Population analysis of Aedes albopictus (Skuse) (Diptera:Culicidae) under uncontrolled laboratory conditions.

Abstract

A semi laboratory experiment using 3 cohorts of Aedes albopictus adults was performed to obtain age-specific mortality and fecundity information and to derive statistical estimates of some population growth parameters. Life expectancy was calculated for both males and females. The following population parameters were estimated: intrinsic rate of increase (rm= 0.21), net reproductive (replacement) rate (Ro= 68.70), age at mean cohort reproduction (To=10.55 days), birth rate (B=0.23), death rate (D=0.02) and generation time (G=20.14 days). The high rm/B (0.91) and B/D (11.50) ratios indicated the high colonizing ability of Ae. albopictus in nature.

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Tropical Biomedicine 25(2): 117–125 (2008)
Population analysis of Aedes albopictus (Skuse)
(Diptera:Culicidae) under uncontrolled laboratory
conditions
Nur Aida, H., Abu Hassan A., Nurita, A.T., Che Salmah, M.R. and Norasmah, B.
School of Biological Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia.
Corresponding author E-mail: aahassan@usm.my
Received 8 August 2007; received in revised form; accepted
Abstract. A semi laboratory experiment using 3 cohorts of Aedes albopictus adults was performed
to obtain age-specific mortality and fecundity information and to derive statistical estimates of
some population growth parameters. Life expectancy was calculated for both males and females.
The following population parameters were estimated: intrinsic rate of increase (rm= 0.21), net
reproductive (replacement) rate (Ro= 68.70), age at mean cohort reproduction (To=10.55 days),
birth rate (B=0.23), death rate (D=0.02) and generation time (G=20.14 days). The high rm/B (0.91)
and B/D (11.50) ratios indicated the high colonizing ability of Ae. albopictus in nature.
INTRODUCTION
Survival is an important parameter when
estimating the vectorial capacity of
arthropod vectors of pathogens (Dye, 1992).
The ability to determine the age structure
and the survival rate of female mosquitoes
is of paramount ecological importance
because longevity affects net reproduction
rates and dispersal distance (Service, 1993).
According to Dye (1992), a critical analysis
of the age composition of a population is
also crucial in epidemiological studies.
Furthermore, knowledge of survival rates
can help in assessing the impact of control
measures (Molineaux et al., 1979).
Determination of population increase from
reproductive capacity is another crucial
component in the study of insect
populations. Population increase can be
described by a fertility table presenting the
potential reproductive ability of females at
different times (Harari et al., 1997).
Most of the previous life-table studies
were concerned mainly with estimating
mortalities of the immature stages. However,
after emergence mosquitoes must live
sufficiently long for mating, blood feeding,
maturation of eggs and oviposition before
there is any new population input from the
emerging generation. During all these stages
there will be further mortalities. If sufficient
field data were available on fecundity,
number and duration of gonotrophic cycles
and survival rates, then it may possible to
construct life and fertility tables for adults
(Service, 1993).
Age-specific horizontal life tables
present a succinct tabular summary of
mortality and reproductive schedules
(Reisen & Mahmood, 1980). When
constructed under relatively benign
insectary conditions, with ample resources
the value obtained may approach a
maximum and may be used to study inherent
differences in the survivorship and
reproductive strategies of populations of
different species evolving under different
ecological regimes.
Recently, the life tables and reproductive
strategies of colonized culicine mosquitoes
including domestic Aedes aegypti (L)
(Crovello & Hacker, 1972), Culex
quinquefasciatus Say (Walter & Hacker
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1974), Culex tritaeniorhynchus Giles
(Reisen et al., 1979) and anopheline
mosquitoes, Anopheles albimanus and
Anopheles vestitipennis (Greico et al., 2003)
were compiled. However, complete
survivorship and
schedules have not yet been compiled for
Aedes albopictus.
Because of the interest in the ability of
this species to survive long enough to
become both infected and infective with
dengue virus, studies on longevity have
previously been conducted in the laboratory
under controlled temperature and humidity
regimes and under field conditions using
mark-release-recapture techniques (Gubler,
1970; Gubler & Bhattacharya, 1971; Lee,
2000, Honorio et al., 2003). Difficulty in
colonization has however, precluded a
tabulation of the age-specific reproductive
efforts of this species.
