© 2004 The Netherlands Entomological Society Entomologia Experimentalis et Applicata 110: 115 – 123, 2004 115 Blackwell Publishing, Ltd. Reduced competitive ability due to Wolbachia infection in the parasitoid wasp Trichogramma kaykai M. E. Huigens 1, *, C. L. Hohmann 2 , R. F. Luck 3 , G. Gort 4 & R. Stouthamer 3 1 Laboratory of Entomology, Department of Plant Sciences Wageningen University, PO Box 8031, 6700 EH Wageningen, the Netherlands; 2 Area de Proteção de Plantas, Instituto Agronomico do Parana, IAPAR, 86001-970 Londrina PR, Brazil; 3 Department of Entomology, University of California, Riverside, CA 92521, USA; 4 Biometris, Mathematical and Statistical Methods Group Wageningen University, PO Box 100, 6700 AC Wageningen, The Netherlands Accepted: 28 October 2003 Key words: Wolbachia, parthenogenesis, Trichogramma kaykai, host fitness, larval competition, Hymenoptera, Trichogrammatidae Abstract Several hymenopteran parasitoids are infected with parthenogenesis-inducing (PI) Wolbachia. Infected wasps produce daughters instead of sons from unfertilized eggs. Thus far, little is known about the direct effects of PI Wolbachia on their host’s fitness. Here, we report reduced competitive ability due to Wolbachia infection in a minute parasitoid wasp, Trichogramma kaykai Pinto and Stouthamer (Hymenoptera: Trichogrammatidae). Immature survival of infected individuals in a host parasitized by a single infected female, laying a normal clutch of eggs, was lower than those parasitized by a single uninfected individual. When the offspring of infected and uninfected females shared the same host, the infected immatures had significantly lower survival rates than their uninfected coun- terparts. The survival rate of infected immatures was higher when they competed with other infected immatures from a different infected parent than in competition with uninfected immatures of con- specific wasps. Thus, the host Trichogramma can suffer a substantial reduction in fitness when it is infected with the PI Wolbachia . We discuss why such a reduction is to be expected when populations of infected and uninfected individuals co-occur, and how the reduced competitive ability of PI Wol- bachia influences the spread of the bacteria in the field. Introduction Genetic elements that manipulate the sex ratio of their host do so to enhance their own transmission (Werren et al., 1988). One such manipulative element is Wolbachia , a cytoplasmic inhabiting bacterium first described by Hertig (1936). This reproductive parasite is transmitted vertically only through egg cells. It therefore enhances its transmission by manipulating the reproduction of its host to favor the production of Wolbachia infected females. Wolbachia manipulates this production in one of five ways: cytoplasmic incompatibility (CI) (Yen & Barr, 1971), feminization (Rigaud et al., 1991; Rousset et al., 1992), male-killing (Hurst et al., 1999), increased fecundity of its host (Girin & Boulétreau, 1995), and the induction of parthenogenesis (Stouthamer et al., 1990; Stouthamer et al., 1993). To date, most research has focused on Wolbachia ’s evo- lution, phylogeny, and the mechanisms by which they exert their main effects (O’Neill et al., 1997). Wolbachia may also affect a host’s reproductive success. The presence of large bacterial numbers within host tissues is likely to inflict a physiological cost on their hosts (Stouthamer et al., 1999). In most cases, such costs do not appear to reduce their host’s fitness (Hoffmann et al., 1994, 1998; Giordano et al., 1995; Bourtzis et al., 1996; Poinsot & Merçot, 1997) and, in some cases, Wolbachia even enhances it (Wade & Chang, 1995; Girin & Boulétreau, 1995; Stolk & Stouthamer, 1995; Poinsot & Merçot, 1997; Vavre et al., 1999). Even though it is generally assumed that physiological costs are mini- mized in populations where all individuals are infected, this does not imply the absence of a cost. Parasites con- sume energy, independent of selection pressures favoring their benevolence (Bull et al., 1991). For example, fecun- dity, adult survival, and locomotory activity are reduced in *Correspondence: Martinus E. Huigens, Laboratory of Entomology, Department of Plant Sciences, Wageningen University, PO Box 8031, 6700 EH, Wageningen, the Netherlands. Tel.: +31 317 485118; Fax: +31 317 484821; E-mail: ties.huigens@wur.nl