Evaluating Liquid and Granular Bacillus thuringiensis var. israelensis Broadcast Applications for Controlling Vectors of Dengue and Chikungunya Viruses in Artificial Containers and Tree Holes

Ground Applications of Vectobac® WDG with A1 Super-Duty Mist Sprayer® and Micronair® AU5000 Atomizer for Suppression of Aedes aegypti Populations in the Florida Keys

Since 2011, the Florida Keys Mosquito Control District (FKMCD) has used the WALS® application strategy with VectoBac® WDG containing Bacillus thuringiensis israelensis via helicopter in Key West for the control of Aedes aegypti larval populations. In 2018, FKMCD conducted a study to determine the effectiveness of using a trailer-mounted A1 Super Duty Mist Sprayer® (A1 Mist Sprayers) with a Micronair® AU5000 (Micron Group) atomizer to apply VectoBac WDG by ground at the rate of 0.5 lb/acre (0.56 kg/ha). Bioassay cups were placed in a residential area encompassing open, moderate, and heavy cover scenarios between 0 and 300 ft (0-91.44 m) perpendicular to the spray line. An application rate of 0.5 lbs/acre (0.56 kg/ha) was used. Bioassay cups were collected after application and returned to the laboratory where 100 ml of distilled water and 10 F1 generation Ae. aegypti larvae were added. Laval mortality was monitored at 2, 4, and 24 h. Three separate runs were completed during the summer of 2018. Average larval mortality at 24 h was >90%. The field trial demonstrated sufficient efficacy to introduce this method of larviciding into operational use.

A Review of the Control of Aedes aegypti (Diptera: Culicidae) in the Continental United States

Aedes aegypti (L) is an anthropophilic mosquito involved in the transmission of a variety of viral pathogens worldwide including dengue, chikungunya, yellow fever, and Zika viruses. This species, native to Africa, is well established in the continental U.S. (CONUS) and occasionally contributes to localized outbreaks of viral diseases. In the last seven decades, mosquito control programs in the CONUS have been focused on vectors of eastern equine encephalitis, St. Louis encephalitis, and West Nile viruses, as well as nuisance species. Aedes aegypti receives little control focus except during outbreak periods, which has led to a lack of information on appropriate and effective control options targeting Ae. aegypti in the CONUS. As such, in the event of an Ae. aegypti-borne arboviral outbreak in the CONUS, there are limited evidence-based control recommendations or protocols in place. Autochthonous outbreaks of Ae. aegypti-borne pathogens have occurred recently in the CONUS, including dengue outbreaks in 2010 and 2013, a chikungunya outbreak in 2014, and the 2016 outbreak of Zika virus. The increasing frequency of Ae. aegypti-borne outbreaks necessitates increased attention and research on control of this species to prevent and mitigate future outbreaks. This review consolidates and synthesizes the available literature on control of Ae. aegypti, specifically within the CONUS, focusing on data generated through operational applications as well as field and semifield experiments. The purpose of this review is to identify and highlight areas where additional research is needed. The review covers chemical control and insecticide resistance, biological control, source reduction, trapping, and alternative techniques.

Susceptibility of mosquito vectors of the city of Praia, Cabo Verde, to Temephos and Bacillus thuringiensis var israelensis

Many vector-borne diseases circulate in the Republic of Cabo Verde. These include malaria during the colonization of the archipelago by the Portuguese explorers and several arboviruses such as yellow fever (now eradicated), dengue and zika.

To control these vector-borne diseases, an integrated vector control program was implemented. The main targeted mosquito vectors are Aedes aegypti and Anopheles arabiensis, and in a lesser extent the potential arbovirus vector Culex pipiens s.l. The main control strategy is focused on mosquito aquatic stages using diesel oil and Temephos. This latter has been applied in Cabo Verde since 1979. Its continuous use was followed by the emergence of resistance in mosquito populations.

We investigated the current susceptibility to Temephos of the three potential mosquito vectors of Cabo Verde through bioassays tests. Our results showed various degrees of susceptibility with 24h post-exposure mortality rates ranging from 43.1% to 90.9% using WHO diagnostic doses. A full susceptibility was however observed with Bacillus thurigiensis var israelensis with mortality rates from 99.6% to 100%.

