Studies on the control of Aedes aegypti (L) the vector of urban yellow fever and dengue fever have yielded inconclusive results and therefore are not prescriptive 1 . The ‘dengue belt’ which runs between latitudes of 35°N and 35°S is home to over 3 billion people who are at risk of being infected. Each year, over 50 million cases of dengue fever, 600,000 cases of dengue haemorrhagic fever and its complications and 93 million asymptomatic cases are reported 2 3 . In the Americas, over 1.7 million cases of dengue fever have been reported with 50,000 clinically severe cases and 1,185 deaths reported in 2010, with an incidence of circa 200 cases per 100,000 population in many countries in Latin America and the Caribbean region 1 3 .

Historically, control of Ae. aegypti was based on fumigation and reduction in the number of mosquito breeding sites 4 . With the introduction of DDT, both malaria and Ae. aegypti were controlled in some countries and eradicated in others 5 . However, the underlying mechanisms of how these vector populations were managed have never been scientifically evaluated 1 . Most studies report that these programs failed due to DDT resistance in the early 1960s but, few studies have examined why DDT was efficacious prior to developing resistance 1 . Even today many workers continue to advocate control methods for Ae. aegypti that include environmental sanitation and source reduction to destroy mosquito breeding sites and space spraying and ultra low volume spraying to kill flying adults 6 7 . However, Chadee 8 demonstrated the failure of the ULV application by truck- mounted machines, because the insecticides did not penetrate closed doors and windows and were blocked by barriers such as perimeter fences constructed of solid bricks.

The efficacy of focal treatment of breeding sites is well documented 5 , but success of this strategy depends on numerous factors including; 1) goodwill of communities disposing them to allow access to their homes and to participate in environmental clean-up campaigns 1 ; 2) political will to maintain continuous program funding 1 9 : 3) susceptibility of Ae. aegypti to the insecticides being used 10 and 4) adequate work ethic of vector control management and field staff 1 .

Studies conducted in Africa 11 , South East Asia 12 and Australia 13 have reported on the elusive resting habits of Ae. aegypti 14 . In the Americas, however, reports on the seclusive resting behaviour of Ae. aegypti within homes have come from mainly Panama 7 , Costa Rica 15 , Dominican Republic 16 , Puerto Rico 17 and Mexico 18 . In addition, studies have shown dengue infected Ae. aegypti in and around houses in Asia 19 and in the Americas 20 . These studies suggest that houses provide suitable environment for man-vector contact, dengue transmission and suitable resting sites for adult mosquitoes.

Recently Chadee 1 reviewed emergency control measures for Ae. aegypti and reported the efficacy of indoor residual spraying (IRS) or intradomicillary spraying and insecticide treated curtains 18 . This strategy is based on the behaviour of Ae. aegypti which usually rests on wall surfaces before and after blood feeding, thus bringing the mosquito into contact with the insecticide treated material or wall surface resulting in high mortality rates. To date, three studies have provided indirect support for the re-introduction of IRS to control Ae. aegypti mosquitoes, with two studies providing evidence needed to support this recommendation. Chadee 21 evaluated focal treatment, ULV and IRS and reported effective control but was unable to explain the reasons for the significant reductions in the mosquito populations. The “cardinal points” study reported that 67% of the dengue positive houses harboured Ae. aegypti mosquitoes 22 while the “casa segura” concept reported the collection of dengue positive mosquitoes inside houses 27 days after initial clinical diagnosis 23 . Therefore the “cardinal points” 24 and the “casa segura” 25 studies provide further evidence of the presence of Ae. aegypti within homes. However, these studies did provide information on the mechanisms which control Ae. aegypti resting behaviour, information necessary for developing more scientifically sound evidence-based control strategies for both the present and the future.

The present study was conducted to determine the primary resting places of Ae. aegypti in houses in Curepe, Trinidad, West Indies and how soon after laying eggs females take a blood meal. These results reveal the behavioural and physiological processes which may account for the presence or resting behaviour of Ae. aegypti inside houses and have implications for dengue outbreak interventions.

The results of the present study quantified the resting behaviour of Ae. aegypti in St. Augustine, Trinidad, W.I. with most females collected from bed rooms (81.9%), living rooms (8.7%) and kitchens (6.9%). Most resting mosquitoes were collected from dark wall surfaces close to the floor rather than on the upper surfaces of the walls close to the ceiling. These results are very similar to that observed in Panama 7 , Costa Rica 15 , Dominican Republic 16 , Puerto Rico 17 and Mexico 18 .

