Erratic hatching in Aedes eggs: a new interpretation

Erratic hatching of eggs in Aedes aegypti (and other aedine mosquitoes) has long been something of an enigma, although the temporal scatter it provides, when coupled with drought resistance, is advantageous to species utilizing small, temporary breeding places. It is well known that eggs of Aedes hatch when the amount of dissolved oxygen ( p o 2 ) in the surrounding water is lowered, as when the medium becomes contamina­ted with bacteria. But erratic hatching in small instalments may occur in the absence of any obvious stimulus. We have re-examined this problem, postulating that the eggs themselves harbour microbial colonies and that these, by lowering the ( p o 2 ) in the microenvironment of each egg, quantitatively influence hatching. When laid the eggs are sterile, but each acquires surface colonies of bacteria, which they pick up from the environment. By rearing each egg in isolation we have shown that the number of bacteria acquired in a given time differs markedly from egg to egg and, as predicted, those with the highest counts hatch first. Under natural conditions, with eggs of different ages massed together, the first larvae to hatch browse over the surfaces of the remaining eggs, presumably consuming the bacteria and thus lowering the likelihood of further hatching. Thus, in this simple fashion, and not, it seems, by means of some hypothetical pheromone (as previously suggested), the first eggs to hatch influence the prospects of the other eggs, ensuring that a proportion of them remain unhatched, thus serving as a reserve for the future.

The effect of photoperiod on diapause induction, and temperature on diapause termination in embryos of Aedes capestris Dyar and Dnab (Diptera: Culicidae)

Populations of Aedes campestris from Manitoba and Saskatchewan were found to be either multivoltine or univoltine depending upon the geographical region from which the adults were collected. Three southern populations (from 49–52° N) were found to be totally multivoltine, whereas a northern population (58° N) was found to be partially (36%) univoltine. The latter fraction was designated as univoltine because the eggs entered an obligatory diapause after oviposition. When univoltine eggs were placed at 30 °C for a period of 10 days, 59% of the eggs hatched when subjected to a hatching stimulus.Facultative dispause was induced in eggs of multivoltine populations of A. campestris, by subjecting fully embryonated eggs to photoperiods of less than 14 h per day. For example, after 30 days at 8 h light: 16 h dark 73% of the eggs entered diapause.

The effect of temperature on the development and survival of two populations of Aedes campestris Dyar and Knab (Diptera: Culicidae)

Larvae of A. campestris from Saskatoon, Saskatchewan, and Churchill, Manitoba, were reared at constant temperatures of 15, 20, 23, 25, and 27 °C. Larvae of both populations failed to complete their development to adults at 15 °C, although development to third- and fourth-instar larvae did occur and survival at this temperature was greatest in the population from Saskatoon. The optimum developmental temperature for both populations was found to be 23 °C. The development rate and the survival rate was higher in the Churchill population at temperatures above 20 °C. At 15 °C, contrary to what would be expected, survival of larvae was much greater in the more southern (Saskatoon) population.

[Fauna and ecology of bloodsucking mosquitoes of Evenkia]

This paper presents data on the specific composition, number, hatching sites, seasonal and daily activity of mosquitoes attacking man and reindeer and the influence of weather factors on the attacking activity of mosquitoes in Central Siberia. 23 species of 3 genera are reported from Central Siberia as follows: Anopheles maculipennis, Culiseta alaskaensis, Aedes beklemishevi, A. cantans, A. caspius dorsalis, A. cataphylla, A. communis, A. cyprius, A. diantaeus, A. excrucians, A. fitchii, A. flavescens, A. hexodontus, A. impiger, A. intrudens, A. leucomelas, A. nigripes, A. pionips, A. pullatus, A. punctor, A. rempeli, A. stricticus, A. cinereus.

