Diverse targets and oviposition timings in seed-specialized wasps: insights from Cupressaceae and Pinaceae parasitism strategies

ABSTRACTS of joint IUFRO 7.03.01 "Cone and seed insects" and 7.03.04 "Diseases and insects in forest nurseries" Working Party Meeting. Vilnius, Lithuania, 16 - 20 September 2012 Diverse targets and oviposition timings in seed specialized wasps: insights from Cupressaceae and Pinaceae parasitism strategies Parasitism of discrete resources such as seeds is tightly linked to the synchrony between the timing of oviposition and host development. In seed-specialized wasps of the Megastigmus genus (Hymenoptera: Torymidae), the only nutritive tissues available to larval instars are the female gametophyte and the seed embryo. Because significant storage accumulation begins in the gametophyte if ovule fertilization has taken place, fertilized ovules obviously provide the more desirable oviposition targets. But among conifers, initiation of th e female gametophyte either occurs before pollination, coincides with pollination or is delayed for as much as a year. This study aimed at addressing the following question: do such diverse host ovule development processes involve diverse oviposition patterns in their specialized parasites ? We analysed and compared the timing and the target of oviposition in different Megastigmus species specialized on two different conifers, for which pollination is essential (Cupressaceae: Cupressus sempervirens L.) or not (Pinaceae: Cedrus atlantica Manetti ex Endl.) for gametophytes to develop. Carrying out extensive histological studies on successive stages of seed development before, during and after the oviposition periods of each Megastigmus species, we demonstrated that oviposition of M. schimitscheki Novitzky and M. pinsapinis Hoffmeyer on C. atlantica occurred about one month before ovule fertilization, at one of the earliest stage of gametophyte development (cœnocytic stage ). Conversely, M. watchli Seitner always oviposited after fertilization, during embryogenesis, on C. sempervirens . These results suggest that wasps specialized on Pinaceae seeds can select any of the potential seed within a cone as an oviposition site, and that their fitness does not depend on fertilization success. This may reflect a more simple and efficient oviposition strategy on Pinaceae than on Cupressaceae, where females have to locate a fertilized gametophyte. Ecological and practical implications of these findings are dis