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Abstract. The resurgence of bed bug infestations prior to the late 1990s has triggered the attention of entomologist, pest control operators (PCOs) and the public after being disregarded as occasional pest for almost 50 or 60 years. The related bed bug species refers to the common bed bug, Cimex lectularius L. and the tropical bed bug, Cimex hemipterus (F.). This review paper is produced with the objective to prompt the awareness of the public and to present an informative article regarding bed bug, in particular on the tropical species, C. hemipterus. Geographically, this species dominates the tropical and subtropical region, including Malaysia and Singapore. The outline of this review paper covers the status and possible resurgence factors, the consequences of bed bug bites or infestations, the general biological parameters (life cycle, environmental effects, movement behaviour and longevity) and the general management of bed bug infestation. We have also concluded some suggestions and recommendations to improve bed bug control measures based on our studies and previous literatures.
Among the 74 identified Cimicids, which include 22 genera and 6 subfamilies (Usinger, 1966; Askew, 1971), three are categorized as human bed bugs: Cimex lectularius Linnaeus, Cimex hemipterus (Fabricius), and Leptocimex boueti Brumpt. C. lectularius is distributed widely over temperate region such as Canada, the United States, and the United Kingdom. C. hemipterus is dominant in the warmer tropical and subtropical regions, such as Asia (Malaysia and Singapore), Africa, and the American tropics, whereas L. boueti is found only in West Africa. L.boueti also can parasitize bats, and C. lectularius and C. hemipterus have a wide spectrum of alternative hosts, including bats, chicken, and other mammals (Burden, 1966; Usinger, 1966; Kettle, 1984; Boase, 2004).
Prior to the late 1990s, the bed bug was thought of only as an occasional pest insect. Bed bug infestations generally were considered to be related to hygiene problems and received almost no attention from the public or pest control operators (PCOs). However, the drastic resurgence of bed bug infestations that occurred in the late 1990s awoke the public, and information about bed begs became prevalent in the news media in many countries (Reinhardt & Siva-Jothy, 2007). Increasing numbers of bed bug infestations have been reported in the United States (Krueger, 2000; Cooper, 2006; Miller, 2007; Potter et al., 2008), the United Kingdom (Paul & Bates, 2000; Boase, 2004; Reinhardt & Siva-Jothy, 2007), Denmark (Kilpinen et al., 2008), Europe (Owen 2004, Kilpinen et al., 2008), Canada (Myles et al., 2003, Hwang et al., 2005), Italy (Masetti & Bruschi, 2007), Australia (Doggett et al., 2003; 2004; Doggett, 2006; Doggett & Russell, 2008), and Korea (Lee et al., 2008).
Several hypothetical explanations for the resurgence of bed bug infestations have been suggested (Boase, 2008; Doggett, 2006; Moore & Miller, 2006; Potter, 2005; 2006). First, rapid urbanization and globalization could have provided more comfortable habitats (more developed public accommodations and residential houses), suitable microclimates (optimum temperature and humidity), and widespread migration (well-developed transport vehicles and human travel) for bed bugs. Second, there was a dearth of knowledge about bed bugs among the public and PCOs due to the long 'silent period' (almost 50 or 60 years) during which bed bugs were only an unnoticed and occasional pest. Third, changes in patterns of insecticide application and insecticide resistance issues increased the difficulty in managing bed bug infestations. Therefore, the objectives of this article were to review the available information regarding bed bug resurgence, biology and behaviour, in particular the tropical species, C. hemipterus, which are relatively less data than the common species, C. lectularius. Thus, suggestions and recommendations could be adduced based on the knowledge, in the pursuit of improved prevention, control measures and infestation management, to produce several timely contributions to the pest control industry and to educate the public.
POSSIBLE EXPLANATION OF BED BUG RESURGENCE
As early described, the resurgence of bed bug has been denoted by many publications (Krueger, 2000; Paul & Bates, 2000; Myles et al., 2003; Boase, 2004; Doggett et al., 2003; 2004; Owen, 2004; Hwang et al., 2005; Cooper, 2006; Doggett, 2006; Miller, 2007; Masetti & Bruschi, 2007; Reinhardt & Siva-Jothy, 2007; Doggett & Russell, 2008; Kilpinen et al., 2008; Lee et al., 2008; Potter et al., 2008). The upsurge of bed bug infestations also have been reported by PCOs in Malaysia and Singapore (Lee, 2007; How & Lee, 2010a). In our surveillance study between July 2005 and December 2008 in Malaysia and Singapore, with total of 54 infested sites, only C. hemipterus was found (How & Lee, 2010a).
