above the upper plane of the leaf surface. Her offspring (the second generation) are apterae that leave the gall shortly after birth to each induce their own similarly shaped gall, either on the same leaf or on adjacent leaves of the same shoot. This results in one or more rows of galls forming parallel to the midrib, with each gall containing one aphid (Fig. 17). Most of these aphids become alatae that migrate to other trees. However, some individuals stay in the galls and remain as apterae to produce apterous offspring (the third generation). These apterae leave the gall to move onto adjacent leaves to induce new galls and produce offspring, a portion of which develop into winged sexuparae. Gillette (1913) reports that 10–12 sexuparae may occur in one gall with the parent. The sexuparae exit these galls in late summer to produce male and female sexuales, which mate and produce the overwintering egg from which the stem mother develops in the following spring.

Primary hosts include P. angustifolia and P. balsamifera (Maxson and Knowlton 1929; Gillette and Palmer 1934; Harper 1959 a ; Floate and Whitham 1993). There is likely no secondary host.

Pemphigus populivenae Russo seems to have completely mistaken Thecabius populimonilis for Pemphigus populivenae]
[Page 62]

Hosts: cottonwoods

Often, the galls are located in rows along the midrib or along the edges of leaves.

Thecabius populi-monilis (Riley).

This aphid forms characteristic rows of bead-like cells, particularly on the upper sides of the leaves of narrow-leaved cottonwood, Populus angustifolia, and to a less extent on some of the wider-leaved species of poplars. The stem mother produces a gall similar to that formed by her young. These galls occur singly, usually between the midrib and the margin of the leaf. The first bead-like rows of galls are produced by the young of the fundatrix. Only a single individual is found in each cell.

According to Gillette, most of the second generation lice become winged and leave the galls, thus serving to scatter the species from tree to tree. A portion, especially of the later lice that are born, remain apterous, remain in the galls, and give rise to the third generation. These young, like the young from the stem mother, also migrate to the new leaves to continue the production of galls each of which harbors but one louse at first, but a portion of the young of this brood remain with the mother in the gall and become winged sexupara, of which 10 or 12 may be found in a single gall with the parent. Emergence of these sexupara begins about the first week of August and continues until the last of September; soon after emergence the sexupara give birth to the se.xual forms.

On the narrow-leaved cottonwood, the galls are most commonly in rows, on one or both sides, and rather paralleling the midrib. On P. balsamifera, the rows may follow the margin, midrib. parallel the midvein, or run at different angles on different parts of the leaf. Sometimes three or four rows of galls will occur on the broad part of the leaf.

The stem mother usually has 5-jointed antennæ and is wingless. The winged individuals of the second generation emerge from the galls within a period of a few weeks. The mouth of the gall opens slowly, finally permitting the winged female to emerge. Predators frequently enter the opening galls and kill the pupal nymphs. Several species of syrphid larvæ and a small hemipterous insect were the forms most commonly observed in Utah.

Wingless females were numerous in the galls at St. George, on July 9, 1925.

This species has been collected in Utah from Blacksmith Fork Canyon, Cedar City, Eden, Farmington, Hyde Park, Murray, Logan, St. George, Salt Lake City, Santa Clara, and Smithfield.