The life table characteristics of the invasive snail Physa acuta were assessed in the laboratory using the individuals occurring in a newly colonised area in Burdwan, West Bengal, India. Using the changes in the shell length and the body weight of the snails as surrogate, the population growth of the snails was estimated along with longevity and the fecundity schedule. The cohort of P. acuta lived for a maximum of 22 weeks with a life expectancy (ex) of 7.27 weeks and the age-specific survivorship being 0.825. Increment of the shell length of the snails complied with the von Bertalanffy growth equation, lt = 11.75(1 − exp−0.17(t−0.06)), and the observed and the expected data of the length at time t (lt) did not vary significantly (z score = 0.230; P = 0.818; n=20 pairs). Following attainment of sexual maturity between 28 and 42 days, oviposition continued till 20 weeks time, with 0.1-10 eggs laid by each individual. The eggs present per capsule remained between 01 and 11, whilst the net reproductive rate (R0), intrinsic rate of increase (rm) and the finite rate of increase (λ) were 116.07, 0.1 and 1.11, respectively. The observations are similar to those made earlier on the same species but from African and South American continents. The results of the present observation are pioneer in providing the initial studies about the life history of the invasive snail P. acuta in Indian context. Using the present information as a basis, further studies including long-term population monitoring should be initiated to understand the effects of the invasive snail P. acuta in the freshwater ecosystem of West Bengal, India.
Energy reserves in mosquitoes are an indicator of fitness, linking larval effort in resource acquisition with adult survival and fecundity. In other words, life history strategies and disease transmission potential can be related to the amount of energy reserves. The energy reserves of four mosquitoes – Aedes aegypti, Aedes albopictus, Armigeres subalbatus and Culex quinquefasciatus (Diptera: Culicidae) – were calculated to justify species-specific differences in their life history strategies. Following repeated sampling of pupae from the respective larval habitats, the glycogen, sugar and lipid contents of individual mosquitoes were assessed and corroborated with pupal weight and adult wing length. Discriminant function analysis was used to acquire an initial reflection of the differences of the parameters among the sex and species of the mosquitoes considered in the study. Using logistic regression and ANOVA, the effects of species and sex as contributors to variations in energy reserves could be established. The results indicated that for all the mosquitoes, sex-specific differences were prominent with reference to the energy reserves. Species-specific differences in energy reserves reflect differences in resource acquisition and assimilation in the tissues, and thus the differences in the life history strategies of these four species.
Competitive interactions between coexisting Aedes aegypti and Ae. albopictus have been implied as a crucial factor shaping life history traits and population characteristics. The overlap in resource requirements and similarities in the life history strategies of the two Aedes mosquitoes form a basis for competitive interactions. In the present study, the role of the food quality of the larval habitats in influencing the outcome of competition between Ae. aegypti and Ae. albopictus is evaluated to highlight food quality as a basis for asymmetric competitive outcomes. Instar I larvae of the two mosquitoes were reared using conspecifics or heterospecifics of constant size and equal ratio with four different food types: boiled rice, boiled pulses, a mixture of boiled rice and pulses, and fish food. Competitive interactions were evaluated using age at pupation (AP), pupal weight (PW), dry adult weight (AW) and wing length (WL) with respect to intra- and interspecific competition for the two sexes of each mosquito species. The results show that Ae. albopictus developed faster but achieved a smaller size compared to Ae. aegypti under interspecific competition conditions, the extent of the difference varying significantly with the food type. Given the variety of food resources available in the small container larval habitats, the results of the study imply that food quality may act differentially with respect to larval development and adult body size, depending on the conspecifics or heterospecifics and on the sex of the species concerned. The dominance of one species over the other may also be a consequence of the resource utilization pattern that varies in the larval habitats.