A review of Leucosphyrus Group with particular reference to the Anopheles dirus complex (Diptera: Culicidae) in India.

The classification of the Leucosphyrus Group remained confused for long. The taxonomic events until it was resolved are traced in this review. The significance the description of Anopheles dirus as a new species and the subsequent characterization of its siblings are outlined. The current classification of the Leucosphyrus Group is presented and the various methods available for identifying the members of the dirus Species Complex are highlighted. Based on the limited information available about the members of the dirus Complex in India, the need for the reevaluations of this species, behaviour and vectorial capacity, which are of considerable importance in relation to malaria epidemiology in India, is emphasized.

Insecticidal activity of some new synthetic compounds against different mosquito species.

Insecticidal activity of an organosilane (HOE 84498, OMS 3055), Organophosphorous (MAT 9460, OMS 3052) and a synthetic pyrethroid compound (Trebon, OMS 3002) was evaluated against adult and larval stages of eight species of mosquitoes. Insecticidal activity of these compounds was limited to larval stages only. Highest activity of HOE 84498 was found against both Cx. tritaeniorhynchus and Anopheles culicifacies (LC50-0.0065 mg/I), MAT 9460 against Armigeres subalbatus (LC50-0.00043 mg/I) and Trebon against Cx. quinuefasciatus (LC50-0.00579 mg/I). Adulticidal effect was observed at higher dosages of 25-100 ug/cm2 of MAT 9460 and HOE 84498 against all the species with LT50 ranging from 22.15-33.76 min. No appreciable residual effect was evident for the three compounds on the surfaces treated at the rate of 1-100 mg/m2 against any mosquito species.

Field observations on the swarming behavior of Armigeres subalbatus (COQ) (Diptera : Culicidae).

Swarming behavior of Armigeres subalbatus was observed in Pondicherry, South India. Swarming occurred both at dawn and dusk over stationary markers. The duration of swarming was longer at dust (135 minutes) and shorter at dawn (75 minutes). Higher intensity in terms of relative density of participating males was observed at dusk when compared to dawn. Peak swarming activity was observed at light intensity ranging between 10 and 183 lux. Response to low light intensity of 1-10 lux was different during dawn and dusk. At this intensity initiation took place at dawn and termination was seen at dusk. Mating pairs were seen in the swarm. Analysis of the composition of swarms showed a male to female ratio of 1:0.01 at dusk and 1:0.02 at dawn. Besides nulliparous females, a few one parous and two parous females were also obtained from the swarm.

Use of hexaflumuron, an insect growth regulator in the control of Aedes albopictus (Skuse).

An attempt was made to assess the control potency of an insect growth regulator (IGR), hexaflumuron, against Aedes albopictus, a potent dengue vector, both in the laboratory and under field conditions. Emergence inhibition activity of this IGR against laboratory reared and field collected larvae of Ae. albopictus showed EI50 values of 1.9 x 10(-4) and 1.80 x 10(-4) mg(ai)/l respectively. Under field conditions, no appreciable reduction in immature density occurred at the lowest application rate of 0.001 mg(ai)/l whereas at the rate of 0.01 mg(ai)/l 100% reduction could be seen in earthern pots placed in a garden for 9 days. A reduction of 100% in pupal density was observed for 21 and 18 days at an application rate of 0.1 mg(ai)/l in pots and tyres respectively. Considering that this IGR was effective for about three weeks, it can be used successfully at the rate of 0.1 mg(ai)/l for controlling Ae. albopictus breeding in container habitats.

Observations on population density of Culex quinquefasciatus and transmission indices of Bancroftian filariasis during and after Integrated Vector Management strategy.

An Integrated Vector Management strategy, implemented as an alternative to the conventional control operations that include mainly chemical control in Pondicherry, South India, reduced very substantially the population density of Culex quinquefasciatus. This resulted in drastic decrease in the intensity of transmission of bancroftian filariasis transmitted by Culex quinquefasciatus and consequently the incidence of new infections in children of 0-5 age group was minimized. When the IVM strategy was withdrawn after five years of implementation and conventional control measures were re-adopted, resilience of Culex quinquefasciatus population was observed and human exposure to the risk of infection increased. The results suggest that maintenance of vector density at reduced levels for prolonged periods, is necessary to control infectious diseases like filariasis, which is difficult in the present day urban situations in developing countries. Hence the emphasis should be on chemotherapy to achieve control of lymphatic filariasis.

