The most infamous species of bed bugs is undoubtedly the common bed bug (Cimex lectularius), widely known due to its close, uncalled-for relation to human hosts. Even among the Cimicidae family, this species seems to be extraordinary and indeed deserves an increased attention for a number of reasons. However, there are more than one hundred related species described. Nevertheless, all members of the family Cimicidae are obligatory parasites of endothermic and usually social vertebrates (bats, swallows, swifts, humans). They feed on the blood of their hosts and use their roosts as a shelter. They climb on the host body in order to feed, although it is plausible they occasionally wilfully travel attached to their host (for more information see our article in Vespertilio journal).1 Their habits therefore create unique structure of so-called metapopulations, inhabiting particular host shelters (more about this topic in our publication).2 This unusual population structure then determines the mechanisms of evolution and fixation of parasite adaptations to its host. Studying such adaptations is therefore the principal object of interest of our research group.
The common bed bug is a species originally associated with bat hosts.3 It is a common parasite in the synanthropic colonies of greater mouse-eared bat (Myotis myotis) in the Central Europe. Nonetheless, there possibly might be original populations in the Middle East, which could still be found in bat caves.4 The specialization on the human host occurred deep in the prehistoric times, long before first human settlements were built (according to the mitochondrial dating – see our article).5 However, bed bug infestations have been documented rather from the time of ancient civilizations than from the medieval Europe. In Great Britain for instance, bed bugs didn’t appear before the 16th century and even then, they could have been found only in the seaports.6 The same applies to Scandinavia or Russia where the bed bug has not been effectively recognised since after the 19th century. With the arrival of the Industrial Revolution, followed by urbanization and especially the World War I, the bed bug however became a regular part of a household. Nevertheless, it vanished from the majority of European countries after the World War II, above all due to the extent use of DDT and pyrethroids. Yet, since the late 90’s it has been gradually coming back. The culprit is indeed its high resistance, perhaps obtained by the excessive use of DDT. Still, genetic analyses have revealed that there are more than just a few sturdy surviving lineages behind the surprising comeback of the common bed bug.2,5,7 It seems that the parasite arose from its ashes from local refugia, due to changes in human behaviour. The expansion of travelling might have been at fault, as well as the simple fact that people can no longer recognise bed bugs.8 That naturally means they grant the bed bug plenty of opportunities for spreading. The changes in human behaviour as well as the resistance of the particular bed bug lineages is the main concern of our current population genetics project.
The tropical bed bug (Cimex hemipterus) has walked the same path as the common bed bug. For its connection to the hot climate it can rarely be found in Europe. The original populations on bats are known from India.9 The species also vanished from most of the regions on account of the use of DDT. The fact that in the last years this species has been repopulating the same regions6 supports the idea that there are global changes of human behaviour behind the bed bug resurgence, rather than the expansion of the resistant lineages.
The bat bug (Cimex pipistrelli) shares the greater mouse-eared bat host colonies with the common bed bug in the central Europe. However, it is also frequent among many other bat species of which the common noctule (Nyctalus noctula) is probably of the highest importance.10 This bat species typically inhabits tower block buildings, which can occasionally lead to the bat bug-human encounter (the listing of such cases is presented in our article).11 Nevertheless, permanent populations on human host are not known and according to our experience the completion of this parasite life cycle can be hardly achieved on human blood.
Due to its appearance, the martin bug (Cimex hirundinis) used to be classified in a separate genus Oeciacus. Our genetic analyses12 however showed it had evolved within the genus Cimex, which comprises parasites associated with bat hosts. Their appearance thus diverted as a result of their relationship with bird hosts. In Europe, it is a common parasite of the martin nests and to a lesser extent it is also associated with swallows and swifts. This way it often enters human dwellings, although it seems unable to feed on a human host. Long-term associations with humans are therefore impossible.
Other bed bug species form local populations more or less on every continent. African Leptocimex boueti presumably forms stable association with human host.9 Before the World War II, the species Haematosiphon inodorus used to be a common poultry pest in the New World. Nowadays, it can be found only in association with its former host – birds of prey. Anyway, it is also possible it historically used to form a human-related population, in particular with Indian Hopi Tribe, which would make it the fourth human-associated bed bug species. There is an extensive and quite convincing linguistic support13 behind this idea.
Ectoparasitic life strategy is only one of many bed bug peculiarities. Next in line is undoubtedly their unique mating strategy – the traumatic insemination. Male penetrates the female body using a modified left paramere (instead of a penis) and breaches her epidermis in between the abdominal segments. It is also a place of female’s specialised organ which has evolved as an adaptation to mechanic trauma and possible infection caused by the penetration. Yet in spite of this adaptation, the copulation is stressful for the female. Intraspecific copulation can even have fatal consequences for the female, as it has been documented in the common bed bug and tropical bed bug crossings.14, 15 In our studies, we focus on the effects of interspecific mating in the crossings of the common bed bug (Cimex lectularius) and the bat bug (C. pipistrelli). These two species both occupy the greater mouse-eared bat host colonies and the traumatic insemination could be therefore serving as one of the mechanisms of their interspecific competition.