Generally, the resurgence of bed bug infestations can be categorized into three large groups of issues: social, environmental, and pest control (Potter, 2005; 2006; Doggett, 2006; Moore & Miller, 2006; Boase, 2008). The environmental issues involve global warming and central heating; increased temperature within the bed bug's optimum range could improve the reproductive rate and numbers, or even nymph growth, thereby increasing the spread and severity of infestations. The ubiquitous application of temperature-control devices, such as air conditioners, may create a comfortable zone for both humans and bed bugs.
The social issues involve human activity, which can help disperse bed bugs from place to place. For example, trading of infested second-hand items or global travel and migration can accidentally transport bed bugs from hotel to hotel or from hotel to home. Globalization and advanced transport systems can spread bed bugs to anywhere humans go (Whitfield, 1939). The increasing human population gradually leads to overcrowding in urbanized areas, which also maximizes the food source for bed bugs. Public awareness about bed bug infestations remains low, as infestations have not been a serious problem for almost 60 years.
PCOs and entomologists also may lack adequate knowledge about how to handle some urban and medical pest infestations (Matthews, 2008), and bed bug infestation is a good example of this problem. Changes in patterns of insecticide application also may have contributed to the spread of bed bug infestations. The bait method of controlling cockroaches is not efficient for bed bug management. Thus, more studies and evaluations on various insecticides are needed to determine insecticides that can be used effectively to manage bed bug infestations.
Insecticide resistance is one of the major causes of the resurgence of bed bug infestations (Potter, 2005; 2006; Moore & Miller, 2006; 2009; Romero et al., 2007; Boase, 2008). Insects may develop resistance mechanisms after long-term exposure to a particular insecticide, which could render it ineffective for use in pest control. Some researchers reported that bed bugs developed DDT (dichlorodiphenyltrichloroethane) or analogous insecticide (example, Î³-BHC) resistance when DDT was extensively used as a control chemical soon after World War II (Johnson & Hill, 1948; Brown, 1958; Busvine, 1958; Lofgren et al., 1958; Gratz, 1959; Mallis & Miller, 1963; Usinger, 1966; Cha et al., 1970). However, use of DDT was banned due to its persistent in the environment.
Pyrethroids are a common group of active ingredients in home-use or pest control products up to date. This group is commonly used for bed bug management because infestations are always associated with human living or resting places, thus safe insecticides or insecticides at low concentrations must be used. However, reports of pyrethroid resistance exist (Myamba et al., 2002; Moore & Miller, 2006; Karunaratne et al., 2007; Romero et al., 2007; Boase, 2008; Kilpinen et al., 2008). There were publications reported that the pyrethroid resistance detected on bed bugs would probably characterized as kdr-type resistance other than monooxygenase-type resistance, as nerve insensitivity blocked the insecticide action of pyrethroid (Yoon et al., 2008; Romero et al., 2009). In more detail, Yoon et al. (2008) reported that this kdr-type resistance on C. lectularius was related to the mutations within the sodium channel Î±-subunit gene.
IMPORTANCE OF BED BUG
Many people suspect that bed bugs, as blood-feeding insects, have great potential to transmit blood-borne diseases, such as filariasis, Kala-azar, yellow fever, relapsing fever, plague, Hepatitis B and AIDS (Burton, 1963; Nelson, 1963; Ogston et al., 1979; Jupp & Lyons, 1987; Webb et al., 1989; Jupp et al., 1991; Blow et al., 2001). However, although bed bugs have been proved capable carrying many pathogens or infectious particles (Strand, 1977), no reports of natural cases of disease transmission by bed bugs exist (Usinger, 1966; Reinhardt & Siva-Jothy, 2007; Potter et al., 2008; Stucki & Ludwig, 2008; Goddard & deShazo, 2009; Heymann, 2009). Bed bug bites can cause secondary infection if they are scratched and exposed to infectious pathogen accidentally. Known secondary infections include folliculitis, cellulitis, and eczematoid dermatitis (Allington & Allington, 1954; Crissey, 1981; Goddard & deShazo, 2009).
