Autophagostimulation by mosquito larvae

Rates of feeding of larvae of Culex pipiens in distilled water and larval water (water in which larvae were held overnight at a density of 1–2 larvae/ml) were compared by two methods. One method depended on assessing the relative rate of displacement of kaolin from the gut when kaolin-glutted larvae were provided with charcoal powder in the test liquids. The other method involved observation of changes in the time spent filtering by individual larvae transferred between distilled and larval water. Compared with plain water, larval water stimulates increased rates of feeding by the same or other conspecific larvae.

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Résumé L'eau dans laquelle on a maintenu des larves pendant une nuit, à la densité de 1–2 larves ml, stimule l'activité de filtration et la prise de nourriture de larves introduites isolément dans ce milieu, par comparaison avec des larves témoins placées dans de l'eau distillée. La comparaison de l'activité alimentaire a été faite selon deux méthodes. La première méthode utilise des larves gavées de poudre de kaolin ajoutée à l'eau du milieu d'élevage; ces larves sont transférées ensuite dans de l'eau distillée ou de l'eau ayant hébergé de nombreuses larves (eau de larves) eau à laquelle on ajoute de la poudre de charbon. La quantité de charbon filtrée et absorbée peut alors être appréciée et comparee pour l'eau distillée et pour l'eau de larves. La seconde méthode est basée sur la mesure du temps passé en activité de filtration, pour des larves isolées introduites dans de l'eau distillée ou de l'eau, ayant hébergé, de larves.Ces expériences démontrent que l'eau d'êlevage des larves groupées renferme un principe phagostimulant apporté par les larves elles-mêmes.

     

The alarm reaction of mosquito larvae

When Aedes larvae are alarmed they swim actively to the bottom; they recover by floating passively upwards. Culex and Anopheles larvae sink passively when alarmed; they swim actively to the surface on recovery. Repeated stimulation produces stimulus satiation, but a different type of stimulus can then give a complete response. Geotaxis and phototaxis are concerned in the alarm reaction in Aedes; phototaxis can overrule geotaxis. The chill coma temperature of these larvae is several degrees cooler than that at which the alarm reaction takes place. Acclimatization can alter the position of both.

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Zusammenfassung Bei drei Arten von Stechmücken-Larven wurde die Schreck-Reaktion untersucht. Alle Larven verschwinden bei Reiz von der Wasseroberfläche, aber der Reaktions-Mechanismus unterscheidet sich bei den verschiedenen Arten. Aedes aegypti ist leichter als Wasser. Die Larven schwimmen aktiv zum Boden und kehren durch Aufwartstreiben passiv wieder zur Wasseroberfläche zurück. Die Larven von Culex und Anopheles sind schwerer als Wasser. Sie sinken bei Reiz passiv zum Boden, die Rückkehr zur Wasseroberfläche ist ein aktiver Prozess, welcher Schwimmbewegungen der Larven notwendig macht.Wiederholter Reiz führt zur Reizsättigung. Hat eine Larve aufgehört, auf einen Anreiz (z.B. Schwingung) zu reagieren, reagiert sie aber sofort auf einen anderen Reiz (z.B. Licht). Dieses deutet an, dass sich die Sinnes-Rezeptoren angepasst haben, dass aber die den Effekt bewirkenden Muskeln nicht ermüdet sind. Aedes-Larven sind nach Reiz negativ phototaktisch. Sie sind ausserdem positiv geotaktisch, aber die negative Phototaxis ist die stärkere Reaktion. Culex- und Anopheles-Larven erschlaffen, wenn sie gereizt werden, sie sinken in diesem Zustand passiv, sie reagieren nicht auf Wechsel in der Licht-Intensität.Die zu Aktivitätslosigkeit führende Temperatur ist um einige Grade kälter als die tiefste Temperatur, bei welcher Schreck-Reaktion noch stattfindet. Anpassung, welche vor 24 Stunden erfolgt, kann sowohl die zu Aktivitätslosigkeit führende Temperatur, als auch die Temperatur ändern, bei welcher noch eine Schreck-Reaktion ausgelöst wird.

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This work was started in the Entomology Department of the London School of Hygiene & Tropical Medicine, and was financed by the Medical Research Council.     

Competition between tadpoles and mosquito larvae

Tadpoles and mosquito larvae often co-occur, and may compete for scarce resources. However, competition between such distantly related organisms has attracted less scientific attention than have interactions among closely related taxa. We examined ecological interactions in two tadpole-mosquito systems in southeastern Australia, one from freshwater ponds (Limnodynastes peronii and Culex quinquefasciatus) and one from brackish-water habitats (Crinia signifera and Ochlerotatus australis). Diets of these tadpoles and mosquito larvae overlap considerably, potentially leading to competition for food. Laboratory experiments show that, in both study systems, mosquitoes reduced the growth rates of tadpoles, and tadpoles reduced the growth rates and survival of mosquito larvae. These negative effects were seen even at high food levels. Thus, our study suggests that tadpoles and mosquito larvae affect each other strongly, and do so via pathways other than simple consumptive competition. Because mosquitoes are important vectors for human diseases, the global decline in amphibian populations may have more impact on human health than has generally been anticipated.     

