Need for an efficient adult trap for the surveillance of dengue vectors.

The emergence and re-emergence of arboviral diseases transmitted by Aedes aegypti and Ae. albopictus continue to be a major threat in the tropics and subtropics. Associations between currently used indices and dengue transmission have not been proven to be satisfactorily predictive of dengue epidemics. Classical larval indices in dengue surveillance have limited use in assessing transmission risk and are a poor proxy for measuring adult emergence. Besides, collection of larval indices is labour intensive and plagued by difficulties of access particularly in urban settings. The re-emergence of dengue disease in many countries despite lower immature indices has warranted the need for more effective indices in dengue vector surveillance and control. Reliable and highly useful indices could be developed with the help of efficient and appropriate entomological tools. Most current programmes emphasize reduction of immature Ae. aegypti density, but it is of little value because its relation to transmission risk is weak. More attention should be paid to methods directed toward adult rather than immature Ae. aegypti. Collection of sufficient numbers of adult mosquitoes is important to understand disease transmission dynamics and to devise an appropriate control strategy. Even though, use of certain traps such as BG-Sentinel traps has been attempted in monitoring Ae. aegypti population, their utility is limited due to various setbacks which make these insufficient for entomological and epidemiological studies. Thus, there is an urgent need for the development of an ideal trap that could be used for adult vector surveillance. The present review critically analyzes the setbacks in the existing tools of entomological surveillance of dengue vectors and highlights the importance and necessity of more improved, more sensitive and reliable adult trap that could be used for surveillance of dengue vectors.

A novel method of controlling a dengue mosquito vector, Aedes aegypti (Diptera: Culicidae) using an aquatic mosquito predator, Diplonychus indicus (Hemiptera: Belostomatidae) in tyres

An investigation was undertaken to evaluate the biocontrol potential of a mosquito predator, Diplonychus indicus, against the immatures of dengue vectors breeding in used tyres. Two gardens were selected and ten tyres each filled with water were kept in different sites of both gardens. One was considered as “control” where no predator was used, whereas in the tyres kept in the “experimental” garden, two second instars of D. indicus were released into each tyre. The absolute density of larvae, pupae and man-landing rate of dengue vector were monitored at weekly intervals. After one year, the predators released in the tyres in the experimental garden were removed and it became the control garden whereas two second instars predators were released into the tyres kept in the control garden which then became the experimental garden. When D. indicus was introduced into the tyres kept in the experimental garden, a drastic reduction of 95% and 98% respectively of late instars and pupae of Ae. aegypti was observed. But there was no reduction in the tyres kept in the control garden. The density of early instars of Ae. aegypti fluctuated in the tyres kept in both the gardens during different months of the year. There were significant differences between the densities of early, late instars and pupae of Ae. aegypti in experimental and control gardens (early: p

Do colour and surface area of ovitrap influence the oviposition behaviour of Aedes aegypti, the vector of dengue and DHF?

A newly developed ovitrap made from a fiberglass tray (FGTO) was compared with conventionally used black jar ovitrap (BJO) in field condition in terms of number of eggs received. The number of eggs laid in FGTO was consistently higher than in BJO. The number of eggs collected in FGTO was significantly higher and it was 5-94 times more than that of BJO (t =9.45; p<0.0001). This finding has implication in designing lethal ovitraps for the control of dengue vectors.

Population interactions between Culex vishnui mosquitoes and their natural enemies in Pondicherry, India.

