Life History and Gall Development of Mordwilkoja vagabunda (Homoptera: Aphidae) on Populus deltoides. Part II—Gall Development1

Fundatrices of this aphid cause the formation of galls from leaf stipules of the cottonwood. Prior to gall stimulation they inhibit elongation of the stem terminals, and in Minnesota such elongation is brought to a complete stop in about 30 days. Galled stem terminals averaged only 79 mm. long at May 30 and showed no growth thereafter, while nongalled stems continued growing to an average length of 252 mm. at August 30. Infested stems average 8 leaves, noninfested ones 23 leaves per stem. Swelling and reddening of infested stems were the first observed symptoms of gall development. Most of the gall tissue was formed during early stages of gall growth, and subsequent development was confined to cell enlargement, changes in cell shape, and development of intercellular spaces. Galls, at first bright red, became yellow-green and finally turned dark brown at maturity, when their height ranged from 40 to 60 mm. Their greatest growth occurred from June 8 to June 26. Natural exit holes developed prior to the appearance of alate fundatrigeniae within the galls. There were three general phases in the formation of these galls: (1) initial stem inhibition, resulting in cessation of terminal growth; (2) cell formation and differentiation, explained on the basis of accumulation of a growth substance; and (3) expansion of the laminae, resulting from increased availability of nutrients that otherwise would be used in stem elongation. Gall tissues maintained a meristematic appearance throughout all stages of gall development.