Influence of light and habitat on predation of Culex quinquefasciatus (Diptera: Culicidae) larvae by the waterbugs (Hemiptera: Heteroptera)

The influence of light and habitat structure on the predation of Culex quinquefasciatus larvae by the common heteropteran water bug, Diplonychus (=Sphaerodema) annulatus, D. rusticus and Anisops bouvieri was assessed in the laboratory. It was revealed that water bugs predate more in presence of light than in dark conditions. While A. bouvieri consumed more prey in structured conditions, D. annulatus and D. rusticus consumed more prey in open conditions. The selection of prey size as well as the respective numbers varied between predators and treatments significantly. Prey vulnerability (PV), an indicator of predatory efficiency, was highest for D. annulatus, moderate for D. rusticus and low for A. bouvieri. Prey consumption and PV values under different treatment conditions indicate that for belostomatid water bugs D. annulatus and D. rusticus, the order of prey consumption under different habitat and light/dark combinations is light open > dark open > light vegetated > dark vegetated. In the case of the backswimmer, A. bouvieri, the order of prey consumption appeared to be light structured > dark structured > light open > dark open. These findings were consistent with resource‐partitioning by water bug species sharing the same guild. If the observed results are extended to natural settings the efficiency of these predators in controlling mosquito populations will vary with the structural complexity of habitats and the intensity of light.     

Flight muscle-specific expression of act88F: GFP in transgenic Culex quinquefasciatus Say (Diptera: Culicidae)

A strategy to engineer a strain of Culex mosquitoes refractory to filarial transmission is described. A requirement for success of the strategy is identification of a flight muscle-specific promoter that functions in the mosquito. A GFP marker gene under the control of the promoter region of the Drosophila melanogaster act88F gene was inserted into the genome of Culex quinquefasciatus. Transformation was confirmed by Mendelian genetics. Hybridization of a genomic Southern blot to a radiolabeled probe verified that the entire donor plasmid integrated into the mosquito genome. GFP expression in the transgenic mosquitoes was restricted to the flight muscles.     

Estimating reaction norms for predictive population parameters,age specific mortality,and mean longevity in temperature‐dependent cohorts of Culex quinquefasciatus Say (Diptera: Culicidae)

Culex quinquefasciatus plays a major role in the transmission of important parasites and viruses throughout the world. Because temperature is an important limiting factor on growth and longevity of all mosquito species, estimating the reaction norms provides very important basic information for understanding both plasticity and individual variations of the population. In the present study, Cx. quinquefasciatus were maintained at five different constant temperatures (15°, 20°, 23°, 27°, and 30°C) for two subsequent generations. Reproductive population parameters in blood‐fed mated females and longevities of virgin and blood‐fed mated adults reared at different temperatures were compared for the two generations. Longevity increased as temperature decreased within a range of 15° to 30°C for the unmated adults, and 15° to 27°C for the mated and blood‐fed adults. Generation times were as long as 124.07 and 106.76 days for two subsequent generations reared at 15°C, and the highest intrinsic rate of increase (r_m) values were estimated at 0.22 and 0.18, respectively, from the cohorts reared at 27°C. For survival rates, reproductive rates (R₀), and r_m values, 30°C was found to be a critical temperature for this species. These cohorts produced the smallest amount of eggs (R₀= 5.06), r_m values decreasing across generations (from 0.11 to 0.06), and the survival rates from egg to adult were found to be insufficient (16.1 and 10.8%). Additionally, the rate of exponential increase with age and age specific mortalities (b) were calculated for the virgin cohorts. Age specific mortality rates increased as temperature decreased. The increase in mortality rates started to accelerate at 27°C and was more pronounced at 30°C, for both females and males. We estimated the coefficients of variation for the b values in which females have smaller coefficients than those of the males at all temperatures.     

Control of Culex quinquefasciatus (Diptera: Culicidae) by Bacillus sphaericus and B. thuringiensis subsp. israelensis, Produced on a New Potato Extract Culture Medium

The efficacy of Bacillus sphaericus and B. thuringiensis israelensis produced on a new potato-based culture medium as well as the conventional culture medium of Luria Bertani was compared against Culex quinquefasciatus in the field. After sporulation, the spores/crystals were harvested and used. The bacterial samples controlled the larvae and pupae for three weeks. Mortality due to the bacterial toxins produced from the new culture medium was very high and comparable to that produced using conventional medium. But the cost for cultivating these bacteria using potato extract was much lower as compared with that of the conventional medium.     

