The problems of high cost and development of resistance in many vector mosquito species to several of the synthetic insecticides have revived interest in exploiting the pest control potential of plants ( Grainge & Ahamed 1988). In addition to application as general toxicants against mosquito immature, phytochemicals may also have potential uses as growth and reproduction inhibitors, repellents, and oviposition deterrents. Plants belonging to six families: Asteraceae, Cladophoraceae, Labiatae, Meliaceae, Oocystaceae, and Rutaceae appear to have potential for providing future mosquito control agents (Sukumar et al. 1991). Atlantia monophylla belonging to family Rutaceae has been earlier reported for insecticidal activity against pests of agricultural importance (Grainge & Ahamed 1988, Sukumar et al. 1991, but there has been no report on its activity against public health pests. The plant is available year round in Pondicherry, India. A study was therefore undertaken to test the extract of this plant for its efficacy against immature stages of selected mosquito species. stock solution was added to 249 ml of tap water in a 500 ml enamel bowl. MATERIALS AND METHODS Preparation of stock solution of plant extract -Effects on target species -Larvicidal and pupicidal activities of the methanolic extracts of A. monophylla were determined by following the standard procedure (WHO 1975). Twenty early fourth instar larvae and 20 pupae of three vector mosquito species, Culex quinquefasciatus, Anopheles stephensi, and Aedes aegypti (obtained from the cyclic colony that has been maintained for the past 24 years at the Centre) were transferred to 249 ml of tap water taken in 500 ml bowls. Five replicates were set up for each test concentration and the plant extract was tested at concentrations ranging from 0.001 to 5 mg/l with two replicates of control with addition of 1 ml of acetone alone to 249 ml of tap water. Bioassay was conducted at room temperature 27 ± 3 o C with 85% relative humidity. In the case of experiment for determining pupicidal activity, the mouth of each bowl containing pupae was covered with muslin cloth to prevent the escape of any emerged adult mosquitoes. Mortality in larvae/pupae was recorded 24 h posttreatment.Insect growth regulating (IGR) activity -Methanolic extract of A. monophylla was also tested for IGR activity against the three mosquito species. Twenty early third instar larvae of the three species were transferred into 500 ml enamel bowls containing 249 ml of tap water. The plant extract was tested at concentrations ranging from 0.001 to 2 mg/l. Each test concentration was replicated five times. Two replicates of control were also maintained. Mortality of the larvae and pupae was recorded at 24 h intervals. Observation was continued in treated and control bowls until the last immature pupated. The treated and untreated bowls containing pupae were kept separately in one-foot cages for adult emergence. Morphological abnormalities in pupae and adults and partially emerged adults were recorded as ...
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
Frequency dependent mosquito larval size (II and IV instars) and species selection by the water bug Diplonychus indicus against three mosquito species Culex quinquefasciatus, Aedes aegypti and Anopheles stephensi was studied in the laboratory. The different frequencies used for each species selection were 20:30:50, 30:50:20, 50:20:30, 25:35:40, 35:40:25 and 40:25:35 of fourth instars of the respective three prey species. All nymphal water bugs (I–V instars) selected IV instar mosquito larvae and the mean proportion of late (larger) larvae eaten by the predator instars was significantly higher than the mean proportion of early (smaller) larvae eaten (F= 2.28; P < 0.001). In all six ratios used to determine the frequency dependent mosquito species selection, all the stages of the water bug selected Ae. aegypti over the other two species (F= 452.43; P < 0.001). The mean number of mosquito larvae eaten increased as its density increased based on various ratios of larvae offered. The study indicated that the predatory efficiency of D. indicus was high when Ae. aegypti was offered as prey, suggesting the utility of this mosquito predator in the control of dengue vectors.
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