Field evaluation of the establishment potential of wMelPop Wolbachia in Australia and Vietnam for dengue control.

BACKGROUND: Introduced Wolbachia bacteria can influence the susceptibility of Aedes aegypti mosquitoes to arboviral infections as well as having detrimental effects on host fitness. Previous field trials demonstrated that the wMel strain of Wolbachia effectively and durably invades Ae. aegypti populations. Here we report on trials of a second strain, wMelPop-PGYP Wolbachia, in field sites in northern Australia (Machans Beach and Babinda) and central Vietnam (Tri Nguyen, Hon Mieu Island), each with contrasting natural Ae. aegypti densities. METHODS: Mosquitoes were released at the adult or pupal stages for different lengths of time at the sites depending on changes in Wolbachia frequency as assessed through PCR assays of material collected through Biogents-Sentinel (BG-S) traps and ovitraps. Adult numbers were also monitored through BG-S traps. Changes in Wolbachia frequency were compared across hamlets or house blocks. RESULTS: Releases of adult wMelPop-Ae. aegypti resulted in the transient invasion of wMelPop in all three field sites. Invasion at the Australian sites was heterogeneous, reflecting a slower rate of invasion in locations where background mosquito numbers were high. In contrast, invasion across Tri Nguyen was relatively uniform. After cessation of releases, the frequency of wMelPop declined in all sites, most rapidly in Babinda and Tri Nguyen. Within Machans Beach the rate of decrease varied among areas, and wMelPop was detected for several months in an area with a relatively low mosquito density. CONCLUSIONS: These findings highlight challenges associated with releasing Wolbachia-Ae. aegypti combinations with low fitness, albeit strong virus interference properties, as a means of sustainable control of dengue virus transmission.

Field evaluations of disposable sticky lures for surveillance of Aedes aegypti (Stegomyia aegypti) and Culex quinquefasciatus in Jakarta.

From December 1997 to April 1998, disposable sticky lures (1608 lure days) were trialled in homes in north Jakarta, Indonesia as surveillance tools for Aedes aegypti (Stegomyia aegypti) (Diptera: Culicidae) and Culex quinquefasciatus (Diptera: Culicidae), referenced to indoor resting adult collections (92 × 10 min). The lures collected 89.4% of the total of 1339 Ae. aegypti and 92.1% of the total of 1272 Cx. quinquefasciatus collected by all methods. Because there were no significant differences with respect to numbers collected in bedrooms, living rooms and kitchens, bedrooms were selected for subsequent trials for reasons of convenience. The main trials involved a replicated complete block design with L-lysine and sodium carbonate. Lures without attractant or with four different dilutions of L-lysine collected 3.4-8.5 times more Ae. aegypti and 4.2-8.1 times more Cx. quinquefasciatus than were collected by mouth aspirator. Lures with or without dilutions of sodium carbonate collected 2.7-5.0 times more Ae. aegypti and 1.8-4.2 times more Cx. quinquefasciatus than aspirator collections. The precision associated with catches of sticky lures was better than that for aspirator collections. Although olfactants generally improved the numbers of mosquitoes collected, the differences in catch between lures with and without attractants were usually non-significant. Any deficit in catch may be offset by increasing the surveillance period to ≥30 days to detect all four dengue serotypes from infected mosquitoes.

Evaluations of mosquito age grading techniques based on morphological changes.

Evaluations were made of the accuracy and practicality of mosquito age grading methods based on changes to mosquito morphology; including the Detinova ovarian tracheation, midgut meconium, Polovodova ovariole dilatation, ovarian injection, and daily growth line methods. Laboratory maintained Aedes vigilax (Skuse) and Culex annulirostris (Skuse) females of known chronological and physiological ages were used for these assessments. Application of the Detinova technique to laboratory reared Ae. vigilax females in a blinded trial enabled the successful identification of nulliparous and parous females in 83.7-89.8% of specimens. The success rate for identifying nulliparous females increased to 87.8-98.0% when observations of ovarian tracheation were combined with observations of the presence of midgut meconium. However, application of the Polovodova method only enabled 57.5% of nulliparous, 1-parous, 2-parous, and 3-parous Ae. vigilax females to be correctly classified, and ovarian injections were found to be unfeasible. Poor correlation was observed between the number of growth lines per phragma and the calendar age of laboratory reared Ae. vigilax females. In summary, morphological age grading methods that offer simple two-category predictions (ovarian tracheation and midgut meconium methods) were found to provide high-accuracy classifications, whereas methods that offer the separation of multiple age categories (ovariolar dilatation and growth line methods) were found to be extremely difficult and of low accuracy. The usefulness of the morphology-based methods is discussed in view of the availability of new mosquito age grading techniques based on cuticular hydrocarbon and gene transcription changes.