Selection of cyanobacteria isolated from mosquito breeding sites as a potential food source for mosquito larvae.

One way to increase the persistence of larvicidal toxins in mosquito breeding sites is to clone the corresponding genes in microorganisms, such as cyanobacteria, which could serve as a source of food for the larvae. We isolated and cultured 10 strains of cyanobacteria from three mosquito breeding sites along the French Mediterranean coast. Most of the strains were tolerant to a relatively wide range of salt concentrations, and all of them were totally or partially resistant to at least four of the five biological or chemical larvicides used in the local mosquito control program. Six unicellular strains from these habitats and Synechococcus strain PCC 7942, a strain maintained for more than 10 years under laboratory conditions, were assessed for ingestion and digestion by larvae Culex pipiens and Anopheles gambiae mosquitoes. The numbers of cells ingested and digested were dependent on the cyanobacterial strain and varied with the mosquito species. Three of the new isolates, Synechococcus strain PCC 8905 and Synechocystis strains PCC 8906 and PCC 8912, were ingested and digested rapidly by larvae of both mosquito species. Since these strains are also tolerant to larvicides and relatively resistant to elevated salt concentrations, they meet the basic requirements for potential recipients of bacterial genes that encode endotoxins.     

Selection of cyanobacteria isolated from mosquito breeding sites as a potential food source for mosquito larvae.

One way to increase the persistence of larvicidal toxins in mosquito breeding sites is to clone the corresponding genes in microorganisms, such as cyanobacteria, which could serve as a source of food for the larvae. We isolated and cultured 10 strains of cyanobacteria from three mosquito breeding sites along the French Mediterranean coast. Most of the strains were tolerant to a relatively wide range of salt concentrations, and all of them were totally or partially resistant to at least four of the five biological or chemical larvicides used in the local mosquito control program. Six unicellular strains from these habitats and Synechococcus strain PCC 7942, a strain maintained for more than 10 years under laboratory conditions, were assessed for ingestion and digestion by larvae Culex pipiens and Anopheles gambiae mosquitoes. The numbers of cells ingested and digested were dependent on the cyanobacterial strain and varied with the mosquito species. Three of the new isolates, Synechococcus strain PCC 8905 and Synechocystis strains PCC 8906 and PCC 8912, were ingested and digested rapidly by larvae of both mosquito species. Since these strains are also tolerant to larvicides and relatively resistant to elevated salt concentrations, they meet the basic requirements for potential recipients of bacterial genes that encode endotoxins.     

Bioassay of three strains of Bacillus sphaericus on field-collected mosquito larvae.

Bacillus sphaericus strains 1593, 1404, and SSII-1 were assayed for infectivity against field-collected larvae of Psorophora columbiae, Culex nigripalpus, and Aedes taeniorhynchus in southwest Florida. Results indicate that all three strains are highly active against the Psorophora and Culex species. A. taeniorhynchus is also susceptible but requires higher dosages to achieve lethal responses. Tests were also conducted on the rate of infection and the differences in susceptibility of different instars to B. sphaericus. These tests indicate that nearly 75% of the mortality that occurs in the course of exposure to B. sphaericus occurs within 48 hr post-incubation with the bacteria. Furthermore, our tests indicate P. columbiae larvae decrease in susceptibility to the Bacillus with increase in larval age (instar). This investigation shows B. sphaericus to be a feasible biological control agent that warrants further study.     

Alkalinity within the midgut of mosquito larvae with alkaline-active digestive enzymes.

Digestive enzyme pH optima were determined in vitro for the larvae of three mosquito spp., Culex pipiens, Aedes aegypti, and Anopheles stephensi. All had optimal amylase activity at about pH 8 and optimal protease activity over a broad range between pH 10 and 12. pH within the digestive tract of intact live larvae was determined from the colours of indicator dyes ingested with kaolin and visible in the gut through the transparent or translucent body tissues. In all three spp. the contents of the anterior midgut were held at a pH just exceeding 10, with an abrupt fall to about pH 7·5 in the posterior third midgut.     

Efficacy of bioactive compounds from Curcuma longa L. against mosquito larvae

To identify larvicidal compounds from the ethanolic extracts of Curcuma longa root, the active compounds were isolated using activity‐guided fractionation with column chromatography and identified based on nuclear magnetic resonance (NMR) and mass spectrometry (MS) data. The dipping method was used to determine the larvicidal activities of each compound against 4th‐instar larvae of Culex pipiens pallens. Two compounds were isolated and identified, ar‐turmerone and 8‐hydroxyl‐ar‐turmerone. The two compounds exhibited larvicidal activities against the 4th‐instar larvae of C. pipiens pallens after 24 hr of treatment with LC₅₀ values of 138.86 and 257.68 ppm, respectively. The larvicidal activities of ar‐turmerone and 8‐hydroxyl‐ar‐turmerone against C. pipiens pallens are reported herein for the first time. The elucidation of the structure of these phytochemicals and their insecticidal activities are important for assessing the potential of this plant as a botanical insecticide.     

