Seasonal Development and Emergence of Two Species of Gall-Forming Aphids, Pemphigus bursarius and P. nortoni, Associated with Poplar Trees in California
Several phases of the biology of Pemphigus bursarius (L.), on the primary host, Populus nigra italica, and Pemphigus nortoni Maxson on Populus fremonti were studied at Davis, California. Seasonal development within the galls, formed on leaf petioles, was determined by periodic dissections of the galls, and periods of emergence of the aphids, and numbers emerging were determined by caging galls. Fundatrigeniae of Pemphigus bursarius began to emerge in early May, approximately 1–2 weeks before P. nortoni. Emergence of P. bursarius continued until the end of June. A maximum number of 341 alates emerged from a single gall. August 26 was the latest date of emergence of P. nortoni from a gall. It yielded 1091 alates. The numbers of aphids emerging from the galls of both species were considerably less than the numbers of first instars produced. Physical factors may have contributed to this mortality, but biological agents, such as a fungus and several kinds of predators, were most important. Alienicolae of P. bursarius colonized several species of secondary hosts in the family Compositae. The alienicolae were offspring of fundatrigeniae collected from the caged poplar galls. Attempts to establish colonies of P. nortoni on potential secondary hosts were unsuccessful.