Fate of Bacillus thuringiensis subsp. israelensis in the field: evidence for spore recycling and differential persistence of toxins in leaf litter

A review of the potential impacts of coastal mosquito control programs on Australian Stingless Bees (Apidae, Meliponini)-likely exposure pathways and lessons learned from studies on honey bees

The impact of the programmatic use of larvicides for mosquito control on native stingless bees (e.g., Apidae, Meliponini) is a growing concern in Australia due to heightened conservation awareness and the growth of hobbyist stingless bee keeping. In Australia, the two most widely used mosquito larvicides are the bacterium Bacillus thuringiensis var. israelensis (Bti) and the insect hormone mimic methoprene (as S-methoprene). Each has a unique mode of action that could present a risk to stingless bees and other pollinators. Herein, we review the potential impacts of these larvicides on native Australian bees and conclude that their influence is mitigated by their low recommended field rates, poor environmental persistence, and the seasonal and intermittent nature of mosquito control applications. Moreover, evidence suggests that stingless bees may display a high physiological tolerance to Bti similar to that observed in honey bees (Apis mellifera), whose interactions with B. thuringiensis-based biopesticides are widely reported. In summary, neither Bti or methoprene is likely to pose a significant risk to the health of stingless bees or their nests. However, current knowledge is limited by regulatory testing requirements that only require the use of honey bees as toxicological models. To bridge this gap, we suggest that regulatory testing is expanded to include stingless bees and other nontarget insects. This is imperative for improving our understanding of the potential risks that these and other pesticides may pose to native pollinator conservation.

Eco-friendly bio-larvicidal and antimicrobial activity of isolated bioactive compound from Kurthia gibsonii (Bacillales)

The targeted organisms include mosquito vectors, bacterial pathogens and non-targeted organisms. Preliminary mosquito larvicidal activity was conducted using cell-free supernatants (CFSs) from 11 gut bacteria. Among them, the bacterium SS11 exhibited promising results and was identified as Kurthia gibsonii based on its 16S rRNA sequence (1350 bp). The diethyl ether extract (DEE) of K. gibsonii demonstrated significant larvicidal effects, with LC50 values of 5.59 µL/mL and 8.59 µL/mL for 3rd instar larvae of Aedes aegypti and 2nd instar larvae of Anopheles stephensi, respectively. Analysis of the DEE using FT-IR, and GC-MS revealed the presence of 16 functional groups, and 7 bioactive compounds, respectively. A molecular docking study identified GC-MS compounds against odorant receptors from A. aegypti and odorant-binding proteins from A. stephensi was performed to assess the interaction and binding affinity. Overall, these findings suggest that the bioactive compounds 2, 4, 6-tribromoaniline from the DEE of K. gibsonii hold potential as an environmentally compatible alternative for biocontrol purposes, and compounds 9-tricosene and didecyl phthalate can be used for mosquito traps.

Recommendations to reduce the streetlight effect and gray areas limiting the knowledge of the effects of plant protection products on biodiversity

Preserving biodiversity against the adverse effects of plant protection products (PPPs) is a major environmental and societal issue. However, despite intensive investigation into the ecotoxicological effects of PPPs, the knowledge produced remains fragmented given the sheer diversity of PPPs. This is due, at least in part, to a strong streetlight effect in the field of ecotoxicology. Indeed, while some PPPs have been investigated in numerous ecotoxicological studies, there are many for which the scientific literature still has little or no information on their ecotoxicological risks and effects. The PPPs under the streetlight include a large variety of legacy substances and a more limited number of more recent or currently-in-use substances, such as the herbicide glyphosate and the neonicotinoid insecticides. Furthermore, many of the most recent PPPs (including those used in biocontrol) and PPP transformation products (TPs) resulting from abiotic and/or biotic degradation are rarely addressed in the international literature in the field of ecotoxicology. Here, based on a recent collective scientific assessment of the effects of PPPs on biodiversity and ecosystem services in the French and European contexts, this article sets out to illustrate the limitations and biases caused by the streetlight effect and numbers of gray areas, and issue recommendations on how to overcome them.

Effects of mosquito control using the microbial agent Bacillus thuringiensis israelensis (Bti) on aquatic and terrestrial ecosystems: a systematic review

BACKGROUND

The bacterium Bacillus thuringiensis serovar israelensis (Bti) is commercially produced in various formulations for use as a larvicide worldwide, targeting especially the aquatic larval stage of mosquitoes. However, there is a concern that repeated Bti treatments may have both direct and indirect impacts on non-target organisms (NTOs) and the ecosystems they inhabit. This review evaluates the evidence for such impacts.

METHODS

Literature was searched using six bibliographic databases, two search engines, and on specialist web sites. Eligibility screening was performed in two steps on (1) title/abstract, with consistency among reviewers assessed by double-screening 557 articles and (2) full text. Articles included after full text screening were critically appraised independently by two reviewers. Disagreements were reconciled through discussions. Key parameters of included studies are presented in narrative synthesis tables, including risk of bias assessments. Meta-analyses comparing treated with untreated ecosystems and using either the raw mean difference or log response ratio as effect size were performed.

REVIEW FINDINGS

Ninety-five articles covering 282 case studies were included in the review. From these, we identified 119 different response variables, which were divided into nine outcome categories. Most studies investigated NTO abundance or life history (reproduction related outcomes), but diversity and community composition are also well represented as outcome categories. The studies are highly variable in methodology, rigor, and spatio-temporal scale, spanning 1 day to 21 years and from < 1m2 to > 10,000 m2. Our metanalyses revealed a consistent negative effect of Bti treatment on abundances of Chironomidae and Crustacea, and also on chironomid emergence, although from a more restricted set of studies and regions. For most remaining response variables, we judged meta-analysis unfeasible, due to low study numbers or insufficient reporting of methods and results.

