Thecabius gravicornis (Patch)

Pseudo Galls. Fundatrix in small pocketlike swelling on upperside of leaf about halfway between midrib and edge of leaf, opening on under side; progeny on underside of leaves which are folded downwards together on midrib.

Collections. As winter host on Populus tacamahaca, in folded leaf pseudo galls. Colorado: Pennock Pass (above Masonville), Steamboat Springs and at Red Feather. Fundatrices Aug. 24, alate fundatrigeniae Aug. 18 and 24; rare, perhaps on account of rarity of host.

Thecabius (Parathecabius) gravicornis (Patch)

Galls are formed by folding leaves downward along the mid-rib to form a yellowish-green, swollen pocket which becomes filled with the developing emigrant alatae. The fundatrix inhabits a separate small pocket on the upper side of a leaf lamina (Palmer 1952). On Populus balsamifera in North America, widely distributed but apparently rather rare; also collected on P. angustifolia in Utah (BMNH collection, leg. R. Danielsson). Alatae (BL 1.7-2.8 mm, with olive-yellow abdomen) migrate in July-August, presumably to an unknown secondary host.

Thecabius gravicornis (Patch, 1913) (Pemphigus) [Folded-leaf balsam aphid].

Feeding by the stem mother adjacent to the midrib on the lower surface of the leaf blade initiates a small cavity in which apterae are produced. The apterae leave the cavity to feed on expanding terminal leaves. Their feeding causes the leaves to fold ventrally along the midrib to form a purse-shaped pseudogall similar to that of P. pseudobyrsa . In Quebec, alatae appear in the pseudogalls from mid-June to early July (Alleyne and Morrison 1977).

Primary hosts include P. balsamifera (Gillette and Palmer 1934) and P. angustifolia (Blackman and Eastop 1994). There has been one report of T. gravicornis on P. nigra L. var. italica (Alleyne and Morrison 1977). This aphid is apparently widespread but uncommon (Blackman and Eastop 1994).

Thecabius gravicornis (Patch)

Galls on Lombardy poplar (Populus nigra L. var. italica Muenchhausen) Pseudogalls formed by folding of young terminal leaves of Lombardy poplar [also keys to Thecabius populicondupliflius]

Pseudogalls of this aphid were found on most young, terminal leaves of a single Lombardy poplar plant at Pincourt, on Ile Perrot, just off the west tip of the Island of Montreal, Que., in the spring and summer of 1973. The stem mother feeds on the mid-ribs on the lower surface of the leaf. This results in the formation of a cavity (Fig. 12) in which the first generation apterae are produced. These apterae leave the confines of this cavity to feed on the young terminal leaves. The leaves fold ventrally (Fig. 13), and winged migrants are produced within the cavity between mid-June and early July.