Bed bug bites caused a cutaneous reaction in humans, and the severity of the reaction depends on individual sensitivity, exposure time, and numbers of bites (Reinhardt et al., 2009). Reinhardt et al. (2009) reported that almost 80% of their study population was sensitive to bed bug bites, and they concluded that the prevalence of bed bug bite sensitivity would increase if infestations spread.
Individuals with hyposensitivity to bed bug bites develop only a hemorrhagic punctum on the skin and experience few delayed reactions. In contrast, individuals with hypersensitivity to the bites may experience anaphylactic-like reactions, including pruritic macupapular rashes, erythematous lesions, itchiness, urticaria, inflammation, and bullous rashes (Cestari & Martignago, 2005; Heukelbach & Hengge, 2008; Stucki & Ludwig, 2008; Goddard & deShazo, 2009; Heymann, 2009). Throughout the four years involvement in direct blood feeding providing for bed bug cultures growth, the first author had experienced those formerly described signs and symptoms of bites, resulted in succession from hyposensitive to hypersensitive reactions (Figure 1). High numbers of bites can lead to more severe reactions, such as blister papules or systemic hypersensitivity (Thomas et al., 2004; Heymann, 2009) and loss of haemoglobin or iron deficiency (Venkatachalam & Belavady, 1962). Some reports have suggested that the bed bug bite reaction is probably IgE mediated and caused by the human immunological responses to salivary proteins of the bed bug, such as nitrophorin, factor X, and a 40-kDa apyrase-like nucleotide-binding enzyme (Valenzuela et al., 1996; 1998; Valenzuela & Ribeiro, 1998; Goddard & deShazo, 2009; Heymann, 2009; Klotz et al., 2009).
Bed bug bites and infestations also can lead to a psychological phobia in some people. Delusory parasitosis describes the condition in which a person experiences the delusional belief that they are still surrounded by the insect (example: bed bugs) even through the infestation has been eliminated. This may cause some forms of psychosis, such as formication and insomnia (Hinkle, 2000; Heukelbach & Hengge, 2008; Lee et al., 2008).
Other than medical importance, bed bug infestation was also play a role in contributing to economic losses, especially in the hospitality and tourism industry (Doggett, 2006; Doggett & Russell, 2008; Heukelbach & Hengge, 2008). The multimillion dollars in losses involved pest control costs, replacement of infested infrastructures, reparation for complaints, medical costs, and also reputation damage (Coleman, 2005; Hwang et al., 2005; Doggett, 2006; Miller, 2007; Shavell, 2007; Doggett & Russell, 2008; Heukelbach & Hengge, 2008; Potter et al., 2008). For example, in San Francisco, CA, USA, professional bed bug management reportedly costs $2000 per infested unit, and some hotels had to pay up to $60,000 to control an outbreak (Miller, 2007). Coleman (2005) and Shavell (2007) reported that a victim of bed bug bites won a lawsuit against a motel and was awarded $186,000 in punitive damages. In Australia, the resurgence of bed bug infestations has had an economic cost of an estimated $AUS100 million (Doggett & Russell, 2008)
BIOLOGY AND OTHER ESSENTIAL INFORMATION OF BED BUG
Cimicids are temporary and opportunistic blood-feeding ectoparasites that tend to be host specific. For example, bat bugs feed on bats, chicken bugs and swallow bugs feed on poultry and birds, and bed bugs feed on humans. However, cimicids may have alternative hosts that are needed for survival in the absence of the main host (Usinger, 1966).
The bed bug has a dorso-ventrally flattened body shape, similar to that of cockroaches, that allows it to crawl and hide in narrow cracks and crevices. The head bears pairs of gradually tapering four-segmented antennae and a three-segmented piercing mouthpart. The 11-segmented abdomen has seven pairs of spiracles that are located on segments 2 to 8. Other prominent external morphological features include a broad pronotum, pairs of reduced hemelytral pads (wingless form; this features only appear on adult), and three pairs of slender but well-developed legs for rapid movement (Usinger, 1966; Roy & Brown, 1970; Askew, 1971; Kettle, 1984).