Greater longevity increases the
probability of transmitting diseases (Estrada-
Franco & Craig, 1995). Temperature and
relative humidity are among the factors that
play a vital role in adult survival (Estrada-
Franco & Craig, 1995). However few studies
to estimate longevity of Ae. albopictus adult
in the field or under uncontrolled condition
(but not directly in the field) have been
attempted.
The goal of this study is to determine the
survival of adult Ae. albopictus and to
determine the variables of life and fertility
tables to be used as a basis for understanding
the population dynamics of Ae. albopictus
under uncontrolled laboratory conditions.
fecundity-fertility
MATERIALS AND METHODS
Study site
Aedes albopictus adults used in this
experiment were produced from field-
collected eggs in Lurah Burung, Universiti
Sains Malaysia (USM), Penang. These eggs
were collected by using ovitraps. This
experiment was conducted in the
insectarium located in the School of
Biological Sciences, USM. Throughout the
study, temperature and relative humidity in
the insectarium were allowed to fluctuate
with the weather outside (uncontrolled
conditions). Photoperiod was also
unregulated and changed with the
surrounding environment. Windows were
opened and no temperature controller or air
conditioner was used during this study. The
study was carried out from 10 March to 31
May 2006.
Sampling technique
Adults were allowed to mate freely and were
provided with continuous access to 10%
sucrose solution and daily blood meal from
laboratory mice from the first day of their
emergence. After determining the sex, the
adult mosquitoes were promptly placed in
adult rearing cages measuring 30cm x 30cm
x 30cm. Thirty females and 30 males were
placed simultenously in each cage, 3 cages
of 60 Ae. albopictus were established to
quantify adult longevity.
The cages were examined daily in order
to record and remove dead mosquitoes. An
opportunity to blood feed was provided daily
to all females between 1800h-1900h by
providing them with laboratory mice
confined in a narrow and tight-fitting fine
wire-mesh cage for 30 minutes. Everyday
after feeding, ovipaper (filter paper Whatman
No. 1, folded into a cone and moistened,
served as the ovipositon subtract) were
changed and eggs counted. These data were
recorded daily until all mosquitoes were
dead.
All eggs that were produced from this
experiment were hatched and larvae were
reared until adults emerged in the same
insectarium in the School of Biological
Sciences, USM. The sex ratio of female to
male adult mosquitoes was determined by
counting the number of adults.
Data analysis
The formulae used in this study essentially
followed Reisen et al. (1979).
1.Age-specific survivorship, lx = yx/yo
where yx = the number of mosquitoes
that were alive on day x, and yo = the
starting number of mosquitoes in the
population.
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2. Age specific life expectancy ex = Tx/lx,
w
where Tx = Σ Lx and Lx = (lx=lx+1)/2
x=1
with w = the day the last individual died.
3.Net reproductive rate per cohort,
w
Ro = Σ Lxmx where, mx = the number
x=1
of female offspring produced per female
per age interval. mx were observational
data.
4.Age of mean cohort reproduction,
w
To = ΣxLxmx/Ro starting at x = 1, the
x=1
day of adult emergence.
5. The instantaneous rate of increase in
females per female was estimated by
using Price (1997) equation rm = logeRo ,
To
and then to get the actual rm value,
6. Dobzhansky et al. (1964) modification of
the original Lotka-Euler equation was
used, where e is the base of the natural
logarithms
w
1 = ΣLxmxe-rm(x+To)
x+1
Mean generation time in days, G, could
then be calculated as
7.
G= logeRo/rm
8. To calculate the instantaneous birthrate
(B), the equation used was B = ln(1+b)
w
where 1/b = Σ Lxe-rm(x+1)
x=1
The death rate (D) was then calculated
as D = B-rm.
RESULTS
During this study, mean daily temperatures
in the insectarium ranged from 26.1 ºC to
32.5 ºC in accordance with the outside
temperature. Highest relative humidity
recorded was 86.6% and the lowest was
68.0%.
