Ecological Entomology (2007), 32, 201–210 © 2007 The Authors Journal compilation © 2007 The Royal Entomological Society 201 Introduction Inbreeding depression is common in many taxa (see reviews in Husband & Schemske, 1996; Lynch & Walsh, 1998), and is predicted in large outbreeding populations as a consequence of the accumulation of deleterious recessive mutations (Davenport, 1908; Bruce, 1910; Haldane, 1927). Furthermore, Lynch and Walsh (1998) note that inbreeding depression tends to affect traits directly related to fitness such as viability, rather than morphometric traits. Inbreeding depression may be prevalent for traits that are highly related to fitness, because such traits often have low heritability and are strongly influenced by domi- nance (Mousseau & Roff, 1987; Crnokrak & Roff, 1995; DeRose & Roff, 1999). Historically, inbreeding in bark beetles has been of interest because of its regularity in the tribes Xyleborini, Dryocoetini, Cryphalini, Corthylini, and Hyorrhynchini (Hamilton, 1967; Kirkendall, 1993). These inbreeding scolytids generally feed on ambrosia in xylem of trees or in small plant parts like seeds. Sib mating occurs on hosts before dispersal. Such inbreeding groups are often characterised by female-biased sex ratios, flightless males, and haplodiploid sex determination by arrhenotoky (Kirkendall, 1993) or paternal genome elimination (Brun et al., 1995). Inbreeding-related behaviour has not been investigated outside of these exclusively inbreeding tribes. Many species of scolytids display the ancestral trait of phloem feeding (Normark et al., 1999), where outbreeding is expected (Kirkendall, 1993). Despite similarity in resource utilisation, such bark beetle sys- tems can vary considerably with respect to the sex that initiates host colonisation, the degree of polygamy, and the population dynamics of outbreaks (Wood, 1982). Considering this variation, the engraver beetle, I. pini, is a fairly typical phloem-feeding species that normally does not experience inbreeding. After dispersal, males colonise hosts and begin constructing nuptial galleries under the bark (Pitman et al., 1965), while releasing aggregation pheromones in frass Correspondence: Michael J. Domingue, Department of Entomology, College of Agricultural Sciences, Pennsylvania State University, College Park, PA 16802, US.A. E-mail: mjd29@psu.edu Inbreeding depression and its effect on intrinsic population dynamics in engraver beetles MICHAEL J . DOMINGUE and STEPHEN A . TEALE Department of Environmental and Forest Biology, College of Environmental Science and Forestry, State University of New York, Syracuse, U.S.A. Abstract. 1. Phloem-feeding bark beetles (Coleoptera: Curculionidae, Scolytinae) generally disperse before mating, leading to expectations of outbreeding. New York and British Columbia populations of engraver beetles ( Ips pini) were tested for inbreeding depression using different methods. Among several traits measured, only the number of offspring surviving to adulthood was strongly reduced by inbreeding. 2. There was no evidence of avoidance of inbreeding depression in two possible mechanisms considered: differential male and female emergence times within full sib broods, and early termination of brood construction in forced sib mating. 3. Sib-mated females lay more eggs and have longer galleries than those in outbred crosses, despite a low rate of survival to adulthood for such eggs. This difference may be due to the ability of engraver beetles to assess crowding in broods as larvae begin to feed, and allows partial compensation for the effects of inbreed depression. 4. Population models assuming density-dependent generational effects were modified to account for inbreeding depression. Inbreeding depression makes populations less prone to cyclical behaviour, particularly at lower carrying capacities. 5. Inbreeding depression has not been previously measured in scolytids, nor has inbreeding-related behaviour been explicitly considered outside of exclusively inbreeding tribes. Key words. Bark beetle, genetic load, inbreeding depression, Ips pini, population ecology , sib mating.