Pemphigus populicaulis Fitch

The gall is yellowish green to creamy, formed by swelling of base of leaf lamina, twisted around broadened petiole, rather like that of P. nortonii and P. populiglobuli but with a small, rounded exit hole on the underside (Palmer 1952, Harper 1959a). Especially associated with P. deltoides and P. deltoides ssp. occidentalis (= monilifera), but also recorded from other cottonwoods and P. tremuloides. The fundatrix is pale yellowish to whitish green. Alatae (BL 1.7-2.0 mm) emerge in July-September. The secondary host has not definitely been established; Glendenning (1924) reported that it was Oenanthe sarmentosa, but did not provide evidence for this assertion. Hottes & Frison (1931) suggested that P. brevicornis Hart, described from corn roots, was the sexupara of populicaulis. Widely distributed in North America. Setzer (1985) studied gall mortality. Sokal et al. (1980) and Bird et al. (1982) studied geographical variation in eastern North America using multivariate techniques.

Pemphigus populi-caulis Fitch

Gall. Yellowish green to creamy, formed of swollen base of leaf twisted with broadened petiole; exit a small round hole on underside between leaf and petiole. Size 10-13 mm in diameter

Cellections. As winter host on Populus sargenti and rarely P. acuminata, in twisted petiole and leaf galls; alternate host unknown. Throughout Colorado and nothern Utah. Fundatrices June 12 to Aug. 29, alate fundatrigeniae June 20 to Aug. 29; common.

Chief Distinguishing Characters. Character, of gall with round exit hole.

This species is very close to P. bursarius (Linnaeus) and distinguished most certainly by form of gall. However, a number of collections of specimens ap parently inseparable from P. populicaulis have been taken in petiole galls re sembling those of P. populi-transversus. Galls of P. bursarius according to Tull gren (1909:114) and Theobald (1929:245) differ in more protruding lips. The question of the status of these forms requires further biological work.

Pemphigus populicaulis Fitch

Gall on Populus sargentii [deltoides] formed by swelling and twisting of petiole and leaf blade

P. populicaulis is more common in southern Alberta than any other gall-forming aphid on poplar. The galls are found on trees along river banks and in many of the shelter belts in the irrigated areas. The primary host of this species in Alberta is P. sargentii Dode.

The gall caused by this aphid is also formed at the junction of the petiole and blade of the leaf and is produced by a swelling and twisting at the base of the blade and at the top of the petiole (Fig. 3). In this respect it is similar in formation to the gall caused by P populiglobuli but differs in that it has a small, round exit (Fig. 19) whereas the exit of the gall caused by P. populiglobuli is a long, narrow, angular slit. When two galls of P. populicaulis are found on a single leaf, their shapes vary slightly from those of single galls (Fig. 19).

In 1924 Glendenning reported water parsley, Oenanthe sarmentosa Presl, as the secondary host of P populicaulis. Strickland ( 1953) recorded Pemphigus brevicornis Harr. from potatoes, beets, and celery in August and September. Hottes and Frison (1931) collected P. brevicornis on roots of an umbrella plant, Erigeron sp., and a goldenrod, Solidago sp., and stated that the specimens might be the sexuparae of P. populicaulis. Cutright (1925) considered P. brevicornis as a synonym of Pemphigus lactucae Fitch. Thus, if Hottes and Frison and Cutright are correct, the name P. populicaulis will be replaced by P. lactucae, which has priority.

Records of the distribution of P. populicaulis in Alberta are as follows: 1950, Picture Butte, Lethbridge, Coaldale, Taber, Vauxhall, Grantham, Turin, Iron Springs, Monarch, Macleod, Barnwell, Welling, Populus sargentii; 1953, Medicine Hat; 1957, Nobleford, Populus sargentii.

Pemphigus populicaulis Fitch 1859 [Poplar leaf-petiole gall aphid].

