Pontania proxima is a common sawfly which makes bean-like galls on willows. Oviposition and induction of gall formation occur only on young leaves with very active metabolism. The development of the gall takes place even if no egg was laid. The leaves of Salix fragilis are very heavily infested by the galls of P. proxima , sometimes with 25 galls to a leaf. The species is parthenogenetic, the male being extremely rare. Incubation time and the duration of the different larval instars of P. proxima vary considerably. Hymenopterous parasitoids are of lesser importance as mortality factors of P. proxima . Only the ichneumonids Lathrostizus lugens Gravenhorst and Scambus vesicarius Ratzeburg are more common. But there are three parasitic inquilines, the weevils Curculio crux Fabricius, C. salicivorus Paykull and the tephritid Euphranta (Staurella ) toxoneura (Loew), the larvae of which develop in the galls and eliminate a substantial portion of the host larvae. These insects are partly entomophagous and therefore remarkable outsiders among their biologically quite uniform relatives. none