Pemphigus rhois, Fitch.

Host Rhus typhina L.

A balloon-shaped gall with the regularity of its outline destroyed by the elongated lobes that cover its surface. A gall is shown in Fig. 14. The epidermis is slightly pubescent and coloured red, shading into yellow and green. It originates from the under side of the leaf, and the point of attachment on the upper side is indicated by a small papilla covered with a dense pubescence. These galls vary in size from very short types less than i cm. to those that are 4 to 5 cm. in length. composed of the ordinary vascular elements of the normal leaf that have been stimulated to increased activity. There is not a special tracheary system originated for the gall. In the galls the strands occupy a definite position, since in the normal leaf they occupy a definite place in relation to the spongy and the palisade parenchyma. Large glands are present in the gall tissue, as shown in Fig. 14. A gland is found invariably asso- ciated with a fibro-vascular strand and seems to have its counterpart in the very small gland that runs through each vein of the normal leaf. In some cases the abnormal glands have acicular trichomes projecting into their cavities.

Melaphis rhois (Fitch, 1866)

Hosts: Primary hosts: Rhus glabra, Rhus typhina.

[Life history and gall description of both M asafitchi and M rhois; the species differ only by adult anatomy and genetics:]

Migration from moss begins in mid-May in Ontario and west Québec, with sexuparae forming loose aggregations and depositing sexuals on previous year twigs of Rhus at the time that expansion of current year shoot buds begins. Small aggregations tend to occur within a few centimetres below the expanding leaf, but large aggregations may consist of >100 individuals scattered along 0.5 m of stem. There they deposit larvae of the sexual forms and die immediately thereafter. Although sexuals are initially found scattered along the stem, moulting individuals are more densely clustered close to the base of the expanding shoot (but usually still below the lowest of these shoots) suggesting a further active aggregation by first instar sexuals. The sexuals undergo four moults over a period of four or five days.

Deposition of larval fundatrices begins two to three weeks after copulation, i.e. in early to mid-June. The first instar fundatrix moves to the distal unexpanded leaves of the new shoot, where it may remain for several days, apparently initiating feeding only after the leaflets begin to unfold. Within one or two days of settling, feeding by the fundatrix on the upper (adaxial) surface of a leaflet results in the formation of an obvious pocket on the abaxial surface of the leaflet opening onto the adaxial surface. Subsequent instars of the fundatrix occur within the closed gall. As a consequence of aggregation by the sexuparae, galls tend to occur in clusters, so that a single compound leaf usually bears several galls (Fig. 6) and often many leaves of a single shoot are affected.

At least three generations develop within the gall, namely the fundatrix, at least one generation of apterae, and the alate emigrants. We have found living fundatrices containing embryos as late as mid-August. Adult alatae begin to appear within the gall in mid to late August (in North Carolina, alates collected as early as 7 August (slides in USNM)) and continue to develop throughout September; some alatoid nymphs may still be present in galls as late as early October. The gall opens by formation of longitudinal slits in the neck of the gall after the majority of individuals have become adult. In Ontario, migration to moss occurs over an extended period from the mid-September to early October, as the host leaves begin to senesce and fall; in Virginia, United States of America they “may be found flying at the last of October” according to Baker (1917). During this period, infested leaves are often apparent from a distance because they tend to senesce and redden earlier than other leaves of the same plant. Open galls are subject to heavy predation by Syrphidae (Diptera) larvae. The number of alates produced in a single gall is usually in the hundreds, but we have observed up to 3750 developing within large galls.

Gall. Initiated by feeding by the first instar fundatrix on upper surface of the lamina of a leaflet adjacent to major veins (not “am Mittelnerv”, i.e., “on midvein”, as stated by Mordvilko (1935)). A spherical pocket extending above and below the blade forms around the fundatrix and subsequently expands below the blade, usually also with a conical projection above the blade, the opening becoming closed by trichomes at adaxial opening and by tissue expansion within neck. Immature gall globular (Fig. 6A), pale green, about 2.0–2.5 mm in diameter at the time the fundatrix undergoes its final moult to adult. Veins adjacent to gall become more or less swollen and adaxial surface reddened. Mature gall (Fig. 6B–D) globular to pyriform, unilocular, largely red in colour in exposed locations, but usually with pale yellow to yellow-green areas on the side protected from the sun, pale areas more extensive in shaded locations; up to 50 mm in length and up to 45 mm at greatest diameter. The gall surface is often irregular to weakly lobed. Galls distinctly bilobed to near the base occasionally occur, and very rarely galls have several deep lobes. It is not clear if these are the product of a single fundatrix (with lobes possibly resulting from a leaflet vein interfering with gall expansion) or result from the settling of more than one fundatrix at the same site; we have found recently initiated galls with more than one fundatrix, but only one surviving beyond first instar. Vestiture of the gall surface reflects the general vestiture of the host plant: those on Rhus glabra are largely glabrous while those on R. typina are finely setose.

Distribution [M rhois only]: Widespread in United States of America and in southeastern Canada. Canada: southern Ontario, southern Québec, Nova Scotia. United States of America: Arkansas, Arizona, Georgia, New Jersey, Pennsylvania, Washington (all confirmed by COI-5P sequence); Kentucky, Maine, Massachusetts, New Hampshire, North Carolina (based on morphology only).