Effect of fenthion treatment on larval densities of insecticide-resistant Culex quinquefasciatus in an urban area of Sri Lanka

Abstract. In Sri Lanka the national Anti-Filaria Campaign (AFC) has routinely employed fenthion since 1974 for larvicidal control of Culex quinquefasciatus , the vector of Bancroftian filariasis in urban areas, where this mosquito breeds prolifically in polluted waters. During 1994 the efficacy of AFC fenthion treatment against organophosphate-resistant Cx quinquefasciatus was investigated at Dehiwela, near Colombo.
The AFC target rate of fenthion application was 1 mg a.i./l, but the actual concentrations of fenthion in freshly treated pits ranged from 0.64 to 63.2 mg a.i./l. There was significant suppression of larval densities in treated soakage pits, the predominant breeding site of Cx quinquefasciatus , although the mosquito population was 6-fold resistant to fenthion at the LC_W level. Production of pupae was almost completely prevented in soakage pits which were sprayed weekly with fenthion, indicating that adult mosquito emergence from this source was minimal. The rapid decline in concentration of fenthion detected in the water of soakage pits indicated that a weekly treatment schedule is essential for effective control. With the rapid recolonization of treated sites, the weekly schedule must be strictly implemented in order to achieve control of resistant larvae. Fenthion activity levels detected in treated pits suggest that a 7–10 day schedule of retreatment would completely suppress susceptible Cx quinquefasciatus.     

Age modifies the effect of body size on fecundity in Culex quinquefasciatus Say (Diptera: Culicidae)

Fecundity of mosquitoes can vary with many factors and can have a strong effect on population growth. This study reports the effects of body size, blood meal size, and age on the reproductive output of nulliparous Culex quinquefasciatus, a vector of arboviruses and other pathogens. Mated adult female mosquitoes from a colony were reared under standard conditions and fed on chickens at different ages post‐eclosion. Blood meal size and wing length were recorded, as well as the number of eggs in the first‐cycle egg raft. Each of these factors had a significant influence on fecundity considered in a simple regression context. Multiple regression analysis revealed a significant interaction effect between age and body size on fecundity. Up to 13 days of age, fecundity was positively correlated with body size, but in mosquitoes older than 13 days, this relationship was not significantly different from zero. These results are discussed in terms of the known physiology of this and other species.     

Blood-meal preferences and avian malaria detection in mosquitoes (Diptera: Culicidae) captured at different land use types within a neotropical montane cloud forest matrix

Human activities modify environmental conditions, altering ecological interactions that can contribute to the increasing number of vector-borne pathogens affecting both human and wildlife populations. There is a dearth of knowledge about mosquitoes feeding preferences and their role as potential vectors of haemosporidian parasites, particularly in modified habitats. During 2013–2014 we sampled mosquitoes in five different land use types within a cloud forest matrix. From a total of 4107 adult mosquitoes, 90 were engorged. We extracted DNA from mosquito blood-meals, abdomens, and thoraxes, which belonged to seven different species. Seventeen specimens were positive for avian Plasmodium parasites. We were able to identify the blood-meal source of 10 mosquitoes, the identified vertebrate species were: Homo sapiens (Human), Sturnira hondurensis (Bat), and Bos taurus (Cow). Our results show that Culex restuans is positive for avian malaria and it is feeding on both humans and domestic animals at urban and peri-urban habitat types, where it is also an abundant species throughout the year. Furthermore, Aedes quadrivittatus, also positive for avian malaria, is feeding on humans in the well-preserved cloud forest, where this mosquito species is highly abundant. This study is the first in Mexico to provide reference data showing generalist mosquito feeding preferences and presence of avian Plasmodium at locations with different land use types.     