The adult bed bug is 4-7 mm long and reddish-brown in colour. The male is distinguished from the female by the asymmetrical pointed posterior part of the abdomen, which is the part of male sexual organ paramere which covered the real sexual organ (aedegus) of male (Figure 2). In other hand, the female has a broader abdomen and bears a notch-like copulatory organ (spermalege) on the ventral side. Bed bugs reach adulthood by passing through five instar-stage nymphal periods. Nymphs are similar in appearance but relatively smaller in size compared to adults; they also are brighter in colour, have an undeveloped reproductive organ, lack wing pads, have fewer bristles, have two-segmented tarsi (adults have three-segmented tarsi), and have three prominent dorsal abdominal scent glands. The eggs of bed bugs generally are < 1.1 mm in length and elongate-oval shaped with an anterior cap (Usinger, 1966; Roy & Brown, 1970; Mallis, 1990).
The bed bug life cycle has three stages: eggs, nymphs, and adults. Females oviposit eggs individually on rough surfaces, and they can occur singly or glued together into clusters. The first instar dislodges the egg cap to hatch. The growth and moulting of nymphs only progresses if enough blood-feeding occurs. Bed bug mating and oviposition by the female also require blood meals. Thus, all nymphal and adult stages are gregarious and obligate hematophagic ectoparasites: They must ingest blood to survive (Usinger, 1966; Roy & Brown, 1970; Askew, 1971; Mallis, 1990).
All cimicids utilize a unique mode of copulation called traumatic insemination, and females of some species have evolved specific adaptations to it. Traumatic insemination describes the copulation process in which the male pierces the female body cavity using its needle-like external genitalia and injects a mass of sperm into the paragenital system and not through the ordinary reproductive tract or opening as most of the insects. The term "traumatic" refers to the integumental wound in the female abdomen after each mating and to the cost of copulation. As an evolutionary adaptation to traumatic insemination, the female has the spermalege as counter-adapt organ, which is divided into ecto- and mesospermalege to reduce the damage of the integumental wound. The mesopermalege also is responsible for sperm selection; it contains phagocyctic haemocytes that may kill low-quality (low-motility) sperm. The regular female reproductive organ (i.e., not the spermalege) and opening function only in egg laying (Usinger, 1966; Stutt & Siva-Jothy, 2001; Morrow & Arnqvist, 2003; Reinhardt et al., 2003; Siva-Jothy, 2006; Pfiester et al., 2009b).
Cimex hemipterus (Fabricius) (Figure 2)
Both C. lectularius and C. hemipterus exhibit morphology and biology similar to that described above. However, these two species show some differentiation, which likely is due to the variation of their geographical distribution. Compared to C. lectularius, the pronotum of C. hemipterus is not as broad or wing-like (it is < 2.5 times wider than its long), and the pronotum margin hairs are curved backwards. The body of C. hemipterus generally is longer than that of C. lectularius (within 6-7 mm vs. 5-6 mm, respectively). The female spermalege is located at the hind margin of the fifth ventral sternite, and the ectospermalege appears as a transverse dark area (Usinger, 1966; Kettle, 1984; Mallis, 1990). The instar stage of C. hemipterus nymphs could be easily distinguished through the length measurement of their shed skin (cast exoskeleton) (How & Lee, 2010b), together with the microscopic observation of some morphological difference, included number of rows of spines on abdominal tergites, the appearance of the hind margin of the mesonotum, length of antennal segments, and the number of bristles along the edge of the pronotum (Usinger, 1966; Newberry, 1990).
Each of the five nymph stages of C. hemipterus requires about 3-10 minutes to complete the blood feeding at host skin and 0.4-7.6 mg of blood volume as food while the adults spend relatively more time in feeding, about 10 - 15 minutes. After feeding well, the male eagerly and aggressively seeks for a fed female. Stationary copulation takes about 20 seconds, but up to 100 seconds can be required when the female is not in the static mode (Wattal & Kalra, 1961). The authors also showed that C. hemipterus most preferred humans as their blood source, as it produced the highest yield of oviposited eggs, followed by chickens, rabbits, and rats. With sufficient supply of blood meals and optimum temperature, C. hemipterus could attain its adulthood in equal sexual ratio from egg within a month or less (22 to 32 days). Also with continuous availability of blood meals, the mated male tropical bed bug could live between 11 and 99 days and the mated female could live 11-109 days; unmated females and males lived 82-216 days and 47-129 days, respectively (How & Lee, 2010b).