Galls are initiated on the upper leaf surface, with blade and petiole becoming twisted to form a more or less globular structure at the juncture of the leaf petiole and the leaf blade. The structure is composed of petiole tissue but also includes tissues of the leaf blade that usually extend some distance down onto the gall (Figs. 9 and 10). Twisting of the leaf tissues forms a semi-spiral groove that extends two-thirds of the diameter of the gall. One or a few small round openings in the groove of mature galls provide an exit for alatae. An average of 410 alatae per gall was reported for Quebec (Alleyne and Morrison 1977). An average of 123 alatae per gall (range: 8-426) was reported for southern Alberta, with their exiting from galls beginning in late June to mid-July and being 90% complete by September 11 (Harper 1959b).

The primary host is P deltoides (Palmer 1952; Harper 1959; Alleyne and Morrison 1977; Floate and Whitham 1993). Rare occurrences of P populicaulis have been reported on intersectional hybrids (Gillette and Palmer 1934; Floate and Whitham 1993). Secondary hosts include water parsley (Apiaceae: Oenanthe sarmentosa C. Presl ex DC.) and possibly fleabane (Erigeron spp.) and goldenrod (Solidago spp.) (Asteraceae) (refs. in Harper 1959a). Harper (1959a) cited references that raised the possibility that P populicaulis might be synonymous with P brevicornis. This synonymy is not supported in more recent work (Eastop and Hille Ris Lambers 1976).

Pemphigus populicaulis

An irregularly spherical gall at union of blade and petiole. The opening is closed by a twisting of the leaf. Common on several poplars. The insects emerge when the gall dries somewhat and opens, usually about the middle of the summer.

Pemphigus populicaulis

Twisted, subglobular galls with oblique openings at base of cottonwood leaves, diameter .25 to .5 inch, aphid

Pemphigus populicaulis
[Page 61]

Hosts: Populus tremuloides, balsamifera subsp trichocarpa, several cottonwoods

This gall, unlike the preceding species, has a spiral slit along which a series of small holes develop later in the season, through which aphids squeeze out.

Pemphigus populicaulis

Galls on Populus deltoides formed at junction of leaf blade and petiole by twisting and swelling with pore-like openings

This is the most common of the aphids forming excrescences on cottonwood in Quebec. The stem mother feeds at the petiole leaf-blade junction on the underside of the leaf. As a result of this feeding activity the petiole bends, sandwiching the stem mother between petiole and leaf blade. Once completely enclosed the stem mother reproduces, an average of 410 alates being produced from 20 galls examined. As the gall matures the upper surface becomes convex and changes in colour from green to red (Fig. 9). A series of small pores develop along the seam on the underside of the gall (Fig. 10) and migrants squeeze through them. Harper (1959) found this gall throughout Alberta on Populus sargentii Dode. Glendenning (1 924) reported water parsley, Oenanthe sarmentosa Presl., as an alternate host. Colonization trials involving a wide range of annual plants are currently under investigation.

Pemphigus populicaulis

This plant louse has been previously recorded from Illinois by Walsh (1862), LeBaron (1873), Thomas (1879). and Davis (1910d). It causes a gall at the base of the leaves of poplar somewhat similar to P. populi-transversus except that opening is a semi-circular curved slit. Under a discussion of P. brevicornis Hart we have already advanced our supposition that populicaulis is the aerial and gall-making form of the root-feeding plant louse described as brevicornis Hart. and the nomenclatorial changes that must result if this theory is substantiated. Data associated with our specimens collected in galls on species of Populus are as follows: Champaign (July 16). 18S5; Albion (May 26). Cave-in-Rock (May 30). Oakwood (July IS). Shawueetown (May 27). I'r-bana (July 10). 192S; Eldorado (June 19). Elizabethtowu (June 20), Kansas (June 17), Oakwood (June 1), Rock Island (July 7), Starved Rock State Park (July 5). 1929: Seymour (June 20), 1930.

Pemphigus populi-caulis Fitch.

