Pupal productivity & nutrient reserves of Aedes mosquitoes breeding in sewage drains & other habitats of Kolkata, India: implications for habitat expansion & vector management.

Background & objectives: The quality of breeding sites is reflected through the pupal productivity and the life history traits of Aedes mosquitoes. Using nutrient reserves and pupal productivity of Aedes as indicators, the larval habitats including sewage drains were characterized to highlight the habitat expansion and vector management. Methods: The pupae and adults collected from the containers and sewage drains were characterized in terms of biomass and nutrient reserves and the data were subjected to three way factorial ANOVA. Discriminant function analyses were performed to highlight the differences among the habitats for sustenance of Aedes mosquitoes. Results: Survey of larval habitats from the study area revealed significant differences (P<0.05) in the pupal productivity of Aedes among the habitats and months. Despite sewage drains being comparatively less utilized for breeding, the pupae were of higher biomass with corresponding adults having longer wings in contrast to other habitats. The nutrient reserve of the adults emerging from pupae of sewage drains was significantly higher (P<0.05), compared to other habitats, as reflected through the discriminant function analysis. Interpretation & conclusions: The present results showed that for both Ae. aegypti and Ae. albopictus, sewage drains were equally congenial habitat as were plastic, porcelain and earthen habitats. Availability of Aedes immature in sewage drains poses increased risk of dengue, and thus vector control programme should consider inclusion of sewage drains as breeding habitat of dengue vector mosquitoes.

Chironomid midges as allergens: evidence from two species from West Bengal, Kolkata, India.

Background & objectives: Arthropods of different taxonomic identity including chironomid midges are known to induce allergic response in humans. The present study was done to access two common chironomid species Chironomus circumdatus and Polypedilum nubifer for their sensitizing potential as an allergen in atopic patients and controls. Methods: Following preparation of allergenic extracts of the two chironomid species separately, 198 atopic patients attending an allergy clinic and 50 age matched controls were tested along with a routine panel of allergens to assess sensitization. Results: The skin prick test (SPT) results revealed that 189 of the 198 patients (95.4%) demonstrated sensitization to both the chironomid species. Higher levels of total IgE was observed in atopic subjects than in the control group. Interpretation & conclusions: The results suggest that the chironomid midges Chironomus circumdatus and Polypedilum nubifer can elicit sensitization in humans. A potential risk for allergic reactions by susceptible individuals exists due to these chironomid species, owing to their abundance and chances of contact with human beings. Further studies may be initiated to characterize the nature of the allergens and to assess their clinical relevance.

Pupal productivity of dengue vectors in Kolkata, India: implications for vector management.

Background & objectives: Entomological surveillance of the dengue vectors using pupal productivity as indicators can be helpful in effective management. On this basis, an assessment was made on the relative importance of the larval habitats of Aedes mosquitoes in Kolkata, an endemic zone for dengue in West Bengal, India. Methods: Monthly collection of larvae and pupae of Aedes from larval habitats categorized as earthen, plastic and porcelain containers and tyres, was carried out from selected sites. Pupal weight was recorded and degree of sexual dimorphism was calculated. The data on pupal weight, sexual dimorphism and immature density were used for regression analysis. Results: The number of positive sites for each type of larval habitats varied with months and mosquito species. Based on mean density per month, the plastic containers were the most productive habitats and the tyres were least productive for both Aedes species. The pupal weight of both Ae. aegypti and Ae. albopictus varied with the relative density and type of larval habitats. Significant differences in pupal productivity, positive sites and the proportion of pupae were observed in the habitats. Species-specific differences in the degree of dimorphism were noted with the females being larger in size than males, irrespective of the habitats. Interpretation & conclusions: Pupal productivity of Aedes mosquitoes in Kolkata differed in terms of the type of the larval habitats with the immature density affecting the body size of the adults. This habitat-based study is a pioneer effort considering Kolkata and calls for a management plan for source reduction of these habitats to minimize Aedes mosquitoes and thus potential risk of dengue.

Efficacy of indigenous larvivorous fishes against Culex quinquefasciatus in the presence of alternative prey: Implications for biological control.

Background & objectives: Indigenous larvivorous fishes bear potential for regulating vector mosquitoes through trophic interactions. The mosquito prey preference of five indigenous larvivorous fishes in the presence of alternative food items was assessed to highlight their use in mosquito vector management. Methods: Laboratory experiments were carried out using the larvivorous fishes Ambassis (=Chanda) nama, Parambassis (=Chanda) ranga, Colisa fasciatus, Esomus danricus and Aplocheilus panchax, as predators and IV instar Culex quinquefasciatus larvae as target prey. Mosquito prey preference of these fishes in the presence of chironomid larvae, tubificid worms and artificial fish foods, in varied proportions, were assessed using preference index. Results: The fishes consumed considerable amount of mosquito larvae both in absence and presence of alternative food items. However, the positive selectivity for mosquito larvae at all densities were significantly (p <0.05) affected by the alternative foods. The chironomid larvae and tubificid worms were consumed proportionately higher than expected (p <0.05), while the artificial fish food was consumed at lower than expected proportions (p <0.05). The niche overlap was significantly similar among the fish species suggesting likeliness in predation pattern and prey preference. Interpretation & conclusion: The results reflect that the alternative food items influence the mosquito prey selectivity and thus the efficacy of indigenous larvivorous fishes. While use of these fishes in the wetlands and allied mosquito larval habitats in different parts of the country is suggested, impact of the alternative prey may affect the successful regulation of mosquitoes. Assessment of appropriate predator-prey ratio under natural habitat conditions is recommended to enhance successful mosquito control by these fishes.