Most biological studies of C. hemipterus were conducted before World War II (Dunn, 1924; Mellanby, 1935; Geisthardt, 1937; Hase, 1930; 1931; Usinger, 1966). Omori (1941) conducted an extensive biological study and Wattal & Kalra (1961) made laboratory observations, but otherwise there is a dearth of information about the biology of C. hemipterus. Wattal & Kalra (1961) also noted that limited information exists about the quantitative bionomics of C. hemipterus compared to C. lectularius. This has illustrated that some essential biological parameters should be studied to fill in the numerous blanks in knowledge. Denote of that, our studies showed that 1st instars of C. hemipterus in the embryo took an average of five days to hatch from eggs, and the hatchability generally was > 90% successful rate. Before reaching adulthood, bed bug nymphs went through five stadia within 17-20 days. In general, the complete life cycle of the tropical bed bug at room temperatures and humidity took place within a month; thus, if an infestation is ignored for > 1 month, it could become complicated, with two or more generations of bed bugs in residence (How & Lee, 2010b).
Of note is the fact that the increase in bed bug infestations in the ASEAN region (Malaysia and Singapore) has been caused solely by C. hemipterus, and infestations by this species also have been reported in Australia. Although both C. hemipterus and C. lectularius tend to follow their dominant and allopatric territory distribution, some exceptions exist in which the two species co-occur or found in a same country. Wattal & Kalra (1961) reported co-occurrence in the hilly regions of India; Newbery (1988; 1989; 1990) and Walpole & Newberry (1988) reported the discovery of both species in a domestic infestation in northern KwaZulu, South Africa; and collected insect samples from Australia suggested the intrusion of both species (Doggett et al., 2003; 2004). In addition, when both C. hemipterus and C. lectularius have occasionally occurred in sympatry, interspecific mating could occur, as a hybrid of the two species has been reported (Newberry, 1988; 1989; Walpole & Newberry, 1988).
Environmental Effects on Bed Bugs: Temperature and Humidity Studies
Edney (1957) stated that the relationship between water and terrestrial arthropods (insects in particular) is an important component of ecological study. This relationship involves water loss, water gain, and the effects of water on body temperature and thermal resistance. Because insects are poikilothermic animals (Romoser & Stoffolano, 1998), environmental factors play an essential role in the insect-water relationship. Among the critical extrinsic factors that affect biological parameters such as longevity and development, temperature and humidity are of great interest to many entomologists.
The literature contains few articles about temperature, humidity, and bed bugs. A few studies were conducted for C. lectularius (Hase, 1930; Usinger, 1966; Mellanby, 1935; Kemper, 1936; Johnson, 1940a; 1940b; 1941; Omori, 1941; Usinger, 1966; Benoit et al., 2007; Roberto et al., 2009) and C. hemipterus (Hase, 1930; Mellanby, 1935; Omori, 1941; Usinger, 1966). Usinger (1966) reported that temperature is the most important of the various extrinsic factors, as it has extensive influence on all aspects of bed bug activities.
Omori (1941) found that C. lectularius and C. hemipterus preferred aggregation and clustering within 28-29Â°C and 32-33Â°C, respectively. These two species exhibited different temperature tolerances, and thus their biology and/or physiology may also differ when exposed to various temperatures. When the lower thermal lethal limit was investigated, C. hemipterus was found to be less resistant to cold temperature (up to 10Â°C), whereas C. lectularius could survive in temperatures as low as 6Â°C (Usinger, 1966). For the upper thermal lethal limit, both species died within 1 hour at 44Â°C and within 24 hours at 40Â°C (Mellanby, 1935). Generally, bed bugs will survive for a shorter length of time at higher temperature and a longer period at lower temperature (Johnson, 1941; Omori, 1941; Usinger, 1966).
