Modification of galls ofDiplolepis triforma(Hymenoptera: Cynipidae) by the parasitoidsEurytoma spongiosa(Hymenoptera: Eurytomidae) andGlyphomerus stigma(Hymenoptera: Torymidae)

Stem galls induced by the cynipidDiplolepis triformaShorthouse and Ritchie are frequently inhabited by several species of parasitic chalcids and ichneumonids that consume inducers or other parasitoids. Two of these parasitoids, the eurytomidEurytoma spongiosaBugbee and the torymidGlyphomerus stigma(Fabr.), consume gall tissues after they have consumed their insect hosts and are considered entomophytophagous. Both are koinobiont ectoparasitoids.Eurytoma spongiosaoviposits within gall chambers and attacks larvae of the inducer throughout the summer season, from the early growth phase of the galls until the galls mature. If the inducer is killed byE. spongiosain early summer, nutritive gall cells degenerate into vacuolate parenchyma and are consumed. If the inducer is killed later in the summer, when galls begin to mature, nutritive cells persist in the chambers for about 1 week before degenerating.Glyphomerus stigmakills and consumes inducer larvae when galls are maturing. New nutritive cells appear under the influence ofG. stigmaand are then consumed. The ability of entomophytophagous chalcids to promote the formation of gall cells provides insight into the derivation of the gall-inducing guild.