Resistance of field-collected populations of Culex pipiens pallens (Diptera: Culicidae) to insecticides in the Republic of Korea

Toxicities of 10 insecticides were examined against late third instars of Culex pipiens pallens, the northern house mosquito, using a direct-contact mortality bioassay. Several strains of mosquitoes were tested (insecticide-susceptible KS-CP strain and five geospatially distant field-collected strains (DG-CP, US-CP, BS-CP, GS-CP, and SG-CP)) and identified by polymerase chain reaction. Marked regional variations of insecticide susceptibility were observed. Extremely high to low levels of resistance were measured: bifenthrin, resistance ratio (RR) = 1-521; β-cyfluthrin, RR = 16-397; α-cypermethrin, RR = 9-343; deltamethrin, RR = 1-40; etofenprox, RR = 2-42; permethrin, RR = 3-46; chlorpyrifos, RR = 2-675; fenitrothion, RR = 0.5-364; and fenthion, RR = 2-360. All strains were susceptible to one or more of the insecticides examined. These results indicate that careful selection and rotational use of these insecticides may result in continued satisfactory control against field populations of northern house mosquitoes.     

Potential distribution of the Asian disease vector Culex gelidus Theobald (Diptera: Culicidae) in Australia and New Zealand: a prediction based on climate suitability

Abstract  Culex gelidus has a wide distribution throughout Asia, where it is a vector of Japanese encephalitis. It was first detected in Australia in 1999, with archived material revealing an introduction sometime prior to 1994. It is currently widely distributed throughout northern and particularly north-eastern Australia. Using climate matching software (CLIMEX Version 1.1) and the known distribution of Cx. gelidus throughout Asia, a predicted distribution for Australasia based on current climate was developed. A potentially wide distribution throughout coastal Australia, particularly in tropical and subtropical areas, was revealed. Few inland locations were suitable, except in tropical areas of the Northern Territory and Queensland. The predicted distribution presented here is concordant with most recent collection records of Cx. gelidus in Australasia. However, there are a small number of exceptions which highlight some of the limitations of this approach for predicting mosquito distributions. The presence of Cx. gelidus in a large artificial swamp in Alice Springs is one such example. The predicted Cx. gelidus distribution incorporates highly populated areas, in which people may experience an increased risk of mosquito-borne viral encephalitis should this mosquito spread throughout its entire predicted range.     

Actin protein up-regulated upon infection and development of the filarial parasite, Wuchereria bancrofti (Spirurida: Onchocercidae), in the vector mosquito, Culex quinquefasciatus (Diptera: Culicidae)

Detection and identification of humoral proteins, which are up-regulated in Culex quinquefasciatus upon infection by Wuchereria bancrofti, is important in tracing out the biochemical consequences of the filarial parasite development in the vector mosquito. Analysis of the haemolymph of infected mosquitoes through SDS-PAGE and RP-HPLC showed up-regulation of five proteins of molecular weights 40, 66, 22, 14, and 7-kDa. Among these, only the 40-kDa was unknown and the others were comparable with those already reported as transferrin, attacin, lysozyme, and defensin, respectively. In the present study, the 40-kDa protein up-regulated upon infection was identified as actin through nano-LC-MS/MS analysis. Actin is known to be one of the cytoskeletal proteins up-regulated in the haemolymph, as part of the innate immune system, of Escherichia coli challenged Drosophila melanogaster larvae. For the first time, we have observed an increased level of actin in the haemolymph of W. bancrofti-infected Cx. quinquefasciatus. However, the exact mechanism of actin involvement in the immune system of this mosquito is yet to be studied.     

Efficiency of fungus‐impregnated black cloths combined with Imidacloprid for the control of adult Aedes aegypti (Diptera: Culicidae)

Entomopathogenic fungi are potential candidates for use in integrated vector management. However, efficient delivery systems for these fungi need to be investigated. It is known that adult mosquitoes are attracted to dark surfaces, and therefore, black cotton cloths impregnated with Metarhizium anisopliae alone or in combination with the insecticide imidacloprid (IMI) were tested under laboratory conditions. Black cloths impregnated with fungus were also tested in large‐cage trials under natural extradomicile conditions. Blood‐fed Rockefeller and wild‐type strain Aedes aegypti had higher levels of survival when compared with sucrose‐fed counterparts following exposure to fungus‐impregnated cloths. However, when blood‐fed A. aegypti were exposed to a combination of M. anisopliae + IMI, the survival rates were statistically equal to those of sucrose‐fed females. Large‐cage trials showed significant decreases in A. aegypti survival following a minimum 12 h exposure of the mosquitoes to fungus‐impregnated cloths. Increased exposure times results in further reductions in survival. The synergism between M. anisopliae and IMI resulted in reduced survival rates independent of feeding regime under laboratory conditions. Fungus‐impregnated cloths tested under simulated field conditions, considered to be unfavourable for fungal infection, resulted in significant reductions in adult A. aegypti survival. We are currently testing the combined use of fungi and insecticides against blood‐fed insects under simulated field conditions.

