Entomologia Experimentalis et Applicata 96: 253–263, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 253 Spruce stands provide natural food sources to adult hymenopteran parasitoids of bark beetles E. Hougardy 1 & J.-C. Gr´ egoire 2 Laboratoire de biologie animale et cellulaire, C.P. 160/12, Universit´ e Libre de Bruxelles, 50 avenue F.D. Roosevelt, 1050 Bruxelles, Belgique 1 Fonds pour la Formation ` a la Recherche dans l’Industrie et dans l’Agriculture 2 Fonds National Belge de la Recherche, Scientifique Accepted: May 11, 2000 Key words: bark beetles, Scolytidae, Ips typographus, natural enemies, parasitoids, adult feeding, natural food resources, Coeloides bostrychorum Abstract Adults of synovigenic hymenopteran parasitoids of bark beetles need supplemental feeding to maximise their lifetime realised fecundity, and yet little is known about the natural sources that provide this food. Here we show that, in spite of the reduced diversity of flowering plants expected in even-aged spruce plantations, several plant species are present that might provide nectar and pollen to the hymenopteran parasitoid complex of Ips typographus L. (Coleoptera: Scolytidae), throughout the growing season. Conifer aphids are also abundant in these stands and produce large amounts of honeydew, which might also be consumed by the parasitoids. Laboratory exper- iments demonstrate that longevity is significantly increased in Coeloides bostrychorum Giraud (Hymenoptera: Braconidae) when the wasps are exposed to flowers of various species commonly found in spruce stands. Introduction Most hymenopteran parasitoids of bark beetles are synovigenic: females emerge with a small number of large, nutrient-rich eggs and new eggs can be pro- duced if food resources are available (Gauld & Bolton, 1988). Numerous authors have provided evidence of the major role played by adult nutrition in the biol- ogy of synovigenic parasitoids (Leius, 1960, 1961a, b, 1963, 1967; Ryan & Rudinsky, 1962; Doutt, 1964; Bushing, 1967; Jervis & Kidd, 1986; Mendel, 1988; Campos & Gonzalez, 1990; Mathews & Stephen, 1997, 1999). The availability of food and moisture can increase longevity and fecundity of parasitic wasps, increasing their potential impact as biological control agents of insect pest populations. Little is known about the range of natural food sources exploited by most adult parasitoid species (Jervis et al., 1992). Although host feeding is time- and energy-saving – because adult feeding and ovipo- sition occur at the same place (Askew, 1971) – the consumption of host fluids or tissues seems to be un- common in parasitoids of bark beetles (Mendel, 1988; Stephen, 1995). Roptrocerus xylophagorum Ratzeburg (Pteromalidae) is the only parasitoid species known to host feed: females, on apparently rare occasions, build a feeding tube to get access to the haemolymph of the concealed host (Bushing, 1967). Beside this species, bark beetle parasitoids may feed from non- host food resources including flower nectar, pollen or homopteran honeydew. The presence of food sources could be a favourable factor in the effectiveness of parasitoids. The aim of this study is to investigate the natural food re- sources available for forest parasitoids, especially hymenopteran parasitoids of Ips typographus L., a well-known pest of spruce in Eurasia. This study starts from an hypothesis developed by Stephen (1995) to explain the poor ability of hymenopteran parasitoids to regulate Dendroctonus frontalis Zimmermann pop- ulations in pine forests of south-eastern United States. Stephen et al. (1997) noticed in one large infestation