This investigation showed lack of global spatial dependence of the number of female adult Ae. aegypti captured in BG-Sentinel traps that were uniformly spaced (130 m) in each of two neighborhoods during 20 consecutive population samples every 3 weeks in San Juan, Puerto Rico. This finding means that adult females of Ae. aegypti were not clustered in particular areas of the neighborhoods, which is unfortunate; these neighborhoods could not be stratified into areas with varying mosquito densities that would simplify vector control operations. This finding is possibly because of the functional homogeneity in terms of housing type, basic public services, etc. of the residential neighborhoods investigated here.
The analysis of local spatial dependence did reveal local clustering or hot spots scattered throughout the neighborhoods, and the temporal analyses showed a relatively high concordance in the rank order of trap productivity in time, which translates into a pattern of spatial stability of Ae. aegypti
females in both neighborhoods. Spatial stability, expressed as the persistence of hot spots for periods of time at the same locations, has been reported for tsetse flies in Luke Community, Ethiopia.24
A previous study using BG traps revealed significant spatial clustering of adult Ae. aegypti
at the household scale but little temporal clustering in individual traps that were operated for 15 days in Cairns, Australia.25
Other previous studies conducted at the household scale did not show spatial consistency in adult or immature density in time.12,18,19
This study differs from previous ones in that we sampled every 3 weeks for over 1 year, which allows for the observation of how spatial patterns change in time in greater detail, and our sampling was done at the scale of city blocks (130 m). The scale at which observations are made seems to be an important component that merits additional investigations. For example, Getis and others12
showed that the spatial dependence of Ae. aegypti
disappeared beyond 30 m in Iquitos, Peru. Exploring scale effects can help optimize entomological surveillance and vector control.24
The results of the present study also showed significant correlations in the rank order of mosquito abundance per trap at most forward time lags throughout the study (), which means high predictability in the spatial pattern of Ae. aegypti
productivity. Captured mosquitoes were most likely produced nearby, because in most mark–release–recapture studies, Ae. aegypti
adults are captured within 100 m a few days after release,21,26,27
with the exception of gravid females that can fly longer distances in search of containers with water.28
The permanency of the rank orders of abundance of Ae. aegypti
females in time must reflect the existence of persistent, local sources of mosquitoes near the traps.29
The important consequence of the existence of relative stability in the spatial pattern of trap yields is that the hot spots could be targeted for a more efficient vector and dengue control. However, this strategy clearly points out that vector control organizations would need to conduct vector surveillance at similar scales. The advent of mosquito surveillance devices, such as the BG-Sentinel trap or similar devices that reflect the local abundance of adult Ae. aegypti
, provides the opportunity to do this surveillance.
The spatial heterogeneity of Ae. aegypti
females per trap was considerable ( and ). One trap captured 91 females and 153 males of Ae. aegypti
in a single day in the porch of a house. It is conceivable that, if a dengue-infected person stays at one of such hot spots in the study areas, it could initiate the local transmission of dengue viruses. Furthermore, it is reasonable to propose the hypothesis that Ae. aegypti
's hot spots are the most likely places where dengue viruses get established and from which dengue viruses can be exported to other areas. It has been shown that dengue virus transmission is highly focal in nature and associated with the abundance of Ae. aegypti
but it has not been shown if the elimination of local hot spots could prevent the establishment of dengue viruses. There is evidence showing that dengue infections tend to recur at or near the same places in time,31,32
which might be because of persistent Ae. aegypti
's hot spots.
Spatial stability faded during periods of significant increases in rainfall and high Ae. aegypti
adult density in EC, which was revealed by the negative correlations between these variables and the Spearman's correlation coefficients. The negative correlations mean that the rank orders of trap captures drastically changed from one sampling date to the next. This transient change in the spatial pattern of trap captures may be indicative of the recruitment of many containers that were filled with water in the study area, but the spatial pattern in mosquito productivity soon returned to its previous order after reductions in population density ( and ). This observation seems to suggest that mosquito sampling after heavy rains would not necessarily reflect the prevalent spatial pattern of productivity. From a vector control perspective, it implies that vector surveillance should be conducted more frequently during periods in which the population of Ae. aegypti
expands. Our results are strikingly similar to the results of Sciarretta and others,33
who have recently described patterns of spatial stability in tsetse flies that were transiently disrupted after significant increases in the size of the fly populations; this stability was followed by a quick return to the previous spatial pattern associated with lower fly densities.
The effect of rain and mosquito density on the dissimilarity of rank order trap captures was not observed in VC (). This neighborhood was more intensely subjected to spatial spraying of insecticides than EC, and perhaps for that reason, Ae. aegypti
adult abundance during the peak of the rainy season in VC was also smaller than in EC: (CDC, unpublished). There is evidence that effective vector control changes the spatial pattern of adult Ae. aegypti
. For example, immature control measures targeting surface containers in a southern Puerto Rican town changed the spatial pattern of adult mosquitoes from one in which there was no clustering before control to one in which significant clusters appeared around untreated, underground aquatic habitats.34
Thus, it is likely that the spatial pattern of Ae. aegypti
is bound to change after the application of effective vector control measures. For this reason, it is recommended that vector control measures be monitored for their effectiveness in reducing adult mosquito abundance and the spatial distribution of mosquitoes.34
Given that hot spots tend to be stationary for periods of time, it is likely that an approach based on adaptive population management could result in an efficient way to reduce the risk of local dengue transmission. Adaptive population management has been successfully applied to reduce stationary hot spots of tsetse flies.24
This management approach relies on the dynamic interaction between entomological surveillance, aimed at identifying hot spots, and application of local vector control in and around hot spots. Clearly, adaptive management depends on efficient vector surveillance, prompt data analysis, and mapping capabilities. Future research on novel ways to control dengue could focus on developing inexpensive but efficient traps for adult Ae. aegypti
, establishing proper scales for trap deployment, and testing the effectiveness of adaptive control.