"poison in my food during Ferrara and during the Great Big Siege of Bologna"

Ferrara is a city and province of the same name in north central Italy. It’s west of Venice, considerably north of Rome, but only a short distance north/northeast of Bologna.

Both cities are situated in the Po River valley, although Ferrara stands on one of its main tributaries while Bologna is only near the river. Ferrara has been designated a World Heritage Site by UNESCO for its beauty and cultural significance, while Bologna is highly industrial and cosmopolitan, and home to the oldest university in the Western world, the University of Bologna, founded in 1088.

When Yossarian refers to these two cities here, however, he is speaking only of his squadron’s bombing missions against them.