Sublethal effects of mosquito larvicides on swimming performance of larvivorous fish Melanotaenia duboulayi (Atheriniformes: Melanotaeniidae).

Laboratory studies were conducted to determine the sublethal effects of exposure to selected larvicides on the critical swimming speed (Ucrit) of crimson-spotted rainbowfish, Melanotaenia duboulayi (Castlenau). This native fish is common throughout southeastern Queensland, and it is increasingly being distributed as a biological control agent of mosquitoes. The selected larvicides included, two organophosphate (OP) compounds (temephos and pirimiphos-methyl), two microbial larvicides (Bacillus thuringiensis spp. israelensis [Bti] de Barjac and Bacillus sphaericus [Bs] Neide), and an insect growth regulator (IGR) (s-methoprene). Exposure to the OP temephos at 10 times the effective field concentration (EFC; 0.33 mg/liter), and OP pirimiphos-methyl at the EFC (0.50 mg/liter), resulted in a significant reduction in the Ucrit of M. duboulayi under controlled conditions. Conversely, exposure to the microbial (Bti and Bs) and IGR (s-methoprene) larvicides at 10 times the EFC had no effect on the Ucrit of M. duboulayi. Accordingly, these products are suitable for integrated pest management programs in Australia.

Ross River virus disease clusters and spatial relationship with mosquito biting exposure in Redland Shire, southern Queensland, Australia.

The spatial heterogeneity in the risk of Ross River virus (family Togaviridae, genus Alphavirus, RRV) disease, the most common mosquito-borne disease in Australia, was examined in Redland Shire in southern Queensland, Australia. Disease cases, complaints from residents of intense mosquito biting exposure, and human population data were mapped using a geographic information system. Surface maps of RRV disease age-sex standardized morbidity ratios and mosquito biting complaint morbidity ratios were created. To determine whether there was significant spatial variation in disease and complaint patterns, a spatial scan analysis method was used to test whether the number of cases and complaints was distributed according to underlying population at risk. Several noncontiguous areas in proximity to productive saline water habitats of Aedes vigilax (Skuse), a recognized vector of RRV, had higher than expected numbers of RRV disease cases and complaints. Disease rates in human populations in areas which had high numbers of adult Ae. vigilax in carbon dioxide- and octenol-baited light traps were up to 2.9 times those in areas that rarely had high numbers of mosquitoes. It was estimated that targeted control of adult Ae. vigilax in these high-risk areas could potentially reduce the RRV disease incidence by an average of 13.6%. Spatial correlation was found between RRV disease risk and complaints from residents of mosquito biting. Based on historical patterns of RRV transmission throughout Redland Shire and estimated future human population growth in areas with higher than average RRV disease incidence, it was estimated that RRV incidence rates will increase by 8% between 2001 and 2021. The use of arbitrary administrative areas that ranged in size from 4.6 to 318.3 km2 has the potential to mask any small scale heterogeneity in disease patterns. With the availability of georeferenced data sets and high-resolution imagery, it is becoming more feasible to undertake spatial analyses at relatively small scales.

Toxicity of VectoLex (Bacillus sphaericus) products to selected Australian mosquito and nontarget species.