CONCLUSIONS

There is now a significant body of studies documenting effects of mosquito control using Bti on NTOs or other ecosystem properties, especially associated with negative effects on Chironomidae, as apparent from our meta-analyses. Accordingly, we suggest the potential for negative NTO or other ecosystem effects of Bti treatment should not be discounted a priori. Once a decision to proceed with Bti treatment has been taken, priority should be given to a well-designed program of ongoing monitoring and assessment. The paucity of rigorous studies conducted with low bias risk for most response variables undermines our capacity for evaluating how common many of the effects documented might be. Future research would benefit from a rigorous and well-replicated approach to studying Bti impacts in semi-field mesocosms or in the field, combined with a greater rigor in reporting key methodological details. A greater focus is needed on understanding the environmental factors which regulate the wider effects of mosquito control using Bti on NTOs and ecosystems, to enhance our capacity for predicting where and when Bti is most likely to have additional, negative and indirect ecological impacts.

Stress Response of European Common Frog (Rana temporaria) Tadpoles to Bti Exposure in an Outdoor Pond Mesocosm

The biocide Bacillus thuringiensis var. israelensis (Bti) is applied to wetlands to control nuisance by mosquitoes. Amphibians inhabiting these wetlands can be exposed to Bti multiple times, potentially inducing oxidative stress in developing tadpoles. For biochemical stress responses, ambient water temperature plays a key role. Therefore, we exposed tadpoles of the European common frog (Rana temporaria) three times to field-relevant doses of Bti in outdoor floodplain pond mesocosms (FPM) under natural environmental conditions. We sampled tadpoles after each Bti application over the course of a 51-day experiment (April to June 2021) and investigated the activity of the glutathione-S-transferase (GST) and protein carbonyl content as a measure for detoxification activity and oxidative damage. GST activity increased over the course of the experiment likely due to a general increase of water temperature. We did not observe an effect of Bti on either of the investigated biomarkers under natural ambient temperatures. However, Bti-induced effects may be concealed by the generally low water temperatures in our FPMs, particularly at the first application in April, when we expected the highest effect on the most sensitive early stage tadpoles. In light of the global climate change, temperature-related effects of pesticides and biocides on tadpoles should be carefully monitored - in particular since they are known as one of the factors driving the worldwide decline of amphibian populations.

Biotoxicity comparison of Bacillus thuringiensis to control vector borne diseases against mosquito fauna

The current study was designed to evaluate the biotoxicity of screened echo-friendly Bacillus thuringiensis strains from different areas of Pakistan. Out of 50 samples, 36% Bt. isolates were quarantined from soil containing cattle waste after morphological, biochemical, and molecular characterization. The toxicity bioassays with Bt. spores and protein diet proved that 11 Bt. isolates were utmost noxious to 3rd instar larvae of mosquitoes Aedes aegypti, Anopheles stephensi, and Culex pipiens. The entopathogenic activity of first 4 Bt. toxins against A. aegypti was highly lethal as compared to the other dipteran larvae. The toxicity (LC₅₀) of spore diet of Bt. strains GCU-DAB-NF4 (442.730 ± 0.38 μg/ml), NF6 (460.845 ± 0.29 μg/ml), NF3 (470.129 ± 0.28 μg/ml), and NF7 (493.637 ± 0.70 μg/ml) was quite high against A. aegypti as compared to the C. pipiens after 24 h of incubation. The highest toxicity of total cell protein was shown by GCU-DAB-NF4 (LC₅₀ = 84.10 ± 50 μg/ml), NF6 (95.122 ± 0.40 μg/ml), NF3 (100.715 ± 06 μg/ml), and NF5 (103.40 ± 07 μg/ml) against A. aegypti after 24 h. So, these strains a have great potential to be used as biological control especially against A. aegypti as compared to the C. pipiens.

Control of Aedes mosquito populations using recombinant microalgae expressing short hairpin RNAs and their effect on plankton

New biocontrol strategies are urgently needed to combat vector-borne infectious diseases. This study presents a low-cost method to produce a potential mosquito insecticide that utilizes the microalgae released into suburban water sources to control mosquito populations. Chlorella microalgae are ubiquitous in local waters, which were chosen as the host for genetic transfection. This species facilitated the recombinant algae to adapt to the prevailing environmental conditions with rapid growth and high relative abundance. The procedure involved microalgae RNAi-based insecticides developed using short hairpin RNAs targeting the Aedes aegypti chitin synthase A (chsa) gene in Chlorella. These insecticides effectively silenced the chsa gene, inhibiting Aedes metamorphosis in the laboratory and simulated-field trials. This study explored the impact of recombinant microalgae on the phytoplankton and zooplankton in suburban waters. High-throughput sequencing revealed that rapid reproduction of recombinant Chlorella indirectly caused the disappearance of some phytoplankton and reduced the protozoan species. This study demonstrated that a recombinant microalgae-based insecticide could effectively reduce the population of Aedes mosquitoes in the laboratory and simulated field trials. However, the impact of this technology on the environment and ecology requires further investigation.