Habitat heterogeneity and prey selection of Aplocheilus panchax: an indigenous larvivorous fish.

Objective: The ability of the native larvivorous fish Aplocheilus panchax (Hamilton, 1822) (Cyprinodontiformes: Aplocheilidae) as predator of mosquito larvae was assessed under laboratory conditions using multiple prey and habitat conditions. Methods: The consumption of larvae of the mosquito Culex quinquefasciatus Say 1823 (Diptera: Culicidae) by A. panchax was evaluated in the presence of tubificid worms and chironomid larvae under complex and simple habitat conditions. The number of mosquito larvae consumed in comparison to other prey types was noted and an index of prey selectivity was used to evaluate the preference for mosquito larvae. Results: Aphlocheilus panchax consumed 53 to 65 mosquito larvae in a three hour feeding bout contrast to 29– 38 tubificid worms and 43–62 chironomid larvae depending on the habitat conditions. The prey consumption differed significantly between the habitats and the prey type. The index of prey selectivity was positive for Cx. quinquefasciatus larvae over other alternative prey in all the habitat conditions. Conclusion: It is apparent from the study that the larvivorous fish A. panchax can be employed for biological regulation of mosquitoes in rice-fields and similar wetlands where the multiple prey choices are available under complex habitat conditions. However, field studies including other prey species will be required to substantiate this finding.

Vulnerability of the mosquito larvae to the guppies (Poecilia reticulata) in the presence of alternative preys.

BACKGROUND & OBJECTIVES: The predatory potential of the larvivorous fishes can be affected by the presence of alternative preys. In the present study the predation pattern of the sewage dwelling Poecilia reticulata (Peters 1872) on the larvae of Culex quinquefasciatus Say 1823 (Diptera: Culicidae) was evaluated in the presence of alternative preys. METHODS: The predation of Cx. quinquefasciatus larvae by different size groups of R. reticulata fishes was evaluated. In addition to this, the niche breadth (N) and diet breadth (B) were measured following Manly's selectivity index (Si) as an indicator of variation of such predation pattern in the presence of alternative prey types, like chironomid larvae and tubificid worms. RESULTS: The consumption of IV instar Cx. quinquefasciatus larvae by individual P. reticulata ranged between 65 and 84 in a 3 h feeding period and varied with the size of fish (F2,33 = 34.91; p < 0.001). The selectivity coefficient revealed a significantly low preference for the Cx. quinquefasciatus larvae (0.16, CL: 0.05-0.27; p < 0.05) compared to the chironomid larvae and tubificid worms, when all the three prey types were present. The niche breadth (N) and diet breadth (B) ranged from 0.77 to 0.92 and 0.69 to 0.93, respectively. The total consumption of all the prey types varied with the predator density, but the selectivity index for the mosquito larvae was significantly low in all the instances. INTERPRETATION & CONCLUSION: P. reticulata can consume a good number of mosquito larvae, with the consumption rate varying with the body size. P. reticulata fishes exhibit low preference for mosquito larvae as prey in the presence of alternative controphic preys like chironomid larvae and tubificid worms. Though establishment and sustenance of P. reticulata in new habitats will be favoured by the presence of alternative preys, but vulnerability of mosquito larvae may be reduced with availability of multiple preys in natural conditions.

Predatory efficiency of the sewage drain inhabiting larvae of Toxorhynchites splendens Wiedemann on Culex quinquefasciatus Say and Armigeres subalbatus (Coquillett) larvae.

The rate of predation by stage IV instar Toxorhynchites splendens larvae on the equivalent instar stage larvae of Culex quinquefasciatus and Armigeres subalbatus, co-occurring in sewage drains, were noted for a period of three consecutive days in the laboratory using different prey densities and combinations. The rate of predation varied by age of the predator, density of prey, and prey type. The number of Ar. subalbatus larvae consumed by a single Tx. splendens larva ranged between 0.50 +/- 0.71 and 16.40 +/- 2.01; while for Cx. quinquefasciatus larvae, the number consumed ranged from 0.20 +/- 0.42 to 20.40 +/- 1.43 per day. The pupation rates of the prey species varied in respect to control, with a minimum of 0.20 +/- 0.42 pupa/day to a maximum of 12.20 +/- 2.30 pupa/day in the presence of Tx. splendens. The values for the controls were 1.00 +/- 0.87 and 14.44 +/- 2.83 pupa/day, respectively. Irrespective of prey densities and combinations, a single Tx. splendens fourth instar larvae was found to consume on average 10.07 larvae on the first day 16.57 larvae on the second day and 4.38 larvae on the third day, killing a total of 17.70 to 45.10 larvae, in three days. In the presence of Tx. splendens, the cumulative pupation, irrespective of prey, remained between 12.20 and 45.10, and differed significantly from control where the values were between 13.90 and 54.70. The results indicate that Tx. splendens can significantly reduce immature numbers and lower the rate of pupation of Cx. quinquefasciatus and Ar. subalbatus. Tx. splendens may be a potential biological resource in the control of mosquitoes inhabiting sewage drains.