Laboratory and field bioassay studies were conducted in southeast Queensland, Australia, on the efficacy of VectoLex Control Granule (CG; active ingredient [AI]:50 Bacillus sphaericus [B.s.] International Toxic Units [ITU]/mg) and VectoLex Water Dispersible Granule (WDG) (AI: 650 B.s. ITU/mg) formulations against third-instar larvae of Culex annulirostris Skuse, Culex quinquefasciatus Say, Culex sitiens Wiedemann, Ochlerotatus rigilax (Skuse), Ochlerotatus. notoscriptus (Skuse), and Aedes aegypti (L.). The Gompertz model for log-linear mortality data were used to determine laboratory 48-h LC95 values. As with foreign evaluations, the B.s. formulations were most effective against Culex spp., with the WDG 10-100 times more effective than the CG on an ITU/mosquito basis. Consequently, Cx. annulirostris and Cx. quinquefasciatus were selected as target species for small-plot field evaluation of WDG efficacy over time. Weekly cohorts of caged third-instar Cx. annulirostris were exposed to replicated low (250 g/ha), medium (500 g/ha), and high (1,000 g/ha) dosages of WDG. Concurrent assessment of Cx. quinquefasciatus mortality outside the cages was also conducted. In water with high organic content, the low rate produced > 99% Cx. annulirostris mortality at 48 h, decreasing to 79% at week 3 and no control at week 4. The medium and high rates resulted in 100% Cx. annulirostris mortality for 2 wk posttreatment, decreasing to 95% at week 3, and no control at week 4. The WDG was equally effective against Cx. quinquefasciatus. Treatment did not affect water quality or nontarget shrimp and fish species survival.

Population structure and dispersal of the freshwater mosquitoes Culex annulirostris and Culex palpalis (Diptera: Culicidae) in Papua New Guinea and northern Australia.

In 1995 and 1998, outbreaks of Japanese encephalitis (JE) virus occurred for the first time in Australia. Virus isolation from pools of mosquitoes indicated Culex annulirostris Skuse was the most likely vector. It was hypothesized that wind-blown mosquitoes introduced JE from Papua New Guinea to the Torres Strait and Cape York, northern Australia. This study used levels of genetic differentiation, as indirect evidence of dispersal of mosquitoes between the two continents. The results have demonstrated that in the region incorporating Western Province in PNG, the Torres Strait and Cape York Peninsula, Cx. annulirostris is represented by a panmictic population, indicating frequent widespread dispersal throughout the region of investigation. The closely related Cx. Palpalis Taylor was also present at some locations but the population structure of this species is uncertain. This supports the hypothesis that wind-blown Cx. annulirostris and possibly Cx. Palpalis could be a possible mechanism for introduction of JE virus into Australia.

Vector competence of Australian mosquitoes (Diptera: Culicidae) for Japanese encephalitis virus.

Australian mosquitoes were evaluated for their ability to become infected with and transmit a Torres Strait strain of Japanese encephalitis virus. Mosquitoes, which were obtained from either laboratory colonies and collected using Centers for Disease Control and Prevention light traps baited with CO2 and octenol or reared from larvae, were infected by feeding on a blood/sucrose solution containing 10(4.5 +/- 0.1) porcine stable-equine kidney (PS-EK) tissue culture infectious dose50/mosquito of the TS3306 virus strain. After 14 d, infection and transmission rates of 100% and 81%, respectively, were obtained for a southeast Queensland strain of Culex annulirostris Skuse, and 93% and 61%, respectively, for a far north Queensland strain. After 13 or more days, infection and transmission rates of > 90% and > or = 50%, respectively, were obtained for southeast Queensland strains of Culex sitiens Wiedemann and Culex quinquefasciatus Say, and a far north Queensland strain of Culex gelidus Theobald. Although infection rates were > 55%, only 17% of Ochlerotatus vigilax (Skuse) and no Cx. quinquefasciatus, collected from far north Queensland, transmitted virus. North Queensland strains of Aedes aegypti L., Ochlerotatus kochi (Dönitz), and Verrallina funerea (Theobald) were relatively refractory to infection. Vertical transmission was not detected among 673 F1 progeny of Oc. vigilax. Results of the current vector competence study, coupled with high field isolation rates, host feeding patterns and widespread distribution, confirm the status of Cx. annulirostris as the major vector of Japanese encephalitis virus in northern Australia. The relative roles of other species in potential Japanese encephalitis virus transmission cycles in northern Australia are discussed.

Autogeny in Ochlerotatus vigilax (Diptera: Culicidae) from southeast Queensland, Australia.

