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The Adaptation of Field Collected Aedes aegypti (L.) and Aedes albopictus (Skuse) in Laboratory Condition

Aedes aegypti and Aedes albopictus has been incriminated in dengue transmission in Malaysia, and all available measures has been taken to reduce the dengue cases and mortality. However, statistical data showed increasing trend in dengue incident. The aim of this study is to evaluate the adaptability F1 offspring of field collected Aedes mosquitoes in a new environment (laboratory). Biological parameter of Aedes mosquito, such as the larval growth, survival and fecundity were scrutinized under laboratory condition. Data obtained from this study showed that both Aedes species were highly adaptable to laboratory condition. The finding of this study is essential for vector control. KeywordsAedes Albopictus; Aedes Aegypti; Larval Growth; Survival; Fecundity


B. The Hatching Rate of Field Strain Aedes Aegypti and Aedes Albopictus
Gravid Ae. aegypti and Ae. albopictus female mosquitoes (n=150 females from each Aedes species) were individually transferred to paper cups containing filter paper for oviposition. Filter papers were removed from the paper cups and air-dried for 24 hours, only filter papers with more than 70 eggs were used in this study. Dried filter papers were divided into 9 groups and labelled according to designated time for hatching.
Filter papers from each group were submerged into dechlorinated water according to the designated time as described below: Group 1: One week after egg collection; Group 2: Two weeks after egg collection; Group 3: Three weeks after egg collection; Group 4: One month after egg collection; Group 5: Two months after egg collection; Group 6: Three months after egg collection; Group 7: Four months after egg collection; Group 8: Five months after egg collection; Group 9: Six months after egg collection. At the designated time, the filter papers were placed in the plastic trays (size: height = 5 cm, length =17 cm, width=12 cm) and flooded with dechlorinated water (200 ml). No food was added during the experiments. All larvae emerged was recorded every day until no hatching observed.

C. The Development Rate of Field Strain Aedes Aegypti and Aedes Albopictus
Aedes eggs were vacuum-hatched to ensure the uniform age of the larvae. One hundred first instar larvae were randomly transferred into 100 individual vials (diameter = 2.5 cm and height = 7.5 cm) filled with dechlorinated water (water volume: 5 ml water on day 1 and 10 ml of water from day 2 onwards). About 50 mg of larval food (dog biscuit, sun-dried beef liver, yeast, milk powder in a ration of 2:1:1:1, ground into a powder) was diluted with 5 ml of dechlorinated water. On day one, only one drop of larval food was dropped into each vial and the drops were increased to two or three drops a day after day two. Water was changed everyday to remove scum. The development rate of the larvae and pupae were monitored at a fixed time and recorded.

D. Survival of Field Strain Aedes Aegypti and Aedes Albopictus Fed on Different Diets
Field strain of Aedes mosquitoes was separated into three groups of adult mosquitoes. The first group consists of male Aedes mosquitoes fed with 10% sucrose, the second group consists of female Aedes mosquitoes fed with 10% sucrose, and the third group consists of female Aedes mosquitoes fed with blood meal and 10% sucrose. Each group consists of 50 adults kept in individual paper cup. Each cup was covered with a piece of muslin cloth with a slit in the middle. Each cup was monitored daily and mortality was recorded.

E. The Length of Gonotrophic Cycle and the Number of Eggs Laid by Field Strain Aedes Aegypti and Aedes Albopictus
Two sets of 30 paper cups were prepared with moist cotton wool and filter paper for oviposition. The top of the cups were covered with muslin cloth. A day after blood meal was given, 30 gravid female Aedes sp. were transferred individually into the first 30 cups. Cotton balls soaked with sucrose were left on the top of the cups as food source. The next day (day 2), the females from the first set were transferred to the second set of cups. Filter paper from the first set of cups were removed and replaced with new set of filter paper. Each set of filter paper removed daily were air-dried before the number of eggs recorded. This experiment was continued until there were no eggs deposited.

F. Data Analysis
Data was expressed as mean ± S.E (Standard Error). Data obtained were analysed for normality test. As the data was found not normally distributed, a non-parametric test (Mann Whitney U test) was used to analyse the data (SPSS analysis version 11.0, all statistical test was considered significant at p = 0.05).

A. The Hatching Rate of Field Strain Aedes Aegypti and Aedes Albopictus
Under laboratory condition, eggs produced by field strain Ae. aegypti and Ae. albopictus (F1 generation) showed a very low hatching rate. Highest hatching rate produced by Ae. aegypti eggs was 49.1% for one month old eggs, while Ae. albopictus eggs achieved highest hatching rate for 3 weeks old eggs at 42.4% (in Table 1, Mann Whitney U test at p = 0.05). The hatching activity starts to decrease after the peak hatching rate for both Aedes eggs. The hatching activities has stopped for 5 or 6 months old Ae. albopictus's eggs. Similarly no hatching was observed for 6 months old s Ae. aegypti's eggs. Laboratory studies by Macgregor [10], Shanon and Putnam [11] and Hien [12] have proved that the age of the Aedes eggs at the time of drying influence the eggs to withstand desiccation but the eggs will die if they become too dry when they were first laid. Eggs with fully developed embryo can withstand dry conditions for long periods of time [13]. The production of eggs that are resistant to desiccation makes Aedes mosquitoes and ideal colonisers of temporary collections of water [14].