Drinking rate regulation in some fresh-water mosquito larvae

ABSTRACT. Drinking rates were determined with four species of freshwater mosquito larvae by colorimetric measurement of the dye ingested after groups of fourth instars were allowed access for set periods to 2% amaranth solutions. The rate of drinking for the saline-tolerant Aedes aegypti, 309±113 nl per h per individual, was comparable with rates given in the literature for several saline-water species, but rates for Culex quinquefasciatus, Culex molestus and Anopheles albimanus were markedly lower (167±30, 48±17 and 108±28m per h, respectively). When larvae of A.aegypti and C. quinquefasciatus were glutted with kaolin (allowed to replace all food in the gut by filter-feeding in kaolin suspension), drinking rates were little affected at first, but after 1 day of fasting (holding in water after glutting), drinking rates were 50% lower and were reduced by a further 20% with fasting for up to 3 days. For A.aegypti, C.quinquefasciatus and C. molestus, drinking rates were approximately doubled with kaolin dispersed in the dye solution, and after fasting, were increased by up to 100% in solutions containing 0.05% of water-soluble yeast extract. A similar phagostimulant effect of 10^-3M adenylic acid was demonstrated for A. aegypti and C. quinquefasciatus. A single experiment indicated similar stimulatory effects of kaolin and adenylic acid for, A. albimanus. With 0.01-0.05% agarose in the dye solutions, drinking rates for A. aegypti and C. quinquefasciatus were more than doubled, and a similar though weaker effect was demonstrated for another colloid, methylcellulose. In constrast, both colloids markedly reduced the rate of drinking with A.albimanus. These findings are discussed in relation to whether drinking and filter-feeding are necessarily coupled. The possible significance of this with respect to larvae that feed in different microhabitats, providing different levels of dissolved and colloidal nutrient organic matter, is considered. The implications of drinking rates for biotests of solubilized bacterial toxins as mosquito larvicides are noted.     

Rapid habituation by mosquito larvae to predator kairomones

Larvae of some species of mosquitoes have been shown to respond to water‐borne kairomones from predators by reducing bottom‐feeding and replacing it with surface filter‐feeding, which uses less movement and is thus less likely to attract a predator. However, if no predator attack takes place, then it would be more efficient to use a risk allocation strategy of habituating their response depending on the predator and the overall risk. The larvae of Culiseta longiareolata Macquart live in temporary rain‐filled pools, where they are exposed to a high level of predation. Within one hour, they responded to kairomones from dragonfly or damselfly nymphs, or to the fish Aphanius, by significantly reducing bottom‐feeding activity. Continued exposure to the predator kairomones resulted in habituation of their response to damselflies, a slower habituation to fish, but no habituation to dragonflies even after 30 h. In contrast, the larvae of Culex quinquefasciatus Say normally live in highly polluted and thus anaerobic water, where the predation risk will be much lower. They also showed a significant reduction in bottom‐feeding after 1 h of exposure to predator kairomones but had completely habituated this response within 6 h of continuous exposure. Some species of mosquito larvae can thus show a very rapid habituation to predator kairomones, while others only habituate slowly depending on the predator and overall predation risk.     

Plasmid stability in Bacillus sphaericus 2362 during recycling in mosquito larvae.

Bacillus sphaericus 2362 strains transformed with the plasmid pUB110 (4.5 kb) and plasmids derived from it, pLDT103 (7.6 kb) and pLDT117 (9.3 kb), were able to recycle (spore germination, vegetative growth, sporulation) in larvae of Culex quinquefasciatus. During the course of recycling, the pUB110 vector and the recombinant plasmid pLDT103 were stable (100 and 99.2%, respectively). However, the recombinant plasmid pLDT117 exhibited 23% segregational instability. Isolates which lost pLDT117 during recycling retained the one large plasmid native to B. sphaericus 2362.     

A new serotype of Bacillus thuringiensis from Colombia toxic to mosquito larvae.

During a survey conducted in Colombia a new isolate of Bacillus thuringiensis that showed toxicity toward Culex quinquefasciatus, Cx. pipiens, Aedes aegypti, and Anopheles stephensi larvae was isolated. Parasporal crystals were spherical in shape and showed a great degree of similarity with those produced by the reference strain of Bacillus thuringiensis subsp. israelensis. Supernatant fraction of the whole culture was not toxic, and heat-stable exotoxin production was negative. Catalase, urease, arginine dihydrolase, amylase, lecithinase, acetyl-methyl-carbinol, and gelatinase production were positive. Hemolysis on sheep blood agar was alpha-type. The isolate 163-131 showed natural resistance to azolocillin and was sensible to cephoperazone, cephalotin, nalidixic acid, and trimetoprin sulfametoxazole. Flagellar agglutination showed a specific H 30 antigen which allows individualization of this strain as a new serotype and the subspecies name of medellin is proposed.