Field and laboratory investigations were undertaken to determine the level of expression of autogeny in the mosquito Ochlerotatus vigilax (Skuse) from southeast Queensland, Australia, and whether there was evidence of seasonal variation. At two field sites in southeast Queensland, Wellington Point and Donnybrook, autogeny rates were determined on six occasions between January 2001 and January 2002. The autogeny rate varied between 71 and 100% at Wellington Point and between 63 and 100% at Donnybrook. Autogenous fecundity ranged from 17 to 63 eggs per female at Wellington Point and from 13 to 88 eggs per female at Donnybrook. Positive relationships were found between adult body size (indicated by wing length), autogeny rate, and fecundity. A laboratory study was conducted to investigate the influence of larval nutrition and adult diet (water versus sucrose) on the expression of autogeny. The autogeny rate at a low-diet treatment was between 73 and 90% when sucrose was withheld from females and 100% when sucrose was provided. All high-diet females were autogenous. Autogenous egg development required 80 +/- 6 h from emergence at 27 degrees C. We conclude that autogeny rates are consistently high in Oc. vigilax from the southeast Queensland region.

Enhanced vector competence of Aedes aegypti (Diptera: Culicidae) from the Torres Strait compared with mainland Australia for dengue 2 and 4 viruses.

Australian Aedes aegypti (L.) mosquitoes colonized from the Torres Strait and three mainland localities (Charters Towers, Townsville, and Cairns) were fed on blood suspensions containing dengue virus type 2 (DEN-2) or dengue virus type 4 (DEN-4). Variation was found in oral susceptibility to DEN-2 (59 -99% infection) and DEN-4 (28-79% infection) among Ae. aegypti assayed for virus at 8, 12, 16, or 20 d after ingestion of infected blood. Torres Strait Ae. aegypti were the most susceptible to DEN-2 and were significantly more efficient in transmission to capillary tube at 16 d (76% transmission) than mainland Ae. aegypti populations (20-28% transmission). Torres Strait Ae. aegypti were also the most susceptible to DEN-4, although transmission did not vary significantly from mainland populations at 16 d (12% compared with 0-4%) or 20 d (16% compared with 4-16%). Disseminated infection (i.e., leg infection) with either DEN-2 or DEN-4 was not an accurate predictor of transmission potential. This study demonstrates differences among Australian Ae. aegypti populations in vector competence for DEN-2 and DEN-4. Torres Strait Ae. aegypti were more frequently infected and able to transmit DEN-2 at higher rates than mainland populations. These data indicate that the Torres Strait region is potentially more receptive to dengue transmission than mainland localities, a finding discussed with respect to past outbreaks.

Assessment of the potential of dogs and cats as urban reservoirs of Ross River and Barmah Forest viruses.

OBJECTIVE: To determine whether dogs and cats are potential reservoirs of Ross River (RR) and Barmah Forest (BF) viruses METHOD: Young seronegative female dogs and cats were experimentally exposed to the viruses using Ochlerotatus vigilax (Skuse) mosquitoes. RESULTS: Only one of the 10 dogs and one of the 10 cats exposed to RR developed neutralising antibody. None of the animals developed detectable viraemia or clinical signs. One dog and three cats exposed to BF developed neutralising antibody. In addition, a serological survey of sera obtained from domestic dogs and cats residing in the Brisbane region indicated that 23.7% and 1.3% of dogs, and 14% and 2% of cats, had neutralising antibodies to RR and BF respectively. CONCLUSIONS: Although dogs and cats are exposed naturally to these viruses, and can become infected, they are unlikely to be important urban reservoirs of either virus.

Point source inoculation of mesocyclops (Copepoda: Cyclopidae) gives widespread control of Ochlerotatus and Aedes (Diptera: Culicidae) immatures in service manholes and pits in north Queensland, Australia.