B. The Development Rate of Field Strain Aedes Aegypti and Aedes Albopictus
The development rates from larval to adult stages of Ae. aegypti were shorter than that of Ae. albopictus. Ae. aegypti took about 6 days to reach the adult stage while Ae. albopictus took about 7 days respectively to reach the adult stage. The pupal stage for both Aedes species lasted 1 to 2 days (in Table 2, Mann Whitney U test). Numerous experiments have been conducted to determine the development rate of Ae. aegypti and Ae. albopictus. In a tropical environment, immature stages of Aedes require about 7 days before adult emergence [15]. Vythilingam et al. [16] and Lee [17] found that the larval period of Aedes sp. was 6-8 days while pupae took about 1-2 days. Abu Hassan and Yap [18] also found that the life cycle of Aedes from the egg to adult stage is 6-8 days.

C. Survival and Longevity of Field Strain Aedes Aegypti and Aedes Albopictus Fed on Different Diets
During the study, females of both Aedes mosquitoes fed with blood and sucrose averaged about 45 to 49 days, while females of both Aedes sp. fed with sucrose only averaged about 49 to 51 days. Male Ae. aegypti fed with sucrose survived 51 days while Ae. albopictus lived only 37 days (in Table 3). Based on the findings, female and male Ae. aegypti fed with sucrose live longer compared to other test groups under controlled laboratory condition. Mean number of days followed by the same letter within the same columns are not significantly difference (P = 0.05, Mann Whitney U-test).
Survival is prolonged by feeding [25], but the type of nourishment provided to the females affects their longevity [26]. Lewis [27] assumed that mosquitoes depend on blood and nectar for survival as they do not possess an efficient mechanism to prevent water loss.
Temperature and relative humidity appears to be the primary factors for adult survival [22,28]. Bates [29] and Hylton [30] observed that longevity of female Ae. albopictus increase under constant humidities and low temperature. At higher temperature, [30] found that the life span of female Ae. albopictus was reduced regardless of humidities. At low humidities, insects are unable to survive probably because of their inability to control water loss [31].

D. The Length of Gonotrophic Cycle and the Number of Eggs Laid by Field Strain Aedes Aegypti and Aedes Albopictus
About 80% of the total eggs were laid during the first four days of experiments. Both females Ae. aegypti and Ae. albopictus took their first blood meal 5 days after emergence. Female Ae. aegypti and Ae. albopictus took about 14 days and 16 days respectively to deposit all their eggs. Furthermore, female Ae. aegypti laid more eggs than female Ae. albopictus. Under laboratory conditions, gonotrophic cycle of both Ae. aegypti and Ae. albopictus are 3.00 and 2.73 days respectively. Mann Whitney U test showed that mean number of eggs deposited by female Ae. aegypti and Ae. albopictus are significantly different (at p=0.05) (in Table 4).  [33]. They found that Ae. aegypti do not always lays all their eggs at once but deposit their eggs in batches over several days. In an earlier study, Macfie [34] found that Ae. aegypti oviposition begins on the 3rd or 4th day after a blood meal.
According to Clements [35] and Pant et al. [36], temperature affects the length of gonotrophic cycle of Ae. aegypti. In tropical conditions, gonotrophic cycle lasts two days for most species [14,35]. At a temperature of 25 °C to 26 °C and relative humidity of 50% to 60%, Gubler and Bhattacharya [37] found that gonotrophic cycle of Ae. albopictus lasted three to five days while Hien [26] found that the gonotrophic cycle lasted three to three and a half days under the same temperature but relative humidity of 60% to 70%. In another experiment, Mori and Wada [38] demonstrated that under natural condition (average field temperature: 25 °C), the duration of gonotrophic cycles for Ae. albopictus was five days. A number of studies conducted in the field and laboratory showed that Ae. albopictus took an average of five days for the first and second gonotrophic cycles [22].

IV. CONCLUSION
Based on the findings, F1 offsprings of field collected Ae. aegypti and Ae. albopictus were able to adapt to laboratory condition. Ae. aegypti is a domestic breeder, while Ae. albopictus is an outdoor breeder. Therefore, it explains that Ae. aegypti has better adaptability than Ae. albopictus. The ability to thrive in any given conditioned of Aedes causes difficulty to vector control agency in controlling Aedes population. Hence, the vector control approach must be revised in order to effectively reduce the dengue vector population.