Egg maturation in an invasive gall wasp, Dryocosmus kuriphilus (Hymenoptera: Cynipidae): An experimental test of the pro-ovigenic and facultatively synovigenic hypotheses

Abstract

There are two hypotheses on egg maturation in the invasive chestnut gall wasp, Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae). That it is pro-ovigenic (most or all of its potential lifetime egg complement is mature upon emergence) or facultatively synovigenic (not all eggs are fully developed upon emergence and may be resorbed when suitable hosts are absent). These hypotheses were tested by determining the effects of adult age and food (honey) on egg maturation in D. kuriphilus wasps with no access to host plants. Egg load (the number of mature eggs per female) neither increased nor decreased with adult age in the presence or absence of honey when deprived of host plants. These findings support the pro-ovigenic hypothesis. Some eggs mature during the adult lifetime of cynipoid parasitoids even without hosts, but the cynipoid gall inducer, D. kuriphilus, is pro-ovigenic, probably due to the abundance of chestnut buds available for oviposition under natural conditions. In addition to no competition for oviposition resources, thelytokous reproduction, unintentional introduction of infested chestnut trees and escape from host-specific parasitoids in introduced countries, pro-ovigeny with a high egg load has presumably resulted in D. kuriphilus becoming a global pest of chestnuts. In addition, body length, mesosomal and metasomal lengths and widths, hind femoral length, hind tibial length, and egg load as well as egg width of this wasp were also measured. There was no variation in egg width, but all the other measurements were positively correlated with egg load. Large females of D. kuriphilus had higher egg loads than small females.