Effect of marker-free transgenic Chlamydomonas on the control of Aedes mosquito population and on plankton

BACKGROUND

More than half of the world's population suffers from epidemic diseases that are spread by mosquitoes. The primary strategy used to stop the spread of mosquito-borne diseases is vector control. Interference RNA (RNAi) is a powerful tool for controlling insect populations and may be less susceptible to insect resistance than other strategies. However, public concerns have been raised because of the transfer of antibiotic resistance marker genes to environmental microorganisms after integration into the recipient genome, thus allowing the pathogen to acquire resistance. Therefore, in the present study, we modified the 3-hydroxykynurenine transaminase (3hkt) and hormone receptor 3 (hr3) RNAi vectors to remove antibiotic resistance marker genes and retain the expression cassette of the inverse repeat sequence of the 3hkt/hr3 target gene. This recombinant microalgal marker-free RNAi insecticide was subsequently added to the suburban water in a simulated-field trial to test its ability to control mosquito population.

METHODS

The expression cassette of the 3hkt/hr3 inverted repeat sequence and a DNA fragment of the argininosuccinate lyase gene without the ampicillin resistance gene were obtained using restriction enzyme digestion and recovery. After the cotransformation of Chlamydomonas, the recombinant algae was then employed to feed Aedes albopictus larvae. Ten and 300 larvae were used in small- and large-scale laboratory Ae.albopictus feeding trials, respectively. Simulated field trials were conducted using Meishe River water that was complemented with recombinant Chlamydomonas. Moreover, the impact of recombinant microalgae on phytoplankton and zooplankton in the released water was explored via high-throughput sequencing.

RESULTS

The marker-free RNAi-recombinant Chlamydomonas effectively silenced the 3hkt/hr3 target gene, resulting in the inhibition of Ae. albopictus development and also in the high rate of Ae. albopictus larvae mortality in the laboratory and simulated field trials. In addition, the results confirmed that the effect of recombinant Chlamydomonas on plankton in the released water was similar to that of the nontransgenic Chlamydomonas, which could reduce the abundance and species of plankton.

CONCLUSIONS

The marker-free RNAi-recombinant Chlamydomonas are highly lethal to the Ae. albopictus mosquito, and their effect on plankton in released water is similar to that of the nontransgenic algal strains, which reduces the abundance and species of plankton. Thus, marker-free recombinant Chlamydomonas can be used for mosquito biorational control and mosquito-borne disease prevention.

Long-term persistence and recycling of Bacillus thuringiensis israelensis spores in wetlands sprayed for mosquito control

Bacillus thuringiensis subsp. israelensis (Bti) is the main larvicide used to control mosquitoes worldwide. Although there is accumulating evidence of Bti having environmental effects on non-target fauna, relatively few field studies have documented the fate of Bti spores in the environment. Spore density was quantified over a 6-yr period (2012-2017) in Mediterranean marshes sprayed with Vectobac 12AS (32 ITU/ha) since 2006 to reduce the nuisance caused by Aedes caspius. Bti spores were naturally found in all habitat types. Spore density expressed as colony-forming units per gram of soil (CFU g^-1) increased significantly at treated sites by a factor of 22 to 500 times relative to control sites, with mean values of 7730 CFU g^-1 in halophilous scrubs, 38,000 in reed beds, 49,000 in bulrush beds and 50 000 in rush beds. Spore density varied little in the first months after the spraying season (April-October), but increased sharply in spring, just before the annual launch of mosquito control. Considering that Bti is an insect pathogen that cannot proliferate without a suitable insect host, this unexpected recrudescence in spring could be related to the warming of water that triggers activity and development of benthic organisms such as chironomids, which may contribute to Bti proliferation by ingesting accumulated spores at the surface of sediments. While spore density tends to decrease over time, presumably during the summer period as a result of increased UV exposure, three to four years were necessary for spore density to return to normal levels after mosquito-control interruption. This study is important because it demonstrates that environmental effects of mosquito-control using Bti can far exceed the short period of Bti efficacy against lentic mosquitoes. Considering that Bti is a microbial agent, these long-term effects should be addressed at multiple levels of ecosystem organization from a one-health perspective.

Frogs Respond to Commercial Formulations of the Biopesticide Bacillus thuringiensis var. israelensis, Especially Their Intestine Microbiota

It is generally believed that Bacillus thuringiensis var. israelensis (Bti) biopesticides are harmless to non-target organisms; however, new research shows controversial results. We exposed acutely and chronicallyLithobates sylvaticusandAnaxyrus americanus tadpoles until metamorphic climax to VectoBac 200G (granules) and VectoBac 1200L (aqueous suspension) at 300-20,000 ITU/L covering field-relevant concentrations and higher. The data show that the exposure parameters tested did not affect significantly the survival, total length, total weight, hepatosomatic index, gonadosomatic index, the expression of genes of interest (i.e., related to xenobiotic exposure, oxidative stress, and metamorphosis), and the intestine tissue layer detachment ofL. sylvaticusandA. americanus in a concentration-response pattern. In contrast, VectoBac 200G significantly increased the median time to metamorphosis ofL. sylvaticus tadpoles by up to 3.5 days and decreased the median by up to 1 day inA. americanus. VectoBac 1200L significantly increased the median time to metamorphosis ofL. sylvaticusandA. americanustadpoles by up to 4.5 days. Also, the exposure to VectoBac 200G and 1200L altered the intestine bacterial community composition inA. americanus at application rates recommended by the manufacturer, which led to an increase in the relative abundance of Verrucomicrobia, Firmicutes, Bacteroidetes, and Actinobacteria. Changes in the intestine microbiota might impact the fitness of individuals, including the susceptibility to parasitic infections. Our results indicate that the effect of Bti commercial products is limited; however, we recommend that Bti-spraying activities in amphibian-rich ecosystems should be kept minimal until there is more conclusive research to assess if the changes in the time to metamorphosis and microbiota can lead to negative outcomes in amphibian populations and, eventually, the functioning of ecosystems.