Observations on the breeding habitats of Aedes aegypti (L.) in Jakarta, Indonesia

A one-year study was done of the breeding habitats of Aedes aegypti (L.) and of the water storage habits of the inhabitants in four crowded districts of urban Jakarta. Immature mosquitoes were found in or near houses in containers of relatively clean water used for drinking or bathing purposes. An average of 185 containers were found per 100 houses, of which 60 were positive for Aedes immatures, resulting in a Container Index of 32%. The mean potential water storage capacity per house was 173 liters of which only 92 liters of water was actually being stored at any one time. Water jars were the most common containers found, but bak mandi (cuboidal or oblong concrete reservoirs) held more water per container. Total water storage per house in bak mandi was twice that of water jars and ten times that of drums, which were uncommon. Ant traps and other miscellaneous containers were unimportant for either water storage or mosquito production. A mean of 0.93 pupae per house was found, pupal production indoors being ten times than outdoors. Indoor water jars produced more pupae per house than all other containers combined. The infestation rate of covered containers was significantly higher than that of uncovered containers, perhaps because loose-fitting lids allowed entrance of gravid females to the attractive darkened interior of the container.

Delineation of La Crosse virus in developmental stages of transovarially infected Aedes triseriatus

The tropisms and development of La Crosse (LAC) virus in stages of transovarially infected Aedes triseriatus were studied with fluorescent antibody (FA) stained dissected organs and titrations of individual arthropods in suckling mice. Viral antigen was detected by FA in 95 of 387 dissected larvae, pupae, and adults. In larvae highest levels of fluorescence were detected in the alimentary tract, followed by ganglia, malpighian tubule, muscle, and other tissues. No specific organs or germ layer-derived tissues appeared to be the sole source of viral replication. Most tissues and organs of A. triseriatus are capable of maintaining LAC virus. Antigen was detected in the identifiable organs immediately upon emergence from the egg. In pupae and adults antigen was detected at high levels in foregut, gonadal and associated tissues, and in salivary glands, which would indicate females could be infective upon emergence. Virus was isolated from all arthropod stages, in 32 of 130 individuals inoculated into suckling mice. Titrations ranged from less than 1.0 log10 SMICLD50 per 0.02 ml for eggs and 1st instar larvae to 3.0 log10 SMICLD50 for 4th instar larvae. Adults and pupae averaged between 2.0 and 3.0 log10 SMICLD50. Increases in titer during maturation were mainly related to increases in size of the organism rather than in titer per unit volume.

The biosynthesis of deoxyribonucleic acid during the life-span of the mosquito

Previous results from this laboratory [Kao et al, (1976) Biochem. J. 154, 471-480] demonstrated that a low-molecular-weight DNA species, "sDNA", present in high concentrations during rapid larval growth of the yellow-fever mosquito, Aedes aegypti, may be an intermediate in DNA replication. We have now determined the life-span profiles of synthetic rates and the steady-state contents of sDNA and total DNA by radiometric and biochemical procedures and by using mosquitoes grown and maintained under defined and controlled conditions, including axenic culture and synchronization of stages. 2. Our results showed exponential increases in DNA contents and synthetic rates which paralleled the changes in weight during larval growth. Of special importance was the finding that in rapidly-growing larvae and newly-emerged adult mosquitoes high contents and rates of synthesis of sDNA were correlated with increases in total DNA/organism, which suggests that sDNA content may be an index of DNA replication. In senescent mosquitos a twofold increase in the synthetic rate of bulk DNA ("pDNA") without an increase in sDNA may reflect repair synthesis. 3. The significance of this investigation is that it may be the first quantitative and systematic description of the synthetic rates and contents of both DNA-replication intermediates and total DNA in an axenic, eukaryotic organism rather than a specific cell type. Equally important is that these findings describe DNA changes during growth and also during aging through senescence in an age-characterized organism.