Population interactions among mosquitoes in the Culex vishnui subgroup, which are vectors of Japanese Encephalitis, and their natural enemies were studied in Pondicherry, India. We tested the hypothesis that the breakdown of interactions between the larvae and their natural enemies due to drought followed by rain was responsible for the sudden increase in the vector population above the threshold for disease transmission during the heavy rainy period. We randomly sampled mosquito larvae and their predators in different breeding habitats and subjected the mean densities of prey, predator, and mosquito larvae infected with parasites/pathogens to covariate analysis to understand the interaction between prey and their natural enemies in relation to environmental factors. In rice fields, neither prey nor predator showed any positive correlation with temperature, RH, or the number of rainy days. However, the pathogen/parasite of mosquito immatures showed a positive correlation with RH. Among the mosquito predators, notonectids exhibited a significant positive correlation with Cx. vishnui larvae. The parasitic Romanomermis iyengari and pathogenic Coelomomyces anopheliscus also showed positive correlations with immatures. No parasites and pathogens of mosquito larvae were recorded in shallow water pools (SWP) or cement tanks (CT) during the study period. Important predators recorded in SWP were notonectids, damselfly nymphs, Diplonychus indicus, and hydrophilids. Dragonfly nymphs, gerrids, and tadpole shrimps were recorded in CT. In CT, prey and their predators were positively correlated with RH and rainy days. In SWP, there was a highly significant correlation between prey, predators and environmental factors. We conclude that rice fields are a stable ecosystem where regular interaction occurs between larvae and their natural enemies and a sudden increase in mosquito populations is uncommon. In transient habitats, no such stability is present and they become more important as breeding habitats in terms of seasonality and number. Shallow water pools should be seriously considered for the control of these vectors.

Effect of food on immature development, consumption rate, and relative growth rate of Toxorhynchites splendens ( Diptera: Culicidae ), a predator of container breeding mosquitoes

Food utilization by the larvae of Toxorhynchites splendens (Wiedemann) was studied in the laboratory by offering larvae of Aedes aegypti Linnaeus, Anopheles stephensi (Liston), and Culex quinquefasciatus (Say). Quantitative analyses of data indicated that immature development was significantly faster with increase in food availability. The regression analysis showed that the degrees of the relationship between immature duration (Id) and food availability were higher when offered early instars of prey (first and second instars) than late instars. Consumption rate (Cr) of the predator increased with increase in food availability and this relationship was highly significant when larvae of An. stephensi were offered as food. Consumption rate to food level decreased with increase in the age class of the prey. There was a significant negative correlation between Id and Cr. This aspect helps to increase population turnover of T. splendens in a shorter period when the prey is abundant. Conversely, the predator compensated the loss in daily food intake at low food level by extending Id thereby attains the minimum threshold pupal weight for adult emergence. There was an increase in the relative growth rate (RGR) of the predator when An. stephensi was offered as prey and this was related to the high protein content of the prey per body weight. There was a positive correlation between Cr and RGR. This adaptive life characteristic strategy of this predator is useful for mass-rearing for large scale field release programmes in the control of container breeding mosquitoes is discussed.

Utility of expanded polystyrene (EPS) beads in the control of vector-borne diseases.

The use of chemicals or bio-larvicides for the control of Culex quinquefasciatus and Anopheles stephensi breeding in pit latrines and overhead tanks (OHT) respectively is discouraged owing to many undesirable impacts in the environment. Due to faecal contamination and poor survival, use of predatory fish in OHTs is not feasible. The use of expanded polystyrene (EPS) beads is a potential alternative in these habitats. EPS beads not only prevent oviposition but also kill the immature by forming a thick blanket on the water surface. A thick layer of 2 cm with beads of 2 mm is sufficient to suppress and prevent mosquito breeding. These are cheap, environmentally safe and do not need frequent application since they remain on the surface for quiet a long time. Successful trials against C. quinquefasciatus breeding in pit latrines, soakage pits, septic tanks, etc., have been carried out in Kenya, Zimbabwe and Tanzania. Certain trials with EPS indicated reduction in microfilaria (mf) rate besides decline in biting density. In India, EPS beads have also been used on small scale for the control of A. stephensi and A. culicifacies breeding in OHTs and unused wells respectively. The polystyrene beads have also been reported to be effective in the control of mosquito breeding in biogas plants and other industrial situations. The practical utility of EPS beads in the control of vector-borne diseases has been discussed in the present review.

Effect of food on immature development, consumption rate, and relative growth rate of Toxorhynchites splendens (Diptera: Culicidae), a predator of container breeding mosquitoes.