Numerous physiological factors may be responsible for the resting behaviour observed during this study. Chadee and Beier 31 reported that Ae. aegypti mosquitoes processed blood by pre-diuresis during blood feeding and by diuresis on completion of blood feeding, especially while resting on clothing or under furniture and on the dark lower part of the wall surface close to the floor. This resting activity is confirmed by studies conducted by Klowden and Briegel 32 who suggested that after a critical threshold in blood-feeding is reached, females usually rest for two physiological processes to occur: first for blood-induced-distension of the gut wall which triggers a refractory period during which females no longer seek or take blood meals 33 ; and second, if the blood meal initiates vitellogenesis, a second inhibition accompanies oocyte development 34 35 and this haemo-lymphborne factor is produced circa 30 hours post blood feeding. This inhibition normally lasts until oviposition and is often referred as the oocyte-induced-inhibition 35 . Based on a preliminary laboratory study females were often observed to remain stationary and seldom move from their resting place until pre-oviposition flight (Chadee, Unpub data). Studies conducted by Bowen 36 , Fuch and Kang 37 , Klowden and Briegel 32 , Klowden 33 and Chadee 38 suggest that Ae. aegypti females required periods of “quiescence” or resting periods (Table 2). These studies clearly demonstrated that females rest for between 36-50 hours post blood-feeding. In fact, Ae. aegypti females collected while resting indoors in Panama were parous (48.5%) and possibly contained oocytes of various Christophers stages and can be considered old females.

During the present study only 10% of the females offered a blood-meal immediately after oviposition, fed but the majority required a rest of at least 12 hours before taking a blood-meal (Figure 1) a similar percentage to that reported by Chadee 38 . Results from these various studies suggest that Ae. aegypti females remain indoors or at resting sites for long periods and this may explain the failure of conventional ultra-low-volume (ULV) spraying using either aerial or truck mounted applications to kill or reduce vector populations. Studies conducted in Panama explained that the resting behaviour of Ae. aegypti was largely responsible for the failure of the spray programs because little or no insecticide aerosol droplets reached these indoor areas especially in bed rooms 7 16 . The location of these resting sites was also a primary reason for the failure to control Ae. aegypti in Trinidad 8 , Suriname 6 , Dominican Republic 16 , Panama 17 and Jamaica 39 since physical barriers such as block fences and closed doors and windows impeded the penetration or movement of the insecticides from the roadside into houses 8 38 .

Conversely, these results suggest that insecticide applications targeting the resting sites of Ae. aegypti mosquitoes may be more effective in reducing or controlling these vector populations. Therefore, the re-introduction of intra-domiciliary spraying or indoor residual spraying (IRS) may prove to be a viable option based on the evidence provided in this study. That is controlling dengue using an old method but with new understanding of the underlying behavioural mechanisms of the mosquito. For example, the success of the early malaria programs in the 1950s and 1960s using IRS for the application of DDT resulted in the successful control/eradication of Ae. aegypti mosquitoes in some countries in the Americas 5 . The initial success of this program was based on the resting behaviour of Ae. aegypti on wall surfaces before and after blood feeding which brings the mosquito into contact with the insecticide treated material or wall surface while the mosquitoes are undergoing diuresis, a physiological process which removes excess liquid from the blood meal and results in compact red blood cells which can be used for egg production. During this post-feeding period females fly to suitable resting sites or remain at the site if not disturbed for the completion of vitellogenesis after which pre-oviposition flight occurs to suitable breeding sites. In addition, this resting notion is supported by studies on Ae. aegypti circadian rhythms in Trinidad which demonstrated that peak blood feeding, oviposition, sugar feeding and copulation occurred between 06-09 hours and at 16-18 hours (Figure 2), with most females resting for circa 19 hours except for some atypical biting activity reported by Chadee and Martinez 29 .

The present study therefore now provides the evidence for the efficacy of IRS programs and explains why some countries in the past were successful in controlling and/or eradicating malaria and Ae. aegypti mosquitoes 5 . Therefore, the challenge for the present and future will be to re-introduce IRS into the dengue control armoury especially as part of an integrated vector management program (IVM). IRS would be suitable for both emergency and routine control programs but IVM managers should ensure the prudent use of insecticides by using 3 options currently available rather than repeatedly applying the same insecticide each year as outlined by World Health Organization 40 . Firstly, by alternating the use of different insecticides in time and space which reduces the frequency of resistance, secondly, by using a combination of different insecticide classes and thirdly using mixtures of two insecticides for coordinated/synergistic action which will further delay the onset of resistance 40 . The use of both old and new insecticides which have been found to have long residual activity would be ideal (e.g. paint-based mixtures of insecticides or entomopathogenic fungi) thus delaying the development of insecticide resistance. In addition, IVM managers should elicit political support for the retraining of staff using the IRS techniques and to expand space spraying (fogging) programs which, under suitable circumstances, can also penetrate inside houses and kill resting mosquitoes.

This study provides the evidence for the efficacy of IRS in controlling the Ae. aegypti mosquitoes and dengue fever transmission by highlighting the major behavioural and physiological factors which contribute to their resting behaviour inside houses in St. Augustine, Trinidad.

IRS: Indoor residual spraying; IGR: Insect growth regulator; DDT: Dichlorodiphenyltrichloroethane; ULV: Ultra-low-volume.

The author declares that he has no competing interests.