A deoxyribonucleic acid-replication intermediate in the growing mosquito

In previous experiments on growth and aging in the yellow-fever mosquito, Aedes aegypti, a low mol. wt. (500000) DNA species was found in the supernatant fraction after ultracentrifugation of homogenates of rapidly-growing larvae. This DNA species, "sDNA", constituted 30-40% of total DNA in 2-4-day-old larvae, but was less than 5% in older larvae, pupae and adults. We have now isolated and characterized sDNA and initiated experiments to determine its metabolic role. Isolated sDNA has the same physical and chemical characteristics as bulk DNA, "pDNA", and differs only in size. In CsCl isopycnic centrifugation the buoyant densities of sDNA and pDNA were 1.700 and 1.697 g/cm3 respectively. The "melting" temperature of both DNA species was 84 degrees C. Base compositions calculated from these data and other methods were 38.9 mol% of guanine-plus-cytosine for sDNA, and 38.5 for pDNA. Also, the size of newly-synthesized DNA was investigated by pulse-labelling and pulse-chase experiments. In neutral sucrose gradients the labelled DNA component after a 2h pulse had a sedimentation coefficient of about 8S, but after a 4h pulse sedimented in a broad band from 10-19S. In alkaline sucrose gradients a single peak around 7S was observed for pulse times up to 4h. After a 6h pulse and a 1 day "chase", labelled DNA species had sedimentation coefficients ranging from 10-15S in alkaline sucrose, and after a 2-day chase the values were 17-31S, similar to those of pDNA under alkaline conditions. These results suggest that sDNA represents an intermediate form in the replication of DNA in mosquito larvae.

[Results of a survey of potential dengue vectors in the Neo-Caledonian archipelago]

The author gives the results of an entomological survey in neo-caledonian archipelago (New Caledonia and Loyalty Islands), november-december 1972. Concerning the distribution and the abundance of Aedes aegypti, the local dengue vector, this study shows the presence of this culicid not only in Noumea, but also all along the western developped coast of New Caledonia, and in Ouvea Island. This constitutes a great risk of outbreak extension from Noumea if a dengue virus is again imported there.

Ecdysone-mediated stimulation of DOPA decarboxylase activity and its relationship to ovarian development in Aedes aegypti

Very little dopa decarboxylase activity is detectable in adult female mosquitoes Aedes aegypti which have not been allowed to engorge blood. However, when such females are injected with the molting hormone beta-ecdysone a marked stimulation of this enzyme's activity is observable. No stimulation is observed in males similarly injected, nor in females injected with cholesterol or a juvenile hormone mimic. In addition, ecdysone injection initiates ovarian development in these anautogenous non-blood-fed mosquitoes. The extent of stimulation in both cases is dependent upon the amount of beta-ecdysone administered. These results suggested that ecdysone may play a role in ovarian development in Aedes and led us to hypothesize that a normal blood meal may trigger the synthesis, activation, or release of this hormone endogenously. Using the radioimmune assay for ecdysone developed by Borst and O'Connor (Science [Wash. D. C.] 178:4-18.), we found that the titer of an antigenic-positive material, presumably ecdysone or a closely related analogue, substantially increased 24 h after blood feeding, thereby supporting our postulation.

Transovarial transmission of LaCrosse virus (California encephalitis group) in the mosquito, Aedes triseriatus

LaCrosse virus (California encephalitis group) was recovered from F(1) eggs, larvae, and adults produced by experimentally infected Aedes triseriatus. The F(1) females transmitted the virus by bite to suckling mice and chipmunks. This, plus isolations of LaCrosse virus from larvae collected from their natural habitats in enzootic areas and from males and females reared from them, suggests that transovarial transmission is the overwintering mechanism for this arbovirus in northern United States.

Breeding places and seasonal incidence of Aedes aegypti, as assessed by the single-larva survey method

The single-larva survey method was employed to study the breeding places and seasonal incidence of Aedes aegypti in Dar es Salaam, Tanzania. From May 1968 to May 1969, 28 462 containers of water-located in approximately equal numbers indoors and outdoors-were investigated. The highest frequency of breeding (8.0%) of A. aegypti was observed in tires and motor parts. Drums, barrels, water-pots, and other receptacles left outdoors showed a higher frequency (3.1%) than those kept indoors (0.6%). Metal containers were infested to a greater extent than those made of mud, wood, or other materials; 2.5% of coconut shells, snail shells, etc. and 1.3% of tree holes, plant axils, and cut bamboos were infested. The seasonal prevalence, expressed as a container index, closely followed and paralleled the fluctuations in rainfall. The value of this survey method for both ecological studies and practical control purposes is discussed.