Field testing & evaluation of the efficacy & duration of effectiveness of a biolarvicide, Bactivec® SC (Bacillus thuringiensis var. israelensis SH-14) in Bengaluru, India

Background & objectives:

Different formulations of Bacillus thuringiensis var. israelensis (Bti) have been tested against different mosquito vectors and other insects for their residual activity. In the present study, the efficacy and residual activity of a new formulation of Bti (Bactivec Suspension Concentrate) were evaluated against immature stages of Anopheles stephensi Liston (Diptera: Culicidae), Aedes aegypti Linnaeus (Diptera: Culicidae) and Culex quinquefasciatus Say (Diptera: Culicidae), in natural habitats in Phase II and Phase III in Bengaluru, India.

Methods:

Preferential breeding habitats of the mosquito species were selected and four dosages (0.25, 0.5, 1 and 2 ml/50 l) were tested in Phase II trial. Two most effective dosages, 0.5 and 1 ml/50 l were selected for Phase III trial. The evaluation was carried out essentially following the guidelines of the World Health Organization Pesticide Evaluation Scheme. Pre-treatment and post-treatment densities were recorded at regular intervals, and >80 per cent reduction in pupae was taken as the duration of effectiveness.

Results:

Bactivec SC treated at the dosage of 1 ml/50 l could produce 10-17 days efficacy (>80% reduction in pupae) in clean water habitats tested, whereas 0.5 ml/50 l dosage showed residual activity from 7 to 14 days against Ae. aegypti and An. stephensi in Phase III studies. In polluted water habitats, 4-7 days efficacy could be recorded against Cx. quinquefasciatus in Phase III.

Interpretation & conclusions:

The Bactivec SC formulation was operationally feasible and easy to handle. For the control of Anopheles and Aedes mosquitoes in freshwater habitats, 1 ml/50 l dosage was found effective, whereas in polluted water habitats against Cx. quinquefasciatus 5 ml/m² was found effective.

Did microbial larviciding contribute to a reduction in malaria cases in eastern Botswana in 2012-2013?

Setting: Larviciding has potential as a component of integrated vector management for the reduction of malaria transmission in Botswana by complementing long-lasting insecticide nets and indoor residual sprays. Objective: To evaluate the susceptibility of local Anopheles to commonly used larvicides. Design: This field test of the efficacy of Bacillus thuringiensis subsp. israliensis vs. Anopheles was performed by measuring larval density before treatment and 24 h and 48 h after treatment in seven sites of Bobirwa district, eastern Botswana, in 2012 and 2013. Vector density and malaria cases were compared between Bobirwa and Ngami (northwestern Botswana), with no larviciding in the control arm. Results: Larviciding reduced larval density by 95% in Bobirwa in 2012, with two cases of malaria, while in 2013 larval density reduction was 81%, with 11 cases. Adult mosquito density was zero for both years in Robelela village (Bobirwa), compared to respectively four and 26 adult mosquitoes per room in Shorobe village (Ngami) in 2012 and 2013. There were no cases of malaria in Robelela in either year, but in Shorobe there were 20 and 70 cases, respectively, in 2012 and 2013. Conclusion: Larviciding can reduce the larval density of mosquitoes and reduce malaria transmission in Botswana. Large-scale, targeted implementation of larviciding in districts at high risk for malaria is recommended.

Transcriptional Profile for Detoxification Enzymes AeaGGT1 and AaeGGT2 From Aedes aegypti (Diptera: Culicidae) in Response to Larvicides

Aedes aegypti (L.) is the vector responsible for transmitting dengue, chikungunya, yellow fever, and Zika viruses, as well as other pathogens. Microbial larvicides based on Bacillus thuringiensis Berliner israelensis (Bti) and Saccharopolyspora spinosa Mertz and Yao, such as VectoBac 12AS and Natular 2EC, have been shown to be effective in reducing larval populations of Ae. aegypti. We examined the gene expression of two detoxification enzymes, glucosyl and glucuronosyl transferases (AaeGGT1 and AaeGGT2), through developmental stages and a time course study in response to larvicide exposure using qualitative real-time polymerase chain reaction (qPCR). AaeGGT1 and AaeGGT2 gene expressions were differentially regulated during development of the immature stages. We also found that male adults had higher expression than female adults after controlling for age effects. AaeGGT1 and AaeGGT2 gene expression were both upregulated in response to VectoBac 12AS and Natular 2EC treatments with the maximum level of expression occurring 24 h after treatment applications. This information sheds light on larvicide-induced changes in the physiology of Ae. aegypti with implications for development of mosquito control strategies.