Seasonal prevalence of Aedes aegypti immatures in Kolkata, India.

Dengue and dengue hemorrhagic fever (DHF) is recurring in India in recent years, affecting many cities including Kolkata (Calcutta). In view of this, a survey of three years was carried out on the prevalence of immatures of the vector mosquito Aedes aegypti in the city of Kolkata. Mosquito immatures were found throughout the year with fluctuating densities in the habitats surveyed. An index of density of immatures (I/S)m employed in the study revealed that in the months of August and September a peak in abundance occurs, irrespective of the habitats, with the values ranging between 1.12 and 11.4. The (I/S)m values were low during the months of April and May ranging between 0.4 and 3.8. As larval habitats, earthen containers and the sewage drains were positive throughout the year, while during the months of December to February, air-coolers and conditioners as habitats remained totally dry, without immature Ae. aegypti. The number of positive sites and monthly density between the habitat types were significantly different with an I/S- value (for a year) ranging between 2.64 (air-coolers) and 4.97 (earthen containers). The survey, while providing preliminary baseline data on immature abundance in Kolkata, calls for continuous entomological surveillance using standard protocols as carried out in Thailand for the vector management program against dengue.

Predatory activity of Rhantus sikkimensis and larvae of Toxorhynchites splendens on mosquito larvae in Darjeeling, India.

BACKGROUND AND OBJECTIVES: Predation potential of the dytiscid beetle, Rhantus sikkimensis Regimbart 1899 and the larvae of Toxorhynchites splendens Wiedemann 1819 occurring along with the larval stages of the mosquitoes in the annual lentic water bodies of Darjeeling was evaluated using the larvae of Culex quinquefasciatus Say 1823 as preys, in the laboratory under simulated natural conditions. METHODS: Field collected R. sikkimensis and larvae of Tx. splendens were offered IV instar larvae of Cx. quinquefasciatus to observe the rate of predation, at varying prey and predator densities. Based on the data obtained on the predation for a period of three consecutive days, two indices of predation, predatory impact (PI) and clearance rate (CR) values were estimated, and compared between the predator species. RESULTS: The rate of predation of IV instar Cx. quinquefasciatus larvae by R. sikkimensis ranged between 21.56 and 86.89 larvae per day, depending on the prey and predator densities. The PI value remained between 18.67 and 35.33 larvae/day depending on prey densities, while the CR ranged between 2.21 and 2.23 larvae litres/day/predator. Compared to these, the Tx. splendens larvae consumed the prey larvae at the rate of 0.67 to 34.22 larvae per day, depending on the prey and predator densities. The PI value ranged between 7.67 and 11.33 larvae/day, and the CR value ranged between 1.41 and 1.76 larvae litres/day/predator. The rate of predation, CR values and PI values of R. sikkimensis and Tx. splendens varied significantly. INTERPRETATION AND CONCLUSION: Both the predators R. sikkimensis and larvae of Tx. splendens can consume a good number of mosquito larvae, though the rate of consumption between the two predators vary owing to the difference in the life history traits and features. It can be assumed that these predators play an important role in larval population regulation of mosquitoes and thereby impart an effect on species composition and interactions in the aquatic insect communities of Darjeeling Hills, India.

Larval habitats and species composition of mosquitoes in Darjeeling Himalayas, India.

BACKGROUND & OBJECTIVES: A preliminary survey of larval mosquito habitats and temporal variation in mosquito diversity in the hill town of Darjeeling, India was made during 2003, for a qualitative and quantitative assessment of mosquito distribution. METHODS: The possible larval habitats of mosquitoes were surveyed and the species diversity in the sites positive for mosquito larvae was noted. Bi-weekly sampling from a particular habitat was carried out to reveal the temporal variation in mosquito species. RESULTS: A good number of lentic aquatic habitats were found to be hosting mosquito immatures, though difference in the physical and biological features of these habitats was prominent. Altogether, immatures of six mosquito species, belonging to four genera--Aedes, Armigeres, Culex and Toxorhynchites were noted with significant difference in temporal variation in their relative and absolute numbers. A positive correlation (r = + 0.707) was found between population of the prey mosquito immatures and the population of immatures of Tx. splendens. The species diversity index (H') for the mosquitoes remained between 0.87 and 1.53. The evenness components ranged between 54.03 and 95.03% and differed significantly. INTERPRETATION & CONCLUSION: In the present study, the aquatic bodies could be categorised into six types depending on the size and structural complexity that may account for the observed variation in the species composition of the larval habitats. In addition to this, other factors like temperature, rainfall and other related climatic attributes may be responsible for the observed species variation, which needs to be confirmed through further studies.