This study details the novel application of predacious copepods, genus Mesocyclops, for control of Ochlerotatus tremulus (Theobald) group and Aedes aegypti (L.) mosquito larvae in subterranean habitats in north Queensland, Australia. During June 1997, 50 Mesocyclops sp. 1 were inoculated into one service manhole in South Townsville. Wet season rainfall and flooding in both 1998 and 2000 was responsible for the dispersal of copepods via the underground pipe system to 29 of 35 manholes over an area of 1.33 km2. Significant reductions in Aedes and Ochlerotatus larvae ensued. In these habitats, Mesocyclops and Metacyclops were able to survive dry periods, when substrate moisture content ranged from 13.8 to 79.9%. At the semiarid inland towns of Hughenden and Richmond, cracking clay soil prevents drainage of water from shallow service pits where Oc. tremulus immatures numbered from 292-18,460 per pit. Introduction of Mesocyclops copepods into these sites during May 1999 resulted in 100% control of Oc. tremulus for 18 mo. One uninoculated pit subsequently became positive for Mesocyclops with resultant control of mosquito larvae.

Pulse-exposure effects of selected insecticides to juvenile Australian crimson-spotted rainbowfish (Melanotaenia duboulayi).

Laboratory toxicity studies were conducted in southeastern Queensland, Australia, to determine the acute lethal effects of a 1-h pulse exposure of selected insecticides to adult and juvenile (<72 h old) crimson-spotted rainbowfish, Melanotaenia duboulayi (Castlenau). In addition, to its ecological significance, this native fish is a predator of mosquitoes. Two organophosphate (OP) compounds (temephos and pirimiphos-methyl), an entomophathogenic bacterium (Bacillus thuringiensis variety israelensis [Bti]) and two insect growth regulators (IGRs) (s-methoprene and pyriproxyfen) were evaluated. Although none of the five insecticides were acutely toxic to adult M. duboulayi under the test conditions, temephos and pirimiphos-methyl were found to be toxic to juveniles, with 24 h pulse exposure LC50 values of 27 and 15 microg/liter (ppb), respectively. Of the two OPs, pirimiphos-methyl was the most toxic, with a lethal dose ratio (pulse exposure LC50 temephos/ pulse exposure LC50 pirimiphos-methyl) of 1.8 (95% CL 0.5-6.4). These pulse exposure LC50 values represented 40 and 4.5% of the estimated environmental concentrations (EEC) for a 15 cm deep water body, respectively. Bti and the two IGRs had no acute toxic effects at up to 10 and 12.5 times the EEC, respectively. Accordingly, in the interests of environmental conservation and integrated pest management (IPM), preference should be given to the latter three insecticides for control of mosquito larvae in juvenile M. duboulayi habitat.

Winter intervention against Aedes aegypti (Diptera: Culicidae) larvae in subterranean habitats slows surface recolonization in summer.

At semiarid Charters Towers, north Queensland, Australia, the importance of Aedes aegypti (L.) in wells was assessed in relation to the colonization of surface habitats during the wet season. From April to July 1999, 10 wells (five positive for Ae. aegypti) were monitored to assess their status and larvae population numbers therein. All surface containers located within a 100 m radius of each well were removed, treated with s-methoprene or sealed to prevent the utilization of these containers by mosquitoes. These inner cores were surrounded by outer zones for a further 100 m in which surface containers were left untreated but all subterranean habitats were treated. Ovitraps were monitored monthly in the inner cores for 36 wk from August 1999 to April 2000 and differences in the proportions of ovitraps positive for Ae. aegypti and Ochlerotatus notoscriptus (Skuse) were analyzed by logistic regression. Analysis of the proportions of ovitraps positive for Ae. aegypti near positive wells indicated significantly greater colonization from November to March (the wet season), compared with those situated near Ae. aegypti negative wells. As Oc. notoscriptus were not produced from subterranean sites, comparisons of the proportions of ovitraps positive for Oc. notoscriptus in positive and negative inner cores provided an indication of the relative productivity of the uncontrolled surface containers in the outer zones. Differences in the utililization of ovitraps by Oc. notoscriptus among positive and negative cores were observed during only one month (March), when oviposition was greater in ovitraps in the negative cores, compared with the positive cores. Best subsets linear regression analysis of the proportion of ovitraps positive forAe. aegypti against meteorological variables (rainfall, mean wind speed, mean relative humidity, mean minimum, and maximum temperature) during the week of ovitrapping indicated that minimum temperature and wind speed accounted for 63.4% of the variability. This study confirms that for semiarid towns such as Charters Towers, the practice of treating a relatively small number of key subterranean habitats during winter will significantly affect Ae. aegypti recolonization of surface container habitats during summer, the period of greatest risk for dengue.