[Two drawings of the gall appear on page 9 of the pdf]

The Gall.—The gall is semi-globular. Large ones attain a diameter of 1/2 inch or more. Normally located galls are at the base of the leaf where the blade and petiole unite. The gall is composed of the thickened and twisted petiole and a very small portion of the base of the leaf blade which frequently extends a short distance down onto the gall. The greater part of the gall extends below the plane of the leaf blade. Viewed from above the mouth is normally not visible and the midrib and the first diagonal veins on either side are seen to merge into the body of the gall.

When viewed from below the midrib is seen to bend abruptly and to merge with a ridge formed by the edge of the petiole which follows the curve of the gall and forms a part of one lip of the opening. The other lip of the opening is formed by a fold in the base of the leaf blade for a part of its length and the balance by the edge of the petiole. The petiole is twisted so that the flattened portion lies in nearly the same plane as the leaf blade instead of at right angles to it as in normal leaves. It is clear that almost the entire gall is made up of the much enlarged and twisted petiole. The mouth is usually full-lipped and extends more or less in a spiral fully 2/3 the distance around the gall. Many times there is a small round opening at some point along the lips through which the winged lice escape. The senior author has collected aphids which correspond in every respect to P. p. -caulis, from galls similar to those produced by P p.-venae at Oak Park MN, and in the Bitter Root Valley in MT. These were from Populus balsamifera trees heavily infested with P p-globuli and P. p-venae. It seems than under certain conditions at least P. p-caulis produces galls on the leaf similar to those produced by P. p.-venae.

This aphid was very abundant in Logan during the spring of 1925. The infestation was so severe that by July 1st many leaves had fallen from the infested poplars, Populus balsamifera and P deltoides. About 3 percent of the leaves remaining on the trees were infested with galls.

The aphids are ordinarily very numerous in these galls, and sometimes a hundred or more pupal nymphs and winged females in addition to the stem-mother are present in a single cell. Ordinarily, there is a large drop of liquid in the bottom of the gall. The waxy material secreted by the wax glands makes the inside of the gall grayish so that when it is opened, the moving aphids appear to be mixed up in a growth of fungus.

In addition to being present in Logan, this aphid was also found in Big Cottonwood Canyon, Utah, on June 29, 1925, where it was producing its characteristic galls. it has also been collected at Hyde Park during early July, 1928.

Notes:

Pemphigus populi-caulis and Pemphigus populi-globuli were described by Dr. Asa Fitch in the " Report of the Noxious and Other Insects of the State of New York," 1859.

Fitch's descriptions of the insects are too meager to make a separation of the species certain without the galls.

The gall produced by the aphis Fitch described as P. p.-caulis is described as a "bullet-like gall " on the middle of a leaf stalk from a Lombardy poplar. This gall was found on the 27th of June, three years before the description of P. p.-caulis was publishéd.

Discussing the gall further, Fitch says: "When the leaves fell from this tree in autumn, a few were found among them, more faded than the others and having these same excrescences, but placed at the base of the leaf instead of on the middle of its stem"

Farther on in his discussion, Fitch states: "Galls analagous to those here described grow upon the leaf stalks of the Lombardy and Black poplar in Europe, from attacks of the Pemphigus bursarius of Linnaeus. "

After comparing it with specimens of that species and its galls, Fitch considered his aphid different from P. bursarius, the fly being paler in its color, and its gall because of spirally coiled somewhat like the shell of a snail."

It is very evident from Fitch's discussion that he had in mind, when he described P. p.-caulis, a gall located on the petiole of the leaf at some distance from the leafblade, and one produced by a twisting of the petiole into a spiral.

It is quite probable that the petiole gall described by Fitch is not typical of P. p.-caulis, but that those at the the base of leaf which he mentions as being taken in the fall are the typically located galls.