Comparison of Mosquito Abundance From Biogents Sentinel 2.0 Traps With and Without Rain Covers

Biogents Sentinel (BGS) traps have rapidly become a standard for adult Aedes aegypti surveillance. Several investigators have found that trap collections can be damaged easily by heavy dew or rain entering the trap intake port. In addition, water entering the trap may temporarily stop the fan, thus reducing the collection potential of the trap. We evaluated the effectiveness of a rain cover designed to minimize damage to mosquito trap collections from BGS 2.0 traps in a residential backyard in Jacksonville, FL. Rain covers consisted of white fiberglass 51-cm² sheets positioned 19, 29, and 39 cm above the air intake of the traps. One BGS trap did not have a rain cover and was used as a control standard for comparison. All traps were baited with the BG-Lure. Generally, traps with rain covers resulted in higher mosquito counts when compared with collections from uncovered traps. Overall mosquito abundance was greater from traps with the rain cover positioned at 29 cm, but this difference was not significantly different when compared with the other cover heights. Covers spaced 29 or 39 cm collected significantly more Culex quinquefasciatus compared with traps at 19 cm and no cover. Species diversity was greatest for BGS traps with the rain cover positioned at 29 cm followed by 39 cm, 19 cm, and no cover; however, differences in diversity among the traps, with or without covers, were not significant.

Controlling Aedes aegypti in Cryptic Environments with Manually Carried Ultra-Low Volume and Mist Blower Pesticide Applications

Because Aedes aegypti , a vector of dengue, chikungunya, and Zika viruses, exhibits resting and reproductive behaviors that present challenges to control them, pesticide application equipment available for vector control must be evaluated for their ability to control this species in a variety of cryptic environments. Five types of pesticide sprayers, representing 3 spray technologies (1 mister, 2 ultra-low volume [ULV] cold foggers, and 2 ULV thermal foggers), were evaluated for their ability to control adult and immature stages of Ae. aegypti in indoor and outdoor environments. Cages holding adult mosquitoes and larvae were placed in cryptic sites for indoor sprays and placed among dense vegetation for outdoor sprays. Adult and pupal mortality data were recorded following applications of a mixture of synergized pyrethrins and pyriproxyfen. We found that no single sprayer was best suited for controlling Ae. aegypti in indoor and outdoor environments, nor was one best for controlling adult and immature mosquitoes. Sprayers producing larger droplets (misters and cold foggers) were more effective in controlling immature mosquitoes indoors and outdoors. Thermal fogging was more effective in controlling adults indoors, whereas cold fogs and misters were more effective for outdoor control.

Effects of larvicidal and larval nutritional stresses on Anopheles gambiae development, survival and competence for Plasmodium falciparum

BACKGROUND

Many studies have shown that the environment in which larvae develop can influence adult characteristics with consequences for the transmission of pathogens. We investigated how two environmental stresses (larviciding and nutritional stress) interact to affect Anopheles gambiae (previously An. gambiae S molecular form) life history traits and its susceptibility for field isolates of its natural malaria agent Plasmodium falciparum.

METHODS

Larvae were reared in the presence or not of a sub-lethal concentration of larvicide and under a high and low food regimen. Development time, individual size, adult survival and competence for P. falciparum were assessed.

RESULTS

Individuals under low food regimen took more time to develop, had a lower development success and were smaller while there was no main effect of larvicide exposure on these traits. However, larvicide exposure impacted individual size in interaction with nutritional stress. Female survival was affected by the interaction between gametocytemia, parasite exposure and larval diet, as well as the interaction between gametocytemia, parasite exposure and larvicidal stress, and the interaction between gametocytemia, larvicidal exposure and larval diet. Among the 951 females dissected 7 days post-infection, 559 (58.78%) harboured parasites. Parasite prevalence was significantly affected by the interaction between larvicidal stress and larval diet. Indeed, females under low food regimen had a higher prevalence than females under high food regimen and this difference was greater under larvicidal stress. The two stresses did not impact parasite intensity.

CONCLUSIONS

We found that larval nutritional and larvicidal stresses affect mosquito life history traits in complex ways, which could greatly affect P. falciparum transmission. Further studies combining field-based trials on larvicide use and mosquito experimental infections would give a more accurate understanding of the effects of this vector control tool on malaria transmission.