Epidemiological significanceof subterranean Aedes aegypti (Diptera: Culicidae) breeding sites to dengue virus infection in Charters Towers, 1993.

The objective of this study wasto determine the epidemiological significance of subterranean mosquito breeding sites to the 1993 outbreak of dengue fever (type 2) in the northern Queensland town of Charters Towers, Australia. In recent studies on subterranean mosquito breeding, containers such as wells and service manholes have been shown to be important breeding sites to Australia's only dengue vector, Aedes aegypti (L.). This study demonstrates a direct epidemiological association between subterranean breeding sites and dengue virus infection. The mean distance between residents seropositive for dengue 2 and the nearest subterranean container (113 m) was significantly less than for a randomly selected control (191 m), (F = 81.9; df = 1, 478; P < 0.001). Residents positive for dengue 2 antibodies was 2.47 (95% confidence interval 1.88-3.24) times higher for those living within 160 m of a well or service manhole, compared with those residing further away. These findings emphasize the importance of incluuding subterranean water containers in Ae. aegypti surveillance and control programs.

Enhancement or modulation of the vector competence of Ochlerotatus vigilax (Diptera: Culicidae) for ross river virus by temperature.

Two different doses of Ross River virus (RR) were fed to Ochlerotatus vigilax (Skuse), the primary coastal vector in Australia; and blood engorged females were held at different temperatures up to 35 d. After ingesting 10(4.3) CCID50/mosquito, mosquitoes reared at 18 and 25 degrees C (and held at the same temperature) had higher body remnant and head and salivary gland titers than those held at 32 degrees C. although infection rates were comparable. At 18, 25, and 32 degrees C, respectively, virus was first detected in the salivary glands on days 3, 2, and 3. Based on a previously demonstrated 98.7% concordance between salivary gland infection and transmission, the extrinsic incubation periods were estimated as 5, 4, and 3 d, respectively, for these three temperatures. When Oc. vigilax reared at 18, 25, or 32 degrees C were fed a lower dosage of 10(3.3) CCID50 RR/mosquito, and assayed after 7 d extrinsic incubation at these (or combinations of these) temperatures, infection rates and titers were similar. However, by 14 d, infection rates and titers of those reared and held at 18 and 32 degrees C were significantly higher and lower, respectively. However, this process was reversible when the moderate 25 degrees C was involved, and intermediate infection rates and titers resulted. These data indicate that for the strains of RR and Oc. vigilax used, rearing temperature is unimportant to vector competence in the field, and that ambient temperature variations will modulate or enhance detectable infection rates only after 7 d extrinsic incubation. Because of the short duration of extrinsic incubation, however, this will do little to influence RR epidemiology, because by this time some Oc. vigilax could be seeking their third blood meal, the latter two being infectious.

Detection of dengue viral RNA in Aedes aegypti (Diptera: Culicidae) exposed to sticky lures using reverse-transcriptase polymerase chain reaction.

Active surveillance for dengue (DEN) virus infected mosquitoes can be an effective way to predict the risk of dengue infection in a given area. However, doing so may pose logistical problems if mosquitoes must be kept alive or frozen fresh to detect DEN virus. In an attempt to simplify mosquito processing, we evaluated the usefulness of a sticky lure and a seminested reverse-transcriptase polymerase chain reaction assay (RT-PCR) for detecting DEN virus RNA under laboratory conditions using experimentally infected Aedes aegypti (L.) mosquitoes. In the first experiment, 40 male mosquitoes were inoculated with 0.13 microl of a 10(4) pfu/ml DEN-2 stock solution. After a 7-d incubation period, the mosquitoes were applied to the sticky lure and kept at room temperatures of 23-30 degrees C. Following 7, 10, 14, and 28 d application, 10 mosquitoes each were removed from the lure, pooled, and assayed for virus. DEN virus nucleic acid was clearly detectable in all pools up to 28 d after death. A second study evaluated sensitivity and specificity using one, two, and five DEN-infected mosquitoes removed after 7,10, 14, 21, and 30 d application and tested by RT-PCR. All four DEN serotypes were individually inoculated in mosquitoes and evaluated using the same procedures as experiment 1. The four serotypes were detectable in as few as one mosquito 30 d after applications to the lure with no evidence of cross-reactivity. The combination of sticky lures and RT-PCR show promise for mosquito and dengue virus surveillance and warrant further evaluation.

