Sexually transmitted diseases of insects: distribution, evolution, ecology and host behaviour Robert J. Knell 1 and K. Mary Webberley 1,2 1 School of Biological Sciences, Queen Mary, University of London, Mile End Road, London E14NS 2 Department of Biology, University College London, 4, Stephenson Way, London NW12HE (Received 27 January 2003; revised 4 August 2003; accepted 20 August 2003) ABSTRACT Sexually transmitted diseases (STDs) of insects are known from the mites, nematodes, fungi, protists and viruses. In total 73 species of parasite and pathogen from approximately 182 species of host have been reported. Whereas nearly all vertebrate STDs are viruses or bacteria, the majority of insect STDs are multicellular ectoparasites, protistans or fungi. Insect STDs display a range of transmission modes, with ‘ pure ’ sexual transmission only described from ectoparasites, all of which are mites, fungi or nematodes, whereas the microparasitic endo- parasites tend to show vertical as well as sexual transmission. The distribution of STDs within taxa of insect hosts appears to be related to the life histories of the hosts. In particular, STDs will not be able to persist if host adult generations do not overlap unless they are also transmitted by some alternative route. This explains the obser- vation that the Coleoptera seem to suffer from more STDs than other insect orders, since they tend to diapause as adults and are therefore more likely to have overlapping generations of adults in temperate regions. STDs of insects are often highly pathogenic, and are frequently responsible for sterilizing their hosts, a feature which is also found in mammalian STDs. This, combined with high prevalences indicates that STDs can be important in the evolution and ecology of their hosts. Although attempts to demonstrate mate choice for unin- fected partners have so far failed it is likely that STDs have other effects on host mating behaviour, and there is evidence from a few systems that they might manipulate their hosts to cause them to mate more frequently. STDs may also play a part in sexual conflict, with males in some systems possibly gaining a selective advantage from transmitting certain STDs to females. STDs may well be important factors in host population dynamics, and some have the potential to be useful biological control agents, but empirical studies on these subjects are lacking. Key words : sexually transmitted disease, STD, insect, host-pathogen, parasite mediated sexual selection, parasite manipulation of host behaviour, evolution of parasitism, sexual conflict. CONTENTS I. Introduction ................................................................................................................................................. 558 II. Methods ........................................................................................................................................................ 558 III. Insect STD diversity .................................................................................................................................... 559 (1) General patterns .................................................................................................................................... 559 (2) Viruses .................................................................................................................................................... 559 (3) Protists .................................................................................................................................................... 565 (4) Fungi ....................................................................................................................................................... 565 (5) Nematodes ............................................................................................................................................. 565 (6) Mites ....................................................................................................................................................... 565 IV. Host diversity and range ............................................................................................................................. 566 (1) Distribution of STDs across host taxa ............................................................................................... 566 Biol. Rev. (2004), 79, pp. 557–581. f 2004 Cambridge Philosophical Society 557 DOI : 10.1017/S1464793103006365 Printed in the United Kingdom