Food utilization by the larvae of Toxorhynchites splendens (Wiedemann) was studied in the laboratory by offering larvae of Aedes aegypti Linnaeus, Anopheles stephensi (Liston), and Culex quinquefasciatus (Say). Quantitative analyses of data indicated that immature development was significantly faster with increase in food availability. The regression analysis showed that the degrees of the relationship between immature duration (Id) and food availability were higher when offered early instars of prey (first and second instars) than late instars. Consumption rate (Cr) of the predator increased with increase in food availability and this relationship was highly significant when larvae of An. stephensi were offered as food. Consumption rate to food level decreased with increase in the age class of the prey. There was a significant negative correlation between Id and Cr. This aspect helps to increase population turnover of T. splendens in a shorter period when the prey is abundant. Conversely, the predator compensated the loss in daily food intake at low food level by extending Id thereby attains the minimum threshold pupal weight for adult emergence. There was an increase in the relative growth rate (RGR) of the predator when An. stephensi was offered as prey and this was related to the high protein content of the prey per body weight. There was a positive correlation between Cr and RGR. This adaptive life characteristic strategy of this predator is useful for mass-rearing for large scale field release programmes in the control of container breeding mosquitoes is discussed.

Laboratory evaluation of methanolic extract of Atlantia monophylla (Family: Rutaceae) against immature stages of mosquitoes and non-target organisms.

Methanolic extracts of the leaves of Atlantia monophylla (Rutaceae) were evaluated for mosquitocidal activity against immature stages of three mosquito species, Culex quinquefasciatus, Anopheles stephensi, and Aedes aegypti in the laboratory. Larvae of Cx. quinquefasciatus and pupae of An. stephensi were found more susceptible, with LC50 values of 0.14 mg/l and 0.05 mg/l, respectively. Insect growth regulating activity of this extract was more pronounced against Ae. aegypti, with EI50 value 0.002 mg/l. The extract was found safe to aquatic mosquito predators Gambusia affinis, Poecilia reticulata, and Diplonychus indicus, with the respective LC50 values of 23.4, 21.3, and 5.7 mg/l. The results indicate that the mosquitocidal effects of the extract of this plant were comparable to neem extract and certain synthetic chemical larvicides like fenthion, methoprene, etc.

Laboratory evaluation of methanolic extract of Atlantia monophylla (Family: Rutaceae) against immature stages of mosquitoes and non-target organisms

Methanolic extracts of the leaves of Atlantia monophylla (Rutaceae) were evaluated for mosquitocidal activity against immature stages of three mosquito species, Culex quinquefasciatus, Anopheles stephensi, and Aedes aegypti in the laboratory.Larvae of Cx. quinquefasciatus and pupae of An. stephensi were found more susceptible, with LC50 values of 0.14 mg/l and 0.05 mg/l, respectively. Insect growth regulating activity of this extract was more pronounced against Ae. aegypti, with EI50 value 0.002 mg/l. The extract was found safe to aquatic mosquito predators Gambusia affinis, Poecilia reticulata, and Diplonychus indicus, with the respective LC50 values of 23.4, 21.3, and 5.7 mg/l. The results indicate that the mosquitocidal effects of the extract of this plant were comparable to neem extract and certain synthetic chemical larvicides like fenthion, methoprene, etc.

Oviposition attractancy of an infusion from a wood inhabiting fungus for vector mosquitoes.

BACKGROUND & OBJECTIVES: Review of literature indicates that none of the chemicals so far studied for oviposition attractancy was found to be promising for operational use. Efforts are being made to find an effective substance to corner a large number of ovipositing females to selective breeding sites for ease of control operation. This will reduce insecticidal consumption and will be cost-effective. This study is aimed to evaluate the oviposition attractancy of aqueous infusion from a wood inhabiting fungus (Polyporus spp.) for vector mosquitoes both in the laboratory and under field conditions. METHODS: An aqueous infusion (10%) was prepared by soaking 10 g of fungal powder in 100 ml of chlorine-free tap water. The mixture was shaken thoroughly and filtered through Whatman No. 1 filter paper. The filtered infusion was tested for oviposition attractancy against vector mosquitoes both in the laboratory and under field conditions. RESULTS: The laboratory study on the dose-effect relationship indicated that at an optimum concentration of 4 ppm, maximum attractancy was observed. Water treated with aqueous fungal infusion (AFI) at 4 ppm received significantly more egg rafts/eggs of vector mosquitoes than other substrates like rearing water, natural breeding water and tap water. Trials conducted in the field showed that mud pots treated with AFI placed in both indoor and outdoor locations received significantly more Aedes aegypti eggs than the control. Similarly, treated pots placed in paddy fields attracted significantly more gravid Anopheles subpictus for oviposition than untreated pots. In contrast, the number of egg rafts of Culex quinquefasciatus laid in AFI treated pots was significantly less than in the control ones owing to strong natural olfactory factors associated with the breeding habitat. INTERPRETATION & CONCLUSION: The results indicated that there is scope for developing a trap incorporating this oviposition attractant along with insect growth regulator(s) (IGR) for surveillance and/or control of Ae. aegypti and other vector mosquitoes breeding in paddy fields.