Johnson (1941), Kemper (1936), and Omori (1941) concluded that humidity was another important environmental factor affecting bed bugs. Different humidity levels were found to affect bed bug fertility, fitness, longevity, and even development period. Generally, bed bugs do not prefer low humidity (Johnson, 1940b; 1941). Johnson (1941) reported that the favourable humidity range may be between 50 and 80% RH. Omori (1941) reported that extremely high humidity can contribute to shortened longevity of the adult bed bug. The lower survival rate of bed bugs in excessively humid conditions may be related to a variety of factors, including uptake of saturated water vapour due to greater critical equilibrium activity (Benoit et al., 2007) and vulnerability to massive growth of viruses or bacteria under highly humid conditions (Romoser & Stoffolano, 1998).
In terms of details about the tropical bed bug, C. hemipterus, Omori (1941) found that the C. hemipterus egg incubation period was as long as 17.1 days at 20Â°C under 83% RH. Mated females could live up to 33 days at 33Â°C under 70-80% RH,whereas unmated females showed longer survival at 33Â°C (107-124 days). Omori (1941) also reported that mated males could survive relatively longer than mated females, for example 24-42 days longer at the two above-mentioned temperatures and RH. However, the author did not report the survival of unmated male bugs, which could be an interesting subject as any detrimental of mating effect to male instead of female. Subsequently, our laboratory observation showed that repeated or continuous copulation jeopardise both male and female bed bugs in survival reduction (How & Lee, unpublished data).
Omori (1941) conducted an outdoor study of longevity of both bed bug species in different seasons (winter, summer, and spring). The longevity exhibited variation that was related to the different temperature and humidity values that characterized the seasonal climate. Omori (1941) also compared longevity between species when they were exposed to various constant temperatures (15, 18, 22, 27, 30, and 33Â°C) at 75-98% RH. Johnson (1940b) reported that C. lectularius longevity was indirectly related to the water loss rate under different humidity and temperature conditions. Overall, the results suggest that temperature and humidity affect not only individual bed bug longevity but also interactions among bed bugs.
Benoit et al.'s (2007) study of the C. lectularius water balance resistance profile explained how bed bugs tolerate water stress by water conservation, aggregation, and quiescence behaviour. Mating effect, strain, sex, and exposure time are factors that need to be considered when examining the effects of temperature and humidity on bed bugs; Johnson (1940b) also considered host blood type and virginity of female bugs.
Dispersal behaviour of bed bugs has been studied extensively due to the importance of tracking and preventing the spread of infestations. However, numerous questions remain about bed bug dispersal behaviour; in particular, why did a resurgence of infestations begin in the 1990s? or how far a bed bug can disperse by itself? Generally, bed bug dispersion can be categorized into two main pathways: active dispersal and passive dispersal (Reinhardt & Siva-Jothy, 2007).
Active dispersal behaviour refers to self-movement from a natural harbourage that is initiated by disturbance (e.g. treatment by pest control operators) (Romero et al., 2009), stress (hunger, temperature, mating avoidance, alarm pheromones, etc.) (Johnson, 1941; Griffiths, 1980; Bell, 1990; Pfiester et al., 2009a), and overgrowth of the population or aggregation (Wertheim et al., 2005). Active dispersion represents one of the main mechanisms for the spread of infestations over short distances, especially inside a room and within contiguous rooms (Reinhardt & Siva-Jothy, 2007; How & Lee, 2010a; 2010c; Wang et al., 2010). However, active dispersion requires a lot of energy, and Mellanby (1938) reported that bed bugs that explored over long distances often starved to death. As a consequence, under optimal conditions (presence of host and no other external or internal forces apply) bed bugs generally remain motionless in their harbourage close to the host (Johnson, 1941) and only search over short distances for a blood host (Usinger, 1966; Potter et al., 2008).
Some studies have shown that females are more likely than males or nymphs to actively disperse (Mellanby, 1939b; Johnson, 1941). All stages of nymphs are inactive stages and do not actively disperse; they prefer to aggregate in their harbourage due to air-borne and contact pheromones. In contrast, the alarm pheromone would be the chemical substance that would promote and activate the active dispersal movement of bed bugs (Benoit et al., 2009a). Our study revealed that the 5th instars, adult males, and adult females of C. hemipterus showed the greatest active movement activity either in the frequency or distance, which the fed females can moved >40 m within 5 days in laboratory testing arena (How & Lee, 2010c).