Figure 1 shows the age specific
survivorship for male and female Ae.
albopictus reared under uncontrolled
laboratory conditions. The average longevity
for male and female Ae. albopictus obtained
from the daily survival curve were 11.29 days
(1-27 days) and 18.86 days (2-36days)
respectively. The males of Aedes albopictus
demonstrated a shorter adult life span than
the females.
Figure 2 represents the reproductive
effort or net reproductive rate of 30 Ae.
albopictus females throughout their life
span. A plot of the number of female
offspring produced by a female of a
particular age (in days) for Ae. albopictus
showed that the curve peaked within the fifth
and seventh day of life and declined as the
age of the female increased.
Table 1 shows the life-history
characteristics of Ae. albopictus. An average
of 221 eggs (220.65) per female was laid
throughout its life span. The Ro value
representing the mean number of female
offsprings produced by a single female from
a cohort during the course of its lifespan was
calculated to be 69 females (68.7). The age
of the mean cohort reproduction (To)
calculated for Ae. albopictus females was
10.55 days. The To was also known as the
time of mean reproductive effort. Higher To
decreased the innate rate of increase (rm).
The intrinsic rate of increase (rm) for Ae.
albopictus was calculated to be 0.21 per
female per day of life. Longer To caused not
only a slower rm, but also a longer generation
time (G) in Ae. albopictus female which was
20.14 days. The birth and death rate of Ae.
albopictus were 0.23 and 0.02 respectively.
Here, it is apparent that there is a huge
difference in the rate. This is also true for
the rm/B and B/D ratio, which were 0.91 and
11.50, respectively.
Figure 3 represents the life expectancy
curve for Ae. albopictus populations
obtained from 3 cohorts. The life expectancy
value of adult females decreased
consistently with age, except for a small
increase on the ninth day of female
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Figure 1. Age-specific survivorship (lx) for colony adult Aedes albopictus under uncontrolled
conditions.
Figure 2. Number of female offspring (Ro) produced by 30 female per generation of Ae.
albopictus in Penang.
emergence. Immediately after emergence as
adults, the females had an average life
expectancy of 19.47 days. However, the ex
value for males did not decrease consistently.
The average life expectancy at the time of
emergence as adults for male Ae. albopictus
was 10.17 days.
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DISCUSSION
In the present study, the mean number of
eggs laid per female during the course of a
life span of Ae. albopictus was 221 (220.65).
This value is lower than that observed by
Gubler & Bhattacharya (1971) where the
total lifetime egg production was from 0 to
784 eggs (total average 283.1 eggs).
According to Del Rosario (1963), the number
of eggs laid per female Ae. albopictus fed
only with human blood was 46 eggs (range 6
to 124). However, comparisons between our
study and the studies by Gubler &
Bhattacharya (1971) and Del Rosario (1963)
would be inaccurate because their studies
were conducted under controlled laboratory
conditions. In the field, egg production is
dependent on environmental factors such as
temperature and relative humidity, since
these factors indeed influence the survival
of adult mosquitoes. The number of eggs laid
by Ae. albopictus also depends on the
physiological age, the body weight after
emergence and particularly the size of the
blood meal (Hien, 1976). In addition, egg
production also depends on the development
of the immature stages. An optimum
condition during the growth of immatures
will result in larger and healthier adults who
can consume more blood from the hosts to
produce eggs.
In this study, females were placed with
males in the cage and this enabled them to
mate freely but flight was limited. Although
the mosquitoes were reared under
uncontrolled conditions, food and an
oviposition site was provided and predators
were nonexistent. Based on the number of
females produced per female per day, almost
Figure 3. Life expectancy of male and female Ae. albopictus from time of emergence.
Table 1: Adult life table characteristics of
Aedes albopictus
Attribute
Aedes albopictus ♀♀
e1: ♀
♂
Tp
Ro
To
rm
G
B
D
rm/B
B/D
19.47
10.17
220.65
68.70
10.55
0.21
20.14
0.23
0.02
0.91
11.50
Attributes: e1 = mean life expectancy from emergence
in days; Tp = Total lifetime eggs production; Ro = net
reproductive rate in living female progeny per female
per generation; To = age in days at mean cohort
reproduction; rm = instantaneous rate of in living female
per female; G mean generation time in days; B =
instantaneous birth and D = death rate.
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