Responses of benthic macroinvertebrate communities to a Bti-based insecticide in artificial microcosm streams

Bioinsecticides based on the bacterium Bacillus thuringiensis subsp. israelensis (Bti) are increasingly being applied directly into aquatic compartments to control nuisance mosquitoes and blackflies and are generally considered environmentally friendly alternatives to synthetic insecticides. Bti-based insecticides are considered highly selective, being Diptera-specific, and supposedly decompose rapidly in the environment. Nevertheless, their safety to non-target species and freshwater ecosystems has been questioned by recent studies, which in fact document possible indirect effects in aquatic food webs such as the decrease of prey availability to predators. This work aimed to evaluate the potential effects of a Bti-based insecticide (VectoBac® 12AS) on a freshwater macroinvertebrate community and on stream ecological functions by using artificial microcosm streams. Artificial microcosm streams were colonized with a macroinvertebrate community plus periphyton collected in a stream together with Alnus glutinosa leaf packs. They were exposed for 7 days to different Bti treatments (0, 12, 120, 1200 μg/L), which are within the recommended concentrations of application in aquatic compartments for blackfly and mosquito control. Besides invertebrate community structure and abundance, effects were evaluated regarding leaf decomposition and primary production as measures of ecosystem functioning. Community structure was significantly altered in all Bti treatments after 7 days of exposure, mostly due to a decline in chironomids, followed by oligochaetes, which both belong to the deposit-feeders' functional group. Direct effects on oligochaetes are surprising and require further research. Also, reductions of leaf decomposition due to Bti-induced sublethal effects on shredders (reduced feeding) or mortality of chironomids (that can also feed on coarse organic matter) observed in our study, represent potential indirect effects of Bti in aquatic ecosystems. Our short-exposure experiment evidenced some negative effects on stream benthic invertebrate communities and on ecosystem functioning that must be considered whenever Bti is used in water bodies for blackfly or mosquito control programs.

Dissecting the Environmental Consequences of Bacillus thuringiensis Application for Natural Ecosystems

Bacillus thuringiensis (Bt), a natural pathogen of different invertebrates, primarily insects, is widely used as a biological control agent. While Bt-based preparations are claimed to be safe for non-target organisms due to the immense host specificity of the bacterium, the growing evidence witnesses the distant consequences of their application for natural communities. For instance, upon introduction to soil habitats, Bt strains can affect indigenous microorganisms, such as bacteria and fungi, and further establish complex relationships with local plants, ranging from a mostly beneficial demeanor, to pathogenesis-like plant colonization. By exerting a direct effect on target insects, Bt can indirectly affect other organisms in the food chain. Furthermore, they can also exert an off-target activity on various soil and terrestrial invertebrates, and the frequent acquisition of virulence factors unrelated to major insecticidal toxins can extend the Bt host range to vertebrates, including humans. Even in the absence of direct detrimental effects, the exposure to Bt treatment may affect non-target organisms by reducing prey base and its nutritional value, resulting in delayed alleviation of their viability. The immense phenotypic plasticity of Bt strains, coupled with the complexity of ecological relationships they can engage in, indicates that further assessment of future Bt-based pesticides' safety should consider multiple levels of ecosystem organization and extend to a wide variety of their inhabitants.

Environmental and socioeconomic effects of mosquito control in Europe using the biocide Bacillus thuringiensis subsp. israelensis (Bti)

Bacillus thuringiensis subsp. israelensis (Bti) has been used in mosquito control programs to reduce nuisance in Europe for decades and is generally considered an environmentally-safe, effective and target-specific biocide. However, the use of Bti is not uncontroversial. Target mosquitoes and affected midges represent an important food source for many aquatic and terrestrial predators and reduction of their populations is likely to result in food-web effects at higher trophic levels. In the context of global biodiversity loss, this appears particularly critical since treated wetlands are often representing conservation areas. In this review, we address the current large-scale use of Bti for mosquito nuisance control in Europe, provide a description of its regulation followed by an overview of the available evidence on the parameters that are essential to evaluate Bti use in mosquito control. Bti accumulation and toxin persistence could result in a chronic expose of mosquito populations ultimately affecting their susceptibility, although observed increase in resistance to Bti in mosquito populations is low due to the four toxins involved. A careful independent monitoring of mosquito susceptibility, using sensitive bioassays, is mandatory to detect resistance development timely. Direct Bti effects were documented for non-target chironomids and other invertebrate groups and are discussed for amphibians. Field studies revealed contrasting results on possible impacts on chironomid abundances. Indirect, food-web effects were rarely studied in the environment. Depending on study design and duration, Bti effects on higher trophic levels were demonstrated or not. Further long-term field studies are needed, especially with observations of bird declines in Bti-treated wetland areas. Socio-economic relevance of mosquito control requires considering nuisance, vector-borne diseases and environmental effects jointly. Existing studies indicate that a majority of the population is concerned regarding potential environmental effects of Bti mosquito control and that they are willing to pay for alternative, more environment-friendly techniques.

Effects of larval exposure to sublethal doses of Bacillus thuringiensis var. israelensis on body size, oviposition and survival of adult Anopheles coluzzii mosquitoes

BACKGROUND

Application of the larvicide Bacillus thuringiensis var. israelensis (Bti) is a viable complementary strategy for malaria control. Efficacy of Bti is dose-dependent. There is a knowledge gap on the effects of larval exposure to sublethal Bti doses on emerging adult mosquitoes. The present study examined the effect of larval exposure to sublethal doses of Bti on the survival, body size and oviposition rate in adult Anopheles coluzzii.