Passive dispersal behaviour refers to the more common spreading route of bed bug infestations that involves various host-associated belongings and activities. Bed bugs can be spread passively in clothing, luggage, belongings, and furniture (Hwang et al., 2005; Doggett, 2006; Kells, 2006; Reinhardt & Siva-Jothy, 2007). This mechanism explains the long-distance dispersion of bed bugs, as these insects are wingless and thus cannot fly. Whitfield (1939) reported that frequent travelling via various type of transportation, including car, van, train, ship, and airplane, may encourage the dispersion of bed bugs from country to country. Globalisation could become the largest pathway leading to a worldwide distribution of bed bugs.
Kells (2006) stated that modern society provides a convenient "hitchhike" system by which bed bugs can move between various potential harbourage and nesting sites. He further classified harbourage sites into two groups: temporary habitation sites (lodging establishments, such as hotels) and permanent nesting sites (long-term residence premises, such as homes and apartments). Other cimicids have different dispersion mechanisms. For example, the swallow bug, Oeciacus sp., can be carried by birds, and the bat bug, Cimex pilosellus (Horvath), can attach to bats and be dispersed when the host establishes a new nest site (Usinger, 1966; Dick et al., 2003).
MANAGEMENT OF BED BUG INFESTATION
Bed bugs have a short life cycle and high reproductive ability (How & Lee, 2010b), so the goal of bed bug management is simply bed bug elimination. Boase (2008) and Doggett (2006) both reported that the persistence of bed bug infestations in public premises is caused by failure to achieve full eradication of the insects. Thus, a comprehensive strategy for eradication of an infestation within a premises must be well planned and must integrate various measures, including inspection, monitoring, control, prevention, and education. After the World War II and before 1990s, the efficient control measure for bed bug infestation referred to application of chlorinated hydrocarbon insecticide (example: DDT, dieldrin) and organophosphate insecticide (example: malathion, diazinon). Following by the resurgence of bed bug infestation prior to late 1990s, there are more and more toxicological and invented management studies been conducted and reported up to date. Relatively, most of the historical studies were referred to C. lectularius compared to that of C. hemipterus (Table 1).
Inspection and Monitoring
Inspection and monitoring are two important procedures in managing bed bug infestations because they help detect the presence of bed bugs within premises and assist in locating the harbourage sites. Inspection and monitoring also provide live bed bug samples for species identification, which is important for designing the control plan (Doggett, 2006; Harlan, 2006). Manual Inspections require well-trained workers with torch light and fine forceps. Other additional studied tools including bed bug-detecting canines (Pfiester et al., 2008), a bed bug intercepting device (Wang et al., 2009a), a pitfall trap baited with carbon dioxide, heat, and chemical lures (Anderson et al., 2009; Wang et al., 2009b). Up to date, however, most inspections still rely on manual inspection by a human due to cost, availability, and other issues.
Education and Prevention
Education about inspection, monitoring, biology, and management of bed bug infestations is current widely available worldwide, in the form of seminars (Barile et al., 2008; Boase, 2008; Doggett & Russell, 2008; Kilpinen et al., 2008; Miller & Fisher, 2008; Naylor et al., 2008; Potter et al., 2008; Turner & Brigham, 2008), professional training and talks, handbooks and publications (Doggett, 2006; Pinto et al., 2007), and internet websites (www.bedbugcentral.com, Bed Bug Central TV in Anonymous, 2009). These activities prominently point out that the cooperation of the public and pest control companies against bed bug infestations is an important element in enhancing the efficacy of pest management strategies.
Prevention of bed bug infestations also can occur by training hoteliers, housekeepers, and the public to detect the presence of bed bugs and to conduct regular monitoring. Infestation prevention measures include improved hygiene maintenance, bed design (fewer cracks and crevices, metal frames), and room design (all possible cracks and crevices are well sealed, not using parquet or carpeted floor, and examination of new or second-hand furniture) (Doggett, 2006).
Physical and Chemical Control
Bed bug infestation control methods can generally be categorized into two groups: physical and chemical. Physical control methods do not use any chemicals or minimize the quantity as much as possible. These methods include bed bug removal through vacuuming (Doggett, 2006, Potter et al., 2008) or trapping (Doggett, 2006; Wang et al., 2009a; 2009b) and heat or cold treatment. In the latter, the objective is to kill the bed bugs using knowledge of their critical upper and lower temperature limits (Doggett, 2006; Roberto et al., 2009; How & Lee, unpublished data). Desiccant dust, diatomaceous gels, and silica gels can be used to cause cuticular damage followed by rapid water loss (Benoit et al., 2009a; Wang et al., 2009a).
