The effect of conspecific density on female reproduction in an egg-carrying bug MARI KATVALA & ARJA KAITALA Department of Biology, University of Oulu (Received 2 July 2003; initial acceptance 1 September 2003; final acceptance 31 March 2004; published online 5 November 2004; MS. number: 7776R) The availability of high-quality egg-laying substrates is likely to determine female reproductive success in insects. The egg-laying substrate may also be a conspecific, which is the case in the golden egg bug, Phyllomorpha laciniata, Vill. Female bugs oviposit mainly on nonparental males and other females. Conspecifics are clearly preferred to the bug’s food plant. We tested the effect of conspecific density on female egg laying in two experiments where an egg-laying focal female was accompanied by males, females or both in two sizes of enclosures for 8 days. Fewer eggs were laid in large enclosures than in small ones and an increase in number of conspecifics resulted in more eggs being laid on them. This study shows that conspecifics are an important resource for female golden egg bugs in terms of egg laying. In addition, egg survival is likely to be affected by the female’s chances of laying eggs on conspecifics. Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Selection of nest or egg-laying site has crucial consequen- ces for female reproductive success (Jaenike & Holt 1991; Martin 1998). Nest site choice is modified by selection caused by nest or egg predation and parasitism and by resource requirements (e.g. availability of nutrients) of the hatched offspring (Jaenike & Holt 1991; Mayhew 1997; Martin 1998). In nature, plenty of nest sites or substrates are available but the best ones may be limited in numbers because of interspecific or intraspecific competition. For example, in open-nesting birds where on average 80% of the nests are lost from predation (Ricklefs 1969; Martin 1992), nest sites with low predation risk may be in limited supply (Martin 1993). Furthermore, females of phytoph- agous insects may oviposit on host plants where egg or larval growth and survival are moderate because females have been unable to find the most suitable host plants for their offspring (e.g. Mayhew 1997). Therefore, safe and high-quality nest sites or egg-laying substrates can be considered important resources for individuals. In general, increasing population density decreases an individual’s fitness because shared resources may become scarce (Begon et al. 1996). However, there are also cases where an increase in the number of conspecifics increases the survival or the reproductive success of individuals. In such cases, increased fitness may result from enhanced predator or parasite defence, more efficient food acqui- sition, increased mating possibilities or social facili- tation (Andrewartha & Birch 1954; Sæther et al. 1996; Courchamp et al. 1999). Thus, in species where the presence of a sufficient number of conspecifics partially determines individuals’ fitness, conspecifics can be con- sidered a resource (Eggleston & Lipcius 1992). Conspe- cifics may be an oviposition resource, for example, in the giant water bugs (Belostomatinae) and pipefish (Sygnathi- dae) where females lay eggs on the backs or in brood pouches of males (Berglund et al. 1989; Smith 1997). In species where conspecifics are a prerequisite for egg laying, the availability of conspecifics is likely to have a significant influence on female reproductive success. If conspecifics are not found, the reproductive success of females may be reduced. We studied the effect of conspecific density on female egg laying in the golden egg bug, Phyllomorpha laciniata, Vill, Heteroptera, Coreidae. Female bugs glue their eggs on to the bodies of conspecific males and females that then carry the eggs until the larvae hatch (Jeanel 1909; Reuter 1909; Kaitala 1996). Males receive eggs during courting from females with whom they have not previously mated (Miettinen & Kaitala 2000; Katvala & Kaitala 2001b). However, accepting an egg to be carried does not neces- sarily lead to mating (Miettinen & Kaitala 2000; Katvala & Correspondence and present address: M. Katvala, Department of Animal Ecology, Evolutionary Biology Centre, Uppsala University, Norbyva¨gen 18D, SE-752 36 Uppsala, Sweden (email: mari.katvala@ oulu.fi). A. Kaitala is at the Department of Biology, University of Oulu, PO Box 3000, FIN-90014 Oulu, Finland. 269 0003–3472/04/$30.00/0 Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2005, 69, 269–273 doi:10.1016/j.anbehav.2004.03.006