Laboratory evaluation of two native fishes from tropical North Queensland as biological control agents of subterranean Aedes aegypti.

The ability of 2 freshwater fishes, eastern rainbow fish Melanotaenia splendida splendida and fly-specked hardyhead Craterocephalus stercusmuscarum stercusmuscarum, native to North Queensland to prey on immature Aedes aegypti was evaluated under laboratory conditions. The predation efficiency of the 2 species was compared to the exotic guppy, Poecilia reticulata, which is commonly used as a biological control agent of mosquito larvae. Of the 3 fish species tested, M. s. splendida was shown to be the most promising agent for the biological control of Ae. aegypti that breed in wells. Melanotaenia s. splendida consumed significantly greater numbers of immature Ae. aegypti than P. reticulata, irrespective of developmental stage or light conditions. Unlike C s. stercusmuscarum, M. s. splendida could be handled, transported, and kept in captivity for extended periods with negligible mortality. However, M. s. splendida was also an efficient predator of Litoria caerulea tadpoles, a species of native frog found in wells during the dry season. This result may limit the usefulness of M. s. splendida as a biological control agent of well-breeding Ae. aegypti and suggests that predacious copepods, Mesocyclops spp., are more suitable. However, the use of M. s. splendida as a mosquito control agent in containers that are unlikely to support frog populations (e.g., aquaculture tanks and drinking troughs) should be given serious consideration.

Evaluation of liquid Bacillus thuringiensis var. israelensis products for control of Australian Aedes arbovirus vectors.

Laboratory bioassay studies were conducted in southeast Queensland, Australia, on the efficacy of Teknar, VectoBac 12AS, and Cybate (active ingredient: 1,200 international toxic units Bacillus thuringiensis var. israelensis [Bti]) against 3rd instars of the arbovirus vectors Aedes aegypti, Ae. notoscriptus, Ae. vigilax, and Ae. camptorhynchus. Probit analyses were then used to determine LD50 (median lethal dose), LD50, and lethal dose ratios (LDR). Aedes aegypti and Ae. notoscriptus, both container-habitat species, tolerated the highest Bti concentrations compared with saltmarsh Ae. vigilax and Ae. camptorhynchus. For example, the LDR for Ae. vigilax versus Ae. notoscriptus exposed to Cybate was 0.14 (95% confidence limit [CL] 0.03-0.61). Similarly, the Cybate LDR for Ae. camptorhynchus versus Ae. notoscriptus was 0.22 (95% CL 0.07-0.70). Teknar produced similar results with an LDR of 0.21 (95% CL 0.04-1.10) for Aedes vigilax versus Aedes notoscriptus. Differences in product efficacy were found when tested against the 2 container-breeding species. Cybate was less effective than Teknar with LDRs of 1.55 (95% CL 0.65-3.67) and 1.87 (95% CL 0.68-5.15) for Aedes aegypti and Ae. notoscriptus, respectively. The significant differences in susceptibility between mosquito species and varying efficacy between products highlight the importance of evaluating concentration-response data prior to contracting with distributors of mosquito control products. This information is crucial to resistance management strategies.

Survival of Aedes aegypti (Diptera: Culicidae) eggs in surface and subterranean breeding sites during the northern Queensland dry season.

The effect of a protracted dry season on the viability of Ae. aegypti (L.) eggs was examined in Townsville, northern Queensland, Australia. Eggs were placed in several different surface and subterranean larval habitats; and after four dry season months, only 1-10% of eggs remained viable in the surface and subterranean sites, respectively. Low humidity and predation by Periplaneta americana (L.) were the major causes of egg mortality in eggs in surface sites. P. americana was the most significant cause of egg predation in subterranean breeding sites but fungi, especially Penicillium citrinum Thom, covered egg batches within 15 d. Mycotoxins produced by the spores of P. citrinum are believed to have killed embryonating eggs. The high mortality rate of Ae. aegypti eggs during the dry season suggests that this survival strategy is unlikely to contribute to rapid and successful recolonization of surface sites at the end of the wet season.