Cannibalistic behavior in Armigeres subalbatus (Diptera: Culicidae).

Phenomenon of natural cannibalism was observed to be exhibited by late (III and IV) instar larvae of Armigeres subalbatus. Cannibalistic behavior in this species was studied in response to food and density. Cannibalism among late instars was found to occur even in the presence of an adequate quantity of food. The rate of cannibalism was enhanced when food was restricted to only the early stages. Even in the total absence of food early instars did not show any cannibalistic behavior. Density had no influence on the rate of cannibalism. Under forced cannibalism and predation fourth instar larvae could not successfully pupate but the duration was prolonged. Cannibalism was thus facultative without any value for the survival of larvae. It may help in maintaining a balance in immature density in their natural habitats.

Larval habitat of Armigeres subalbatus (COQ) and its characteristics in Pondicherry.

Larval habitats of Armigeres subalbatus were delineated in urban and rural areas of Pondicherry. Survey of various potential mosquito breeding habitats revealed that septic tanks were the typical breeding habitat in both areas and the proportion was significantly higher in urban areas. The productive status of septic tanks differed in different months and the overall proportion breeding Ar. subalbatus was significantly higher in urban areas (0.0447) compared to rural areas (0.0181). Sporadic breeding observed in receptacles holding water admixed with cow-dung, was however insignificant. Among the various physico-chemical factors of septic tank habitat analysed in relation to breeding, only ammonia nitrogen was found to be significantly correlated with immature density.

Beta-cyfluthrin, a synthetic pyrethroid for mosquito control.

Beta-cyfluthrin (OMS 3051), a new synthetic pyrethroid and one of the stereoisomers of cyfluthrin, was studied for insecticidal activity against eight mosquito species. Its larvicidal activity with LC50 values of 5.62 x 10(-5) and 1.19 x 10(-4) mg/l respectively for Culex quinquefasciatus and Aedes aegypti was comparable with that of deltamethrin. This pyrethroid was more effective against the larvae of Armigeres subalbatus (LC50 - 7.76 x 10(-7)) and the adults of Anopheles culicifacies LT50 - 27.76 min at 2.0 mu/cm2) than the other species tested. Residual efficacy at 50 mg(ai)/m2 was more persistent (for 14-25 weeks) on thatch and asbestos among the four treated surfaces. This compound also elicited oviposition deterrent activity at 0.001 mg/l against Cx.quinquefasciatus. beta-cyfluthrin is a good insecticide for mosquito control. However, care should be exercised while using it as a larvicide in breeding habitats considering its toxicity to fish.

Influence of short time exposure to an insect growth regulator, hexaflumuron, on mortality and adult emergence of vector mosquitoes

Hexaflumuron, an insect growth regulator (IGR), was found to greatly affect the development of immatures and emergence of adults of three species of vector mosquitoes, Culex quinquefasciatus, Aedes aegypti and Anopheles stephensi, when larvae were subjected to short time exposure of < or = 1h. This IGR could completely prevent adult emergence even at a minimum exposure time of 10 min at 0.001, 0.01 and 0.1 mg/l. On treatment, larval and pupal mortality as well as varying degrees of morphogenetic abnormalities were induced in immatures and adults of the three species. Four weeks of control achieved in a slow moving sullage canal breeding Culex quinquefasciatus indicates that this IGR can be of use in such breeding habitats

Prevalence of bancroftian filariasis & its control by single course of diethyl carbamazine in a rural area in Tamil Nadu.

The prevalence of microfilaraemia, clinical spectrum of bancroftian filariasis and vector potential were studied in Vettavalam village in North Arcot district of Tamil Nadu. The effectiveness of selective therapy with diethyl carbamazine (DEC) in controlling filariasis in rural areas was also evaluated. The prevalence of microfilaraemia (mf rate) and disease (disease rate) was found to be 11.7 and 11.09 per cent respectively. Hydrocele was the dominant clinical sign in males and lymphoedema in females. The density of the vector Culex quinquefasciatus was 25.44 females/man-hour, and the infection and infectivity rates were 18.16 and 1.09 per cent respectively. All mf carriers detected after the mass blood survey were given a single course of DEC at the dosage of 6 mg/kg of body wt/day for 12 days. Only 61.6 per cent of them took the full course of DEC treatment. DEC therapy brought down the mf rate from 11.7 to 5.84 per cent after one month. In the absence of further treatment, there was no significant change in mf prevalence after one year.