Significance and Impact of the Study

The use of fungus‐impregnated cotton cloths is a promising point source application method for the control of adult Aedes aegypti, and this strategy could be incorporated into an integrated vector management programme aiming to reduce the incidence of dengue fever.     

Evolution of resistance toward Bacillus sphaericus or a mixture of B. sphaericus+Cyt1A from Bacillus thuringiensis, in the mosquito, Culex quinquefasciatus (Diptera: Culicidae)

The 2362 strain of Bacillus sphaericus (Bs) Neide is a highly mosquitocidal bacterium used in commercial bacterial larvicides primarily to control mosquitoes of the genus Culex. Unfortunately, Bs is at high risk for selecting resistance in mosquito populations, because its binary toxin apparently only binds to a single receptor type on midgut microvilli. A potential key strategy for delaying resistance to insecticidal proteins is to use mixtures of toxins that act at different targets within the insect, especially mixtures that interact synergistically. We tested this hypothesis for delaying the phenotypic expression of resistance by exposing Culex quinquefasciatus Say larvae to Bs alone or in combination with Cyt1A from Bacillus thuringiensis subsp. israelensis. Two laboratory lines of Cx. quinquefasciatus, one sensitive to Bs and the other containing Bs resistance alleles, were subjected to intensive selection pressure for 20 generations with either Bs 2362 or a 3:1 mixture of Bs 2362+Cyt1A. At the end of the study, the sensitive line had evolved >1000-fold resistance when selected with Bs alone, whereas the parallel line selected with Bs+Cyt1A exhibited only low resistance toward this mixture (RR95, 1.4). Similar results were observed in the lines containing Bs resistance alleles. Both lines selected with Bs+Cyt1A exhibited substantial resistance to Bs in the absence of Cyt1A. Although selection with Bs+Cyt1A did not prevent the underlying evolution of resistance to Bs, these results suggest that a mixture of Bs with other endotoxins, particularly one like Bs+Cyt1A in which the components interact synergistically, will provide longer lasting and more effective mosquito control than Bs alone.     

An α‐amylase is a novel receptor for Bacillus thuringiensis ssp. israelensis Cry4Ba and Cry11Aa toxins in the malaria vector mosquito Anopheles albimanus (Diptera: Culicidae)

Bacillus thuringiensis ssp. israelensis (Bti) produces four Cry toxins (Cry4Aa, Cry4Ba, Cry10Aa and Cry11Aa), and two Cyt proteins (Cyt1Aa and Cyt2Ba), toxic to mosquito‐larvae of the genus Aedes, Anopheles and Culex, important human disease vectors that transmit dengue virus, malaria and filarial parasites respectively. Previous work showed that Bti is highly toxic to Anopheles albimanus, the main vector for transmission of malaria in Mexico. In this work, we analysed the toxicity of isolated Cry proteins of Bti and identified an An. albimanus midgut protein as a putative Cry4Ba and Cry11Aa receptor molecule. Biossays showed that Cry4Ba and Cry11Aa of Bti are toxic to An. albimanus larvae. Ligand blot assays indicated that a 70 kDa glycosylphosphatidylinositol‐anchored protein present in midgut brush border membrane vesicles of An. albimanus interacts with Cry4Ba and Cry11Aa toxins. This protein was identified as an α‐amylase by mass spectrometry and enzymatic activity assays. The cDNA that codes for the α‐amylase was cloned by means of 5′‐ and 3′‐RACE experiments. Recombinant α‐amylase expressed in Escherichia coli specifically binds Cry4Ba and Cry11Aa toxins.