Phytobia is a genus of flies in the family Agromyzidae,[5] with a worldwide distribution principally in Europe and the Americas.

[5] All known larvae of Phytobia feed on the young xylem within stems of woody plants, creating concealed galleries that may reach considerable lengths (reportedly up to 17 m in P.

[6][7] P. betulae lays its eggs on the bark of new-growth twigs of suitable birch trees and, after hatching, larvae tunnel downwards along the shoot within the differentiating xylem layer, sometimes reaching the base of the tree.

[6] Confirmed host plant families for Phytobia species include Betulaceae, Fagaceae, Rosaceae, Salicaceae, Sapindaceae, and Cupressaceae in North America, Europe, and Australia, with Asteraceae also hypothesized to be a host family based on the finding of possible larval sign on the shrub Wedelia calycina in Guadeloupe.

[6][7] Larval tunnels mar the appearance of certain woods used commercially, and thus may be of economic importance.