Aedes albopictus in Bermuda:
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seasonality, spatial correlates and density dependence
David
1
Kendell ,
Camilo
2
Khatchikian ,
Laran
2
Kaplan
and Todd
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2
Livdahl
1
1Bermuda
Ministry of Health
2Department of Biology, Clark University, Worcester MA 01610
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Figure 4: Correlations between
sites counts by yearly intervals. A
decay function was fit to the data
forcing y-intercept to equal 1
(R2=0.636, P<0.001). Intervals (time
lags) range from 1 to 7. Such a decay
function may provide information
about the relative stability of habitats,
a possibly useful control metric.
Temporal Patterns
2002-2007, Mean ± SE
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-The Bermuda Department of Health keep a weekly ovitrap
monitoring program in entry ports, and have increased trap numbers
after detection of small container breeder mosquitoes (135 by 2000,
300 by 2003, and 582 by 2005). Weekly egg counts from these traps
provided the basis for this study.
-Presently, an active mosquito program combines GIS assisted
tracking and monitoring with control measures, including fish and
chemical control of breeding sites and crews in the field removing
discarded containers suitable for breeding mosquitoes.
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E ggs/paddle
-Aedes aegypti occupied Bermuda from the slave trade era until its
eradication in the mid-1960s. Aedes aegypti was rediscovered in
1998, and Aedes albopictus arrived in 2000.
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C o rrela tio n C o efficien t
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Introduction, Mosquitoes in Bermuda
-Bermuda is a territory that consists various islands with a total area
of 53.32 km located off the east coast of USA. It has over 65,000
habitats and receive around half million tourists per year.
GOVERNMENT OF BERMUDA
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Time Lag (y)
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Spatial and Demographic Variables
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Figure 5: Effects of the
distance to roads (a) and
buildings (b) on the
number of egg in ovitraps.
Data are for all years
combined, using annual totals
for each trap. Groups with
common letters are not
significantly different from
one another (p>0.05, Tukey’s
HSD). No differences were
found in (b).
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Week
Figure 1: Weekly egg production, pooled across years (2002-7). Mean number of
eggs per trap, ±SE (based on residual mean square from 1-way ANOVA). This pattern
suggests an early generation, with major reproduction in late June, followed bv a at
least two more distinct peaks, in August and September. These latter peaks may
represent subsequent generations. Eggs were found in traps during every week for at
least one of the years of the study period.
-Laran Kaplan (2006) identified eggs collected in ovitraps in the
period 2003-2005 using molecular methods (PCR, ITS-1), finding a
progressive reduction of Ae. aegypti (from 12.5% prevalence in 2003
to 3.7% in 2005).
-All the analyses conducted in this study assume that the Ae. aegypti
population in the islands is extremely small.
Figure 6: Effect of human
population over total count of
mosquitoes
eggs
collected.
Quadrats were sampled randomly and
without overlap from the database of
egg counts for the entire period of
study (Area = 0.25 Km2). (R2=0.113,
P<0.05).
-The Bermuda case provides a unique opportunity to study
mosquitoes invasions at a very fine scale, where environmental
variables remain constant, with an extended spatial and temporal
sampling program.
Figure 2: Number of eggs per week per year. Groups with common letters are
not significantly different from one another (p>0.05, Tukey’s HSD).
Main Conclusions
-Aedes albopictus appears to be density dependent regulated growth, with
some indication that it overshot its carrying capacity during 2005.
-The early decline in Ae. aegypti, preceding indications of density effects,
suggests that competitive exclusion is an unlikely mechanism for its
displacement by Ae. albopictus.
-Decrease in temporal autocorrelation suggests a progressive decay of
breeding habitats quality with time (habitats remain favorable for less than 7
years, on average).
-Association of counts with roads and human population suggests that humans
are a relevant factor providing breeding habitat for Ae. albopictus in Bermuda.
Reference:
Figure 1: Location of ovitraps used in this study. Solid dots indicate the location of the
ovitraps used in this study (total 299). The criterion for selection was the availability of
multiple years of data in order to allow comparisons to be made. The Department of Health
keeps all ovitrap data georeferenced in GIS databases, in conjunction with infrastructure and
demographic files.
Overlay
Figure 3: Response of per capita rate of change to density. Per capita growth
rates were calculated as annual rates, using (lnNt+1/Nt), using total eggs for each
site as N for all 1 km2 quadrats, chosen by overlaying a grid, excluding quadrats that
contained bodies of water. R2=0.496, P<0.01
Kaplan, L. 2005. Aedes aegypti and Aedes albopictus in Bermuda: The spatial and temporal distribution from 20002005. M.A. thesis, Clark University, Worcester, MA.
Acknowledgments: We appreciate the generosity of the National Institutes of Health (R15
AI062712-01) and the Keck Fundation, the help and fundamental collaboration of the Bermuda
Health Department, the help provided by Clark’s University students in Biol 224, for 2004, 2005,
and 2007. We thank Prof. Deborah Robertson for assisting with Laran Kaplan’s molecular work.