Susceptibility of Phlebotomus argentipes and P. papatasi (Diptera: Psychodidae) to insecticides.

Field collected fully fed adults of Phlebotomus argentipes and P. papatasi were screened in the laboratory for susceptibility to DDT, BHC, malathion, deltamethrin, permethrin, lambdacyhalothrin and bendiocarb. Pondichery strain of P. papatasi and P. argentipes showed variations in their susceptibility to insecticides. Both the species were resistant to permethrin but tolerant to DDT and malathion with LD99.99 values of 13.88, 1.92, 1.08 and 34.63, 4.69, 16.32 times more than the deterministic doses respectively. However, they were susceptible to bendiocarb with LD99.99 7.6 and 1.6 times lower than the deterministic doses. While P. papatasi was susceptible to BHC, P. argentipes showed tolerance (1.6 times). The former showed tolerance to deltamethrin and the latter exhibited resistance (34 times). It was reverse in case of lamdacyhalothrin. High susceptibility of the vector sandflies to bendiocarb suggests that this insecticide could be used effectively against OP and pyrethroid resistant populations for Kalaazar control.

Breeding habitats of vector sandflies and their control in India.

In India Phlebotomus argentipes and Phlebotomus papatasi are the proven and suspected vectors of visceral leishmaniasis respectively. In the past, only a few samples from the field yielded sandfly immatures though there were many adults in the vicinity. This might be because of dearth of information on the various factors influencing the oviposition of the gravid females as well as survival and development of immatures. Therefore, available information on these aspects has been reviewed so as to highlight the imperative of undertaking further studies on the ecology of immatures in field conditions.

Bioefficacy of mosquito mat, coil and dispenser formulations containing allethrin group of synthetic pyrethroids against mosquito vectors.

The bioefficacy of mats, coils and dispensers containing allethrin group of synthetic pyrethroids was studied against laboratory strains of Culex quinquefasciatus, Aedes aegypti and Anopheles stephensi. Except esbiothrin in the mat formulation which was 100 and 178 times more effective against Cx. quinquefasciatus than against Ae. aegypti and An. stephensi with KT50 of 0.005, 0.5 and 0.89 min. respectively, all the three allethrins in mat or coil formulations were equally effective against the three vector mosquitoes. When compared to mat formulations of d-allethrin and bioallethrin, mat formulation of esbiothrin was 156 and 144 times more effective against Cx. quinquefasciatus. It was 162 times more effective against Cx. quinquefasciatus than in the coil formulation. Dispenser containing d-allethrin was the least effective against Cx. quinquefasciatus (KT50: 2.65 min.) and Ae. aegypti (KT50: 4.68 min.) but as effective as coil against An. stephensi. When mat and dispenser heated or coil burnt continuously for 10 hours the knockdown was consistently above 60% in all the three vector species but mortality was not consistently > 60% in Ae. aegypti. Repellent effect of the formulations ranged from 0.0 to 80.3%, 0 to 57.3% and 59.2 to 78.3% against Cx. quinquefasciatus Ae. aegypti and An. stephensi respectively. Repellency was more against An. stephensi than against the other two species. Mats and coils deterred > 50% of the mosquitoes from feeding on the chicken (range: 50 to 99.52%) but in case of dispenser the effect was only 14.72 to 65.01%. The use of these formulations as a control tools in reducing man-vector contact is discussed.