METHODS

Third-instar An. coluzzii larvae were exposed to control and sublethal Bti concentrations at LC20, LC50 and LC70 for 48 h. Surviving larvae were reared to adults under standard colony conditions. Thirty randomly selected females from each treatment were placed in separate cages and allowed to blood feed. Twenty-five gravid females from the blood-feeding cages were randomly selected and transferred into new cages where they were provided with oviposition cups. Numbers of eggs laid in each cage and mortality of all adult mosquitoes were recorded daily. Wing lengths were measured of 570 mosquitoes as a proxy for body size.

RESULTS

Exposure to LC70Bti doses for 48 h as third-instar larvae reduced longevity of adult An. coluzzii mosquitoes. Time to death was 2.58 times shorter in females exposed to LC70Bti when compared to the control females. Estimated mortality hazard rates were also higher in females exposed to the LC50 and LC20 treatments, but these differences were not statistically significant. The females exposed to LC70 concentrations had 12% longer wings than the control group (P < 0.01). No differences in oviposition rate of the gravid females were observed between the treatments.

CONCLUSIONS

Exposure of An. coluzzii larvae to sublethal Bti doses reduces longevity of resultant adults and is associated with larger adult size and unclear effect on oviposition. These findings suggest that anopheline larval exposure to sublethal Bti doses, though not recommended, could reduce vectorial capacity for malaria vector populations by increasing mortality of resultant adults.

Lethal and sublethal toxicity assessment of Bacillus thuringiensis var. israelensis and Beauveria bassiana based bioinsecticides to the aquatic insect Chironomus riparius

Despite being considered environmentally safe, a deeper environmental risk assessment is needed for microbial insecticides; special attention should be devoted to their sublethal toxicity to non-target species. This study evaluated effects of VectoBac® 12AS - VB (based on the bacterium Bacillus thurigiensis var. israelensis) and Naturalis®-L - NL (based on the fungus Beauveria bassiana) on the aquatic insect Chironomus riparius life-history and biochemical responses. Acute tests estimated a 48 h-LC₅₀ (median lethal concentration) of 1.85 μg/L (VB) and 34.7 mg/L (NL). Under sublethal exposure, VB decreased adults' emergence (LOEC - lowest observed effect concentration of 80 ng/L) while NL impaired larval growth (LOEC of 0.32 mg/L) and delayed emergence (LOEC of 2 mg/L for males and 0.8 mg/L for females). Despite not being monotonic, phenoloxidase activity increased (LOEC of 20 ng/L (VB) and 2 mg/L (NL)), suggesting activation of the immune system. There were no indications of oxidative damage nor neurotoxicity. Catalase activity was stimulated with all VB treatments, possibly associated with detoxification of immune response products. Under NL exposure, glutathione-S-transferase activity increased but did not show a dose-dependent response and, total glutathione decreased in the highest concentration. Exposure to both formulations caused the increase in protein content, while carbohydrate and lipids were not altered. This study revealed the susceptibility of C. riparius to VB and NL at concentrations below the ones recommended for field application, with potential population-level effects. These results add important information for the risk assessment of these microbial insecticides in aquatic ecosystems, considering relevant sublethal endpoints and raising concern about the adverse effects on non-target aquatic organisms.

Effect of Chlorine and Temperature on Larvicidal Activity of Cuban Bacillus thuringiensis Isolates

BACKGROUND

The efficacy of biolarvicides may be influenced by species of mosquito, larval age and density, temperature, water quality, bacterial formulation, and others. The aim of this study was to evaluate the influence of temperature and chlorine on larvicidal activity of Bacillus thuringiensis Cuban isolates against Aedes aegypti.

METHODS

The influence of temperature (25, 30, 35 °C) and chlorine (2.25mg/L) on the larvicidal activity of eleven B. thuringiensis Cuban isolates (collected between 2007 and 2009) were tested under laboratory conditions following WHO protocols. Bioassay data were analyzed by Probit program. The effect of chlorine and temperature (25, 30, 35 and 40 °C) on the Cry and Cyt proteins of these isolates was determined by SDS-PAGE polyacrylamide gel electrophoresis.

RESULTS

The pathogenicity of the isolates U81, X48 was affected at 35 °C. However, A21, A51, L910, and R89 isolates increase their entomopathogen activity at 35 °C. No differences were observed in toxicity of M29, R84, R85 and R87 isolates at different temperatures. The Cry 4, Cry 10 and Cry 11 proteins were reduced in A21, X48, R85 isolates at 35 and 40 °C. The Cyt proteins were reduced at 35 and 40 °C in A21, X48, R85, and A51 isolates. In L910 and R84 isolates, the Cyt toxin was degraded only at 40 °C. In chlorinated water, the lethal concentrations 50 and 90 in A21, A51, M29, R84, U81, and X48 isolates were increase.

CONCLUSION

A21, A51, L910, R85, and X48 isolates have a strong larvicidal activity for the treatment of Ae. aegypti breeding's sites exposed to high temperature and chlorine.

