Extensive surveys of weedy plant species among hedgerows of different complexity and floristical composition were conducted with the aim to evaluate the role of weeds as reservoirs of agromyzids and to study the trophic relationships between these Diptera and noncrop plants. Field samplings were carried out weekly collecting foliage infested by leafminers from the representative weedy plants in three sites located in Bologna province, Italy. During 1998–1999, a total of 646 agromyzid specimens were reared, representing eight genera and 24 species. The most abundant species were Chromatomyia horticola (Goureau), Phytomyza lappae Goureau, Phytomyza ranunculi Schrank, Phytomyza plantaginis Robineau-Desvoidy, Liriomyza trifolii (Burgess), and Ophiomyia pulicaria (Meigen). C. horticola was the dominant species accounting for 35.76% of adults reared. Agromyzids mined 25 plant species belonging to nine families. The Asteraceae, in particular Cirsium arvense (L.) Scopoli and Sonchus asper (L.) Hill, were the most important sources of leafminers considering both species richness and population densities. Shannon–Weaver (H′), Simpson (D), and Berger–Parker (d) indices all pointed out that agromyzid species diversity was higher in the site characterized by the highest floristic richness, the highest complexity of ecological net and nonintensive agriculture. Most of the weeds sampled proved to be reservoirs for noneconomically important agromyzids in open-field crops in Italy.