Another unique aspect of cimicids’ biology is the inheritance of their genetic material. The variability in the number of sex chromosomes in the common bed bug and a few other species has been known for a long time. Only recently however, our colleagues from the Charles University have proved that these accessory chromosomes do not have to originate by fragmentation.16, 17 The size of cimicids’ nuclear genome is therefore variable.
Cimicids, and the common bed bug in particular, also have a remarkable mechanism of inheritance of mitochondrial genome. Biparental inheritance of mitochondrial DNA as itself is quite common across the animal kingdom. Nevertheless, with the exception of only one clam family, it concerns a negligible ratio between father and mother mtDNA. These two cases (clams and bed bugs) therefore represent the proverbial exceptions proving the universal rule of the maternal mtDNA inheritance. In most cases, the paternal mtDNA leak occurs in occasions of hybridization of related species or distant populations of the same species. Nevertheless, bed bugs seem to inherit mitochondria from both of their parents very often (more in our article).18 Similarly to clams, the bed bug mtDNA variants recombine, according to the research of our colleague, Warren Booth (unpublished).
1/ Balvín, O. et al. Transport of bugs of the genus Cimex (Heteroptera: Cimicidae) by bats in western Palaearctic. Vespertilio 16, 43 –54 (2012).
2/ Booth, W., Balvín, O., Vargo, E. L., Vilímová, J. & Schal, C. Host association drives genetic divergence in the bed bug, Cimex lectularius. Mol. Ecol. 24, 980–992 (2015).
3/ Horváth, G. La distribution géographique des cimicides et l´origine des punaises des lits. in Extrait du IXe Congres International de Zoologie Tenu a Monaco 294–299 (1913).
4/ Povolný, D. & Usinger, R. L. The discovery of a possibly aboriginal population of the bed bug (Cimex lectularius Linnaeus, 1958). Acta Musei Morav. Sci. Nat. 51, 237–242 (1966).
5/ Balvín, O., Munclinger, P., Kratochvíl, L. & Vilímova, J. Mitochondrial DNA and morphology show independent evolutionary histories of bedbug Cimex lectularius (Heteroptera: Cimicidae) on bats and humans. Parasitol. Res. 111, 457,–469 (2012).
6/ Naylor, R., Balvín, O., Delaunay, P. & Akhoundi, M. The bed bug resurgence in Europe and Russia. in Advances in the Biology and Management of Modern Bed Bugs (John Wiley and Sons, 2018).
7/ Szalanski, A. L., Austin, J. W., McKern, J. A., Steelman, C. D. & Gold, R. E. Mitochondrial and Ribosomal Internal Transcribed Spacer 1 Diversity of Cimex lectularius (Hemiptera: Cimicidae). J. Med. Entomol. 45, 229–236 (2008).
8/ Reinhardt, K., Harder, A., Holland, S., Hooper, J. & Leake-Lyall, C. Who knows the bed bug? Knowledge of adult bed bug appearance increases with people age in three counties of Great Britain. J. Med. Entomol. 45, 956–958 (2008).
9/ Usinger, R. L. Monograph of Cimicidae. (Entomological Society of America, 1966).
10/ Balvín, O., Bartonička, T., Simov, N., Paunovic, M. & Vilímová, J. Distribution and host relations of species of the genus Cimex on bats in Europe. Folia Zool. 63, (2014).
11/ Balvín, O. & Bartonička, T. Cimicids and bat hosts in the Czech and Slovak Republics: ecology and distribution. Vespertilio 17, 23–36 (2014).
12/ Balvín, O., Roth, S. & Vilímová, J. Molecular evidence places the swallow bug genus Oeciacus Stal within the bat and bed bug genus Cimex Linnaeus (Heteroptera: Cimicidae): The genus Oeciacus within the genus Cimex. Syst. Entomol. 40, 652–665 (2015).
13/ Reinhardt, K. Which bed bug the Hopi know? (A present for Robert Leslie Usinger’s 100th birthday). Am. Entomol. 58, 58c–59 (2012).
14/ Omori, N. Experimental studies on the cohabitation and crossing of bed-bugs (Cimex lectularius L. and C. hemipterus F.). Preliminary report. in VII. International Kongres der Entomologie (ed. Uschmann, G.) 2, 895–915 (1939).
15/ Walpole, D. E. Cross-mating studies between two species of bedbugs (Hemiptera: Cimicidae) with a description of a marker of interspecific mating. South Afr. J. Sci. 84, 215–216 (1988).
16/ Sadílek, D., Urfus, T. & Vilímova, J. Am I really so smashed? in Zoologické dny (2018).
17/ Sadílek, D., Urfus, T., Vilímová, J., Hadrava, J., Suda, J. (2019) Nuclear Genome Size in Contrast to Sex ChromosomeNumber Variability in the Human Bed Bug,Cimex lectularius (Heteroptera: Cimicidae). Cytometry Part A.
18/ Robinson, G., Balvin, O., Schal, C., Vargo, E. & Booth, W. Extensive mitochondrial heteroplasmy in natural populations of a resurging human pest, the bed bug. J. Med. Entomol. accepted, (2015).