RNAi-based bioinsecticide for Aedes mosquito control

Zika virus infection and dengue and chikungunya fevers are emerging viral diseases that have become public health threats. Their aetiologic agents are transmitted by the bite of genus Aedes mosquitoes. Without effective therapies or vaccines, vector control is the main strategy for preventing the spread of these diseases. Increased insecticide resistance calls for biorational actions focused on control of the target vector population. The chitin required for larval survival structures is a good target for biorational control. Chitin synthases A and B (CHS) are enzymes in the chitin synthesis pathway. Double-stranded RNA (dsRNA)-mediated gene silencing (RNAi) achieves specific knockdown of target proteins. Our goal in this work, a new proposed RNAi-based bioinsecticide, was developed as a potential strategy for mosquito population control. DsRNA molecules that target five different regions in the CHSA and B transcript sequences were produced in vitro and in vivo through expression in E. coli HT115 and tested by direct addition to larval breeding water. Mature and immature larvae treated with dsRNA targeting CHS catalytic sites showed significantly decreased viability associated with a reduction in CHS transcript levels. The few larval and adult survivors displayed an altered morphology and chitin content. In association with diflubenzuron, this bioinsecticide exhibited insecticidal adjuvant properties.

Environmental Metabolic Footprinting (EMF) vs. half-life: a new and integrative proxy for the discrimination between control and pesticides exposed sediments in order to further characterise pesticides' environmental impact

Pesticides are regularly used for a variety of applications and are disseminated throughout the environment. These substances may have significant negative impacts. To date, the half-life, t_1/2, was often used to study the fate of pesticides in environmental matrices (water, soil, sediment). However, this value gives limited information. First, it does not evaluate the formation of by-products, resulting in the need for additional experiments to be performed to evaluate biodegradation and biotransformation products. T_1/2 also fails to consider the chemical's impact on biodiversity. Resilience time, a new and integrative proxy, was recently proposed as an alternative to t_1/2, with the potential to evaluate all the post-application effects of the chemical on the environment. The 'Environmental Metabolic Footprinting' (EMF) approach, giving an idea of the resilience time, was used to evaluate the impact of botanicals on soil. The goal is to optimise the EMF to study the impact of a microbial insecticide, the Bacillus thuringiensis israelensis (Bti), on sediment. The difficulty of this work lies in the commercial solution of Bti that is really complex, and this complexity yields chromatograms that are extremely difficult to interpret; t_1/2 cannot be used. No methodologies currently exist to monitor the impact of these compounds on the environment. We will test the EMF to determine if it is sensitive enough to tolerate such complex mixtures. A pure chemical insecticide, the α-cypermethrin, will be also studied. The article shows that the EMF is able to distinguish meta-metabolome differences between control and exposed (with Bti) sediments.

Perturbation and delayed recovery of the reed invertebrate assemblage in Camargue marshes sprayed with Bacillus thuringiensis israelensis

Bacillus thuringiensis var. israelensis (Bti) is the most commonly used larvicide to control mosquitoes worldwide. Considered as nontoxic to most organisms, Bti can nevertheless cause trophic perturbations to natural communities by reducing the abundance of Chironomidae, which are a key element of wetland food webs. Since August 2006, up to 8400 of the 33 000 ha of mosquito larval biotopes in the Camargue (Rhône delta, in southern France), are monitored by a public agency and Bti-sprayed (aqueous solution of VectoBac 12AS at 2.5 L/ha) whenever mosquito larvae (Ochlerotatus caspius and Oc. detritus) appeared in water bodies. This resulted in 30-50 aerial treatments/year, in addition to ground spraying of unknown frequency. The sprayed habitats include Phragmites australis reedbeds, which support a specific avifauna of conservation concern. We compared the abundance of invertebrate prey available to passerine birds at treated and control sites relative to the predicted values based on hydrology over a 9-year period. Food available to reed passerines was significantly reduced at treated areas, translating into a 34% decrease in breeding birds based on predictive modeling. The most affected arthropods were Diptera, Aranaea, Coleoptera, and Hymenoptera. No cumulative effects were observed over time, but the recovery of the invertebrate assemblage after the cessation of mosquito control was delayed due to Bti spore persistence and proliferation in the sediments. While hydrology remains a prime factor influencing primary and secondary productivity of the Camargue reed marshes, Bti spraying had significant negative effects on animal communities at several trophic levels.

Do Multi-year Applications of Bacillus thuringiensis subsp. israelensis for Control of Mosquito Larvae Affect the Abundance of B. cereus Group Populations in Riparian Wetland Soils?

Bacillus thuringiensis subsp. israelensis (Bti) is a soil-borne bacterium affiliated to the Bacillus cereus group (Bcg) and has been used in biocontrol products against nematoceran larvae for several decades. However, knowledge is limited on whether long-term Bti application can affect the structure of indigenous communities of Bcg and the overall abundance of Bti. Using species- and group-specific quantitative PCR assays, we measured the Bcg- and Bti-abundances in riparian wetlands in the River Dalälven floodplains of central Sweden. On five occasions during one vegetative season, soil samples were collected in alder swamps and wet meadows which had been treated with Bti for mosquito larvae control during the preceding 11 years, as well as in untreated control sites and well-drained forests in the same area. The average abundance of Bcg in alder swamps was around three times higher than in wet meadows. Across all sites and habitats, the Bti treatments had no effect on the Bcg-abundance, whereas the Bti-abundance was significantly higher in the treated than in the control sites. However, for individual sampling sites, abundances of Bti and Bcg were not correlated with the number of Bti applications, indicating that added Bti possibly influenced the total population of Bti in the short term but had only a limited effect in the longer term. The findings of this study increase the understanding of the ecology of Bti applications for mosquito control, which can facilitate environmental risk assessment in connection with approval of microbiological control agents.

