Sometimes the size of the host where the parasitoids developed are responsible for this (i.e. small hosts > apterous). Not sure if this is true for this species.
Yes that might be an idea too, sometimes wings polymorphy is related to different life strategies (e.g. migratory or dispersing speciemens with little reproduction are fully winged, whereas specimens with not migratory but with high reproduction are brachy- or apterous), or habitat stability (in temporary habitats more fully winged specimens occur). This is e.g. found in Heteroptera.