pdf not downloadable, probably was missing from the website. none Entered From California, U.S.A. and incorrectly identified as Mastrus ridibundus (Gravenhorst), the parasitoid Mastrus ridens Horstmann (Hymenoptera: Ichneumonidae) was introduced in Argentina in 2005 for the control of Cydia pomonella (L) (Lepidoptera: Tortricidae). From 2005 to 2009 a total of 185.268 adult M. ridens individuals, were released in 58 sites in pear, quince, walnut and apple orchards in Mendoza province. Samplings carried out in 2005 and 2007 showed the overwintering as prepupae and a dispersal of approximately 2000 meters from the site of release. The establishment of M. ridens was assessed from 2006 to 2009 through parasitoidism, with a range of 0-45.3%. The parasitoidism average from 2005 to 2009 was 9.89 %, significantly higher than parasitoidism of native species (0.25%).