Isolation and characterization of native Bacillus thuringiensis strains from Saudi Arabia with enhanced larvicidal toxicity against the mosquito vector Anopheles gambiae (s.l.)

BACKGROUND

Worldwide, mosquito vectors are transmitting several etiological agents of important human diseases, including malaria, causing millions of deaths every year. In Saudi Arabia, as elsewhere, vector-control is based mostly on chemical insecticides which may be toxic and cause environmental deprivation. Here, to support the development of bio-pesticide alternatives, a study was conducted to identify native Bacillus thuringiensis (Bt) isolates with improved toxicity against the malaria vector, Anopheles gambiae (s.l.).

METHODS

Sixty-eight Bt isolates were obtained from 300 soil and other samples collected from 16 sites across Saudi Arabia. Bt identification was based on morphological characteristics of colonies, shape of parasporal crystals and biochemical profiles. After characterization of their mosquitocidal activity, larvicidal strains were described through 16S ribosomal DNA gene sequencing, cry, cyt and chi genes PCR-amplification profiles, and SDS-PAGE protein analyses.

RESULTS

Spherical Bt crystals were predominant amongst the 68 isolates (34%), while irregular, bi-pyramidal and spore-attached crystals were found in 32, 13 and 21% of strains, respectively. LC50 and LC90 bioassays showed that 23/68 isolates were larvicidal, with distinct biochemical activity profiles compared to non-larvicidal Bt strains. Eight larvicidal strains showed larvicidal activity up to 3.4-fold higher (LC50 range: 3.90-7.40 μg/ml) than the reference Bti-H14 strain (LC50 = 13.33 μg/ml). Of these, 6 strains had cry and cyt gene profiles similar to Bti-H14 (cry4Aa, cry4Ba, cry10, cry11, cyt1Aa, cyt1Ab, cyt2Aa). The seventh strain (Bt63) displaying the highest larvicidal activity (LC50 = 3.90 μg/ml) missed the cry4Aa and cyt1Ab genes and had SDS-PAGE protein profiles and spore/crystal sizes distinct from Bti-H14. The eight strain (Bt55) with LC50 of 4.11μg/ml had cry and cyt gene profiles similar to Bti-H14 but gave a chi gene PCR product size of 2027bp. No strains harbouring cry2, cry17 + 27, cry24 + 40, cry25, cry29, cry30, or cyt2Ba were detected.

CONCLUSION

This study represents the first report of several Saudi indigenous Bt strains with significantly higher larvicidal efficacy against An. gambiae than the reference Bti-H14 strain. The very high toxicity of the Bt63 strain, combined with distinct cry and cyt genes and SDS-PAGE-protein profiles makes it a promising candidate for future applications in mosquito bio-control.

Fate of insecticidal Bacillus thuringiensis Cry protein in soil: differences between purified toxin and biopesticide formulation

BACKGROUND

Bacillus thuringiensis produces insecticidal proteins known as Cry, and its efficiency and absence of side effects make it the most widely used biopesticide. There is little information on the role of soils in the fate of Cry proteins from commercial biopesticide formulations, unlike toxins from genetically modified crops, which have been intensively studied in recent years. The persistence of Cry in soil was followed under field and laboratory conditions.

RESULTS

Sunlight accelerated loss of detectable Cry under laboratory conditions, but little effect of shade was observed under field conditions. The half-life of biopesticide proteins in soil under natural conditions was about 1 week. Strong temperature effects were observed, but they differed for biopesticide and purified protein, indicating different limiting steps.

CONCLUSION

For the biopesticide, the observed decline in detectable protein was due to biological factors, possibly including the germination of B. thuringiensis spores, and was favoured by higher temperature. In contrast, for purified proteins, the decline in detectable protein was slower at low temperature, probably because the conformational changes of the soil-adsorbed protein, which cause fixation and hence reduced extraction efficiency, are temperature dependent. © 2016 Society of Chemical Industry.

Bioinsecticide and leaf litter combination increases oviposition and reduces adult recruitment to create an effective ovitrap for Culex mosquitoes

Mosquito egg traps, aquatic habitats baited with oviposition attractant and insecticide, are important tools for surveillance and control efforts in integrated vector management programs. The bioinsecticide Bacillus thuringiensis subsp. israelensis (Bti) is increasingly used as an environmentally friendly alternative to chemical insecticides and the combination of Bti with a simple oviposition attractant like leaf litter to create an effective egg trap seems appealing. However, previous research suggests that Bti may itself alter oviposition, and that leaf litter may dramatically reduce Bti toxicity. Here we present results from field experiment designed to link the effects of litter and Bti on mosquito oviposition habitat selection and post-colonization survival to production of adult mosquitoes. Tripling litter increased Culex spp. oviposition nearly nine-fold, while Bti had no effect on oviposition. Neither factor altered egg survival, thus larval abundance reflected the effects of litter on oviposition. Both Bti and litter reduced larval survival by ∼60%. We found no evidence that increased litter reduced Bti toxicity. Adult production was dependent upon both litter and Bti. In the absence of Bti, effects of litter on oviposition translated into three-fold more adults. However, in the presence of Bti, initial increases in oviposition were erased by the combined negative effects of Bti and litter on post-colonization survival. Thus, our study provides field evidence that combined litter and Bti application creates an effective ovitrap. This combined treatment had the highest oviposition and the lowest survival, and thus removed the greatest number of mosquitoes from the landscape.

