Gall Formation in Crofton Weed, Eupatorium adenophorum Spreng (syn Ageratina adenophora), by the Eupatorium Gall Fly Procecidochares utilis Stone (Diptera, Trypetidae)

Abstract

Procecidochares utilis lays eggs on the stem apex of Eupatoriurn adenophorurn and on hatching the larvae tunnel into the stem. In response to the presence of the larvae a gall forms in the stem which may contain from 1 to 23 larvae. Callus tissue differentiates and divides to block the entry passages and seal the larvae in the stem. The normal development of the stem is halted and it swells as the pith cells continue to divide and become gall parenchyma. A layer of highly meristematic nutritive tissue develops around the larval cavity on which the growing larvae feed. New vascular tissue differentiates in the pith region of the gall around the larval cavity. Growth of the gall ceases when the larvae pupate, by which time most of the nutritive tissue has been consumed and the cells in the pith region have enlarged. Just prior to pupation the mature larva cuts a cylindrical tunnel to the edge of the gall, leaving only the epidermis intact; it then returns to the central cavity to pupate. The adult fly escapes by breaking through the epidermal 'window' at the end of the cylindrical tunnel.