The aphid herein figured and described as Pemphigus populi-caulis is from galls located at the base of the leaf and produced by a twisting or spiralling of the petiole. The species described as P. p.-globuli, is considered to be the globuli of Fitch because of the likeness of its gall to the one described by Fitch in his discussion of that species.

Since the original descriptions of P. P.-caulis and P. P.-globuli were published these species have received much attention, if we may judge from the frequent references to both. However, in spite of this, several aphidologists appear to have failed to distinguish between them.

P. p.-caulis and P. p.-globuli are very similar structurally and their galls are, in a general way, much alike, although quite different when studied in detail. This general similarity of both the aphids and their galls appears to have caused considerable confusion in the literature. This was probably increased by a lack of detail in the earlier descriptions.

In studying the two species under discussion, the senior author has collected material from several states and in many localities in some of these states. When collections of both species have been made in the same locality, they rarely have been found on the same host species. In Colorado and Utah P. p.-caulis was taken on Populus deltoides and P. p.-globuli on Populus angustifolia and Populus balsamifera. In California P. p.-caulis was taken on Populus Fremontii and P. p.-globuli on Populus trichocarpa. P. p. -caulis was taken on Populus deltoides at St. Cloud, Minnesota, and both species on Populus balsamifera at Oak Park, Minnesota. East of the Rocky Mountains in Montana P. p.-caulis was taken on Populus deltoides and P. p.-globuli on Populus angustifolia. P. p.-caulis was taken on Populus balsamifera in the Bitter Root Valley of Western Montana and P. p.-globuli on the same species of Populus near the Flat Head Lake. The junior author reports both species on Populus balsamifera. These observations appear to indicate that Populus balsamifera is the only common host of P. p.-caulis and P. p.-globuli east of the Rocky Mountains. Populus trichocarpa may serve as a host for both in the range of this tree.

These collections have been carefully separated according to the type of gall and a large series of the aphids from each gall type critically studied. These studies have brought to light certain structural differences by which P. p.-caulis and P. p.-globuli alate forms from the galls may be separated. These differences are constant in relative magnitude but not in actual magnitude.

Both species under discussion are quite variable in size and in some other characters. This necessitates studying several individuals and comparing the means of characters in order successfully to separate them. When a sufficient number of individuals to make a representative population is studied, separation becomes easy and certain.

Thomas, in the "Third Annual Report on Noxious and Beneficial Insects of Illinois," 1879, pictures a typical Pemp. populi-globuli gall for the gall of Pem. populi-caulis. Dr. O. W. Oestlund described Pem. populi-caulis in his synopsis of the Aphididæ of Minnesota. His description is not sufficiently detailed to determine which of the two species under discussion he really had. The host is given as Populus monilifera.

Essig, in Pomona College Journal of Entomology, 1912 describes and illustrates Pem. populi-caulis and several galls. These galls are of 3 quite distinct types. One is typical galls. of Pem. p.-caulis. Several are typical Pem. p.-globuli and one is typical of what the senior author is calling Pem. p-venae. The senior author has collected all three types of galls on Populus trichocarpa in California. The aphids from the typical Pem. p.-globuli and Pem. p.-caulis were the ones used in the studies already referred to. The ones from the other type of gall do not fit with either Pem. p.-caulis or Pem. p.-globuli. Several collections of these galls have been made in Minnesota. The aphids from Minnesota differ from the species being discussed in the same way as those from California. This species is described later as Penz. P.-venae. The descriptions of Essig are not sufficiently detailed to enable one to determine his species with certainty. However, his figures of the antenna, of the alate viviporous female and the stem mother are typical of Pem. p.-caulis.

It seems quite probable that Essig failed to separate the 3 species in his material.

Dr. Edith M. Patch described Pem. populi-caulis from Populus balsamifera and pictures 1 typical Pem. populi-globuli gall and 2 not typical of this species or of Pem. populi-caulis. The latter are similar to galls taken by the senior author at Oak Park, Minnesota, produced by what he calls Pem. populi- venæ.