Some physical devices can be modified by the addition of a low toxicity chemical as a synergist to improve their efficacy. For example, the addition of alarm pheromone components can prompt the movement of bed bugs and enhance the chances that they will contact the desiccant dust (Benoit et al., 2009a). The addition of carbon dioxide, a heat generator, or aggregation pheromone added to bait in a pitfall trap also can be used (Wang et al., 2009b). However, some traditional physical methods, such as wrapping the mattress in black plastic and exposing it to direct sunlight, may not provided efficient bed bug control (Doggett et al., 2006).
Chemical control especially the residual insecticide treatment, is the most important control measure in battling bed bug infestations. Due to their cryptic nature, bed bugs are able to hide almost anywhere (in particular in obscure cracks and crevices) (Usinger, 1966; Boase, 2004; Potter et al., 2008; Moore & Miller, 2009). Thus, spraying may not last long enough or have enough contact with the bed bugs to kill them. Bed bugs infest human resting places or other places in the home, and this proximity to humans restricts the use of many classes of chemical. Therefore, most of the currently registered insecticide products used for bed bug control are either natural pyrethrins or synthetic pyrethroids (Doggett, 2006; Barile et al., 2008; Doggett & Russell, 2008; Potter et al., 2008; Turner & Brigham, 2008; Moore & Miller, 2009).
SUGGESTION FOR BED BUG MANAGEMENT
The positive correlation between numbers of harborage sites and severity of infestations explains why bed bug infestations can be more serious in rooms or premises with more potential harborage sites. Thus, when PCOs conduct inspections of infested places, they should first inspect the following potential harborages: bedding (including mattress, bed sheet, bed frame, and pillow), headboard, cracks and crevices surrounding the wall and floor, sofa cushions, rattan chairs, and wooden furniture (e.g., cupboards and picture frames) (Hwang et al., 2005; Doggett, 2006; How & Lee, 2010a). In addition, the distribution of infested harborage sites in each place should be recorded by the PCO to build a useful reference profile for faster and easier pre-treatment and monitoring. This reference profile then could be used to track the hypothetical route of bed bug dispersal within a room based on its distribution frequency (How & Lee, 2010a).
Monitoring of bed bug should be conducted one week after treatment to confirm the total eradication of bed bugs, included their 1st instars, as our results showed that C. hemipterus eggs would hatched in the range of 5-7 days (How & Lee, 2010b). Measurement of cast exoskeletons from the apical end of the rostrum to the anal pore was found to be a significant and convenient method to classify the five nymphal stages of the bed bug (How & Lee, 2010b). Using this technique, the composition of bed bug populations in the field could be easily detected and used as an estimation of chronological infestation level.
Our result also indicated that the tropical bed bug, even under starved condition, could survive up to average 100 days. This means that infested premises abandoned or quarantined for three months still likely would contain tropical bed bugs. To apply heat treatment against bed bug infestation, rapidly generated heat and a minimum temperature > 46oC would required (Mellanby, 1935; Johnson, 1941; Usinger, 1966; Benoit et al., 2009b; Roberto et al., 2009; How & Lee, unpublished data). Besides that, the findings from our tropical bed bug movement study suggest that if only late instars and adults are found on the premises during an inspection, the infestation is new; an established infestation likely would consist of mixed stages. This finding also implies that bed bugs can disperse to contiguous rooms during periods of heavy infestation or when treated with insecticides that have strong repellent characteristics, thereby complicating the management plan (How & Lee, 2010c). Accordingly, the inspection or monitoring should be carried out not only in any particular complained or infested room but also on those contiguous rooms as well to confirm and prevent potential dispersal of bed bug.
As mentioned earlier, insecticide resistance is the major concern on bed bug management, particularly on the common pyrethroid products. To overcome these resistance problems, alternative insecticides (Barile et al., 2008; Richard et al., 2008; Turner & Brigham, 2008) that use synergistic chemicals (Romero et al., 2009) or other efficient control measures (Miller & Fisher, 2008; Roberto et al., 2009) should be emphasized and investigated.