Ecological studies on the breeding of Aedes aegypti and other mosquitos in shells of the giant African snail Achatina fulica

The breeding of larvae of Aedes aegypti, Aedes simpsoni, and Eretmapodites quinquevittatus in empty shells of Achatina fulica was studied in the coastal zone of Dar es Salaam, Tanzania. The average density of shells was estimated to be 228 per ha. From 11 to 35% were positive for mosquito larvae. A. aegypti were found in 82-84% of positive shells; A. simpsoni in 8-13%. On Msasani peninsula, during the 3-month rainy season April-June 1970, the larval density of A. aegypti in shells was estimated at 1 100 per ha, that of A. simpsoni and E. quinquevittatus being estimated at 60 and 280 larvae per ha, respectively.Empty shells of A. fulica may contain up to 250 ml of water (average: 56.5 ml). The number of larvae per shell varies from 1 to 35 (average: 8.4) and it was estimated that, depending on the availability of food, and other factors, approximately 10 ml of water are required per larva. Viable eggs of A. aegypti were still to be found in 4% of the shells at the end of the dry season.

Studies on the life budget of Aedes aegypti in Wat Samphaya, Bangkok, Thailand

For a complete understanding of the epidemiology of a vector-borne disease, a knowledge of the bionomics of the vector is needed. The development of Aedes aegypti was studied in Wat Samphaya, Bangkok, Thailand, where work on the adult biology had been carried out the previous year (1966-67). Particular attention was given to the variation in the numbers of immature stages of the mosquito in relation to the known seasonal incidence of dengue haemorrhagic fever. Of the three types of water container in the Wat, water jars were the main source of adults, flower pot plates were less important, and the contribution of ant traps was insignificant.The variation in the numbers of emerging adults depended on changes in the mortality of the immature stages rather than on variations in the numbers of eggs laid. Both early and late larval instar mortalities are important, the former becoming more significant during the period March-August. The mortality between the eggs and second-instar larvae is density-dependent. There was no clear trend of association between mortality and season except for a fall in larval mortality in April-May preceding the increase in annual incidence of haemorrhagic fever, which usually occurs in June.

Aedes aegypti (L.) and Aedes albopictus (Skuse) in Singapore City. 3. Population fluctuations

Fluctuations in the adult Ae. aegypti and Ae. albopictus populations and their relationship to rainfall were studied by weekly collections from a number of stations in the city. Aedes aegypti populations generally fluctuated with the rainfall, with multiple peaks, except in the middle of the year when there was no increase in rainfall at the time of the peak in population. It is suggested that other regulating factors, in addition to rainfall, also determine the fluctuations of this species. Aedes albopictus also fluctuated, with three peaks in a year, and these bore a close relationship to rainfall.In one area, both larvae and adults of Ae. albopictus were studied simultaneously. The larval populations were investigated by exposing tin cans in the field. The population peaks of larvae were found to precede those of adults by almost exactly 2 months. It is suggested that each adult population peak represents the cumulative effect of more than one generation of mosquitos.

Subtle periodicity of pupation in rapidly developing mosquitos: with particular reference to Aedes vittatus and Aedes aegypti

Aedes vittatus and Ae. aegypti are important vectors of yellow fever and other arbovirus diseases in Africa and they complete many generations, through all developmental stages, in a single year. Recent studies on Drosophila have offered precise techniques for analysing periodicities in populations of single developmental events in individuals. If the variability of pupation times is low, periodicities may not be recognizable by an obviously polymodal pattern in a single synchronized population. Analysis of 8 synchronized populations of Ae. vittatus and Ae. aegypti, evenly spaced through 24 hours, provided a sensitive method of recognizing even subtle periodicities in rapidly developing mosquitos. By this means Ae. vittatus has been shown to exhibit a weak diurnal periodicity of pupation while a strain of Ae. aegypti showed not the slightest periodicity of pupation under the same light-dark cycle. The authors stress that periodicity of emergence or pupation in mosquitos can have important consequences for epidemiology and vector control and should be verified for each vector species for which control measures are envisaged.