Monitoring resistance to Bacillus thuringiensis subsp. israelensis in the field by performing bioassays with each Cry toxin separately

Bacillus thuringiensis subsp. israelensis (Bti) is increasingly used worldwide for mosquito control and is the only larvicide used in the French Rhône-Alpes region since decades. The artificial selection of mosquitoes with field-persistent Bti collected in breeding sites from this region led to a moderate level of resistance to Bti, but to relatively high levels of resistance to individual Bti Cry toxins. Based on this observation, we developed a bioassay procedure using each Bti Cry toxin separately to detect cryptic Bti-resistance evolving in field mosquito populations. Although no resistance to Bti was detected in none of the three mosquito species tested (Aedes rusticus, Aedes sticticus and Aedes vexans), an increased tolerance to Cry4Aa (3.5-fold) and Cry11Aa toxins (8-fold) was found in one Ae. sticticus population compared to other populations of the same species, suggesting that resistance to Bti may be arising in this population. This study confirms previous works showing a lack of Bti resistance in field mosquito populations treated for decades with this bioinsecticide. It also provides a first panorama of their susceptibility status to individual Bti Cry toxins. In combination with bioassays with Bti, bioassays with separate Cry toxins allow a more sensitive monitoring of Bti-resistance in the field.

Gene expression patterns and sequence polymorphisms associated with mosquito resistance to Bacillus thuringiensis israelensis toxins

BACKGROUND

Despite the intensive use of Bacillus thuringiensis israelensis (Bti) toxins for mosquito control, little is known about the long term effect of exposure to this cocktail of toxins on target mosquito populations. In contrast to the many cases of resistance to Bacillus thuringiensis Cry toxins observed in other insects, there is no evidence so far for Bti resistance evolution in field mosquito populations. High fitness costs measured in a Bti selected mosquito laboratory strain suggest that evolving resistance to Bti is costly. The aim of the present study was to identify transcription level and polymorphism variations associated with resistance to Bti toxins in the dengue vector Aedes aegypti. We used RNA sequencing (RNA-seq) for comparing a laboratory-selected strain showing elevated resistance to Bti toxins and its parental non-selected susceptible strain. As the resistant strain displayed two marked larval development phenotypes (slow and normal), each phenotype was analyzed separately in order to evidence potential links between resistance mechanisms and mosquito life-history traits.

RESULTS

A total of 12,458 genes were detected of which 844 were differentially transcribed between the resistant and susceptible strains. Polymorphism analysis revealed a total of 68,541 SNPs of which 12,571 SNPs exhibited more than 40% frequency difference between the resistant and susceptible strains, affecting 2,953 genes. Bti resistance is associated with changes in the transcription level of enzymes involved in detoxification and chitin metabolism. Among previously described Bti-toxin receptors, four alkaline phosphatases (ALPs) were differentially transcribed between resistant and susceptible larvae, and non-synonymous changes affected the protein sequence of one cadherin, six aminopeptidases (APNs) and four α-amylases. Other putative Cry receptors located in lipid rafts, such as flotillin and glycoside hydrolases, were under-transcribed and/or contained non-synonymous substitutions. Finally, immunity-related genes showed contrasted transcription and polymorphisms patterns between the two developmental resistant phenotypes, suggesting the existence of trade-offs between Bti-resistance, life-history traits and immunity.

CONCLUSIONS

The present study is the first to analyze the whole transcriptome of Bti-resistant mosquitoes by RNA-seq, shedding light on the importance of studying both transcription levels and sequence polymorphism variations to get a comprehensive view of insecticide resistance.

Pre-selecting resistance against individual Bti Cry toxins facilitates the development of resistance to the Bti toxins cocktail

The bioinsecticide Bacillus thuringiensis subsp. israelensis is a larvicide used worldwide for mosquito control, which contains three Cry toxins and one Cyt toxin. We investigated for the first time in Aedes aegypti (1) the evolution of resistance and cross-resistance of strains selected with each Cry toxin, and (2) the effect of pre-selection with Cry toxin on the evolution of resistance to a mix of Bti toxins. Cross resistance was higher between Cry4Ba and Cry11Aa than between Cry4Aa and either Cry4Ba or Cry11Aa, suggesting both common and specific mechanisms of resistance. Pre-selecting resistance to each Cry toxins facilitated the development of resistance to the full Bti toxins cocktail.

Persistence and recycling of bioinsecticidal Bacillus thuringiensis subsp. israelensis spores in contrasting environments: evidence from field monitoring and laboratory experiments

Sprays of commercial preparations of the bacterium Bacillus thuringiensis subsp. israelensis are widely used for the control of mosquito larvae. Despite an abundant literature on B. thuringiensis subsp. israelensis field efficiency on mosquito control, few studies have evaluated the fate of spores in the environment after treatments. In the present article, two complementary experiments were conducted to study the effect of different parameters on B. thuringiensis subsp. israelensis persistence and recycling, in field conditions and in the laboratory. First, we monitored B. thuringiensis subsp. israelensis persistence in the field in two contrasting regions in France: the Rhône-Alpes region, where mosquito breeding sites are temporary ponds under forest cover with large amounts of decaying leaf matter on the ground and the Mediterranean region characterized by open breeding sites such as brackish marshes. Viable B. thuringiensis subsp. israelensis spores can persist for months after a treatment, and their quantity is explained both by the vegetation type and by the number of local treatments. We found no evidence of B. thuringiensis subsp. israelensis recycling in the field. Then, we tested the effect of water level, substrate type, salinity and presence of mosquito larvae on the persistence/recycling of B. thuringiensis subsp. israelensis spores in controlled laboratory conditions (microcosms). We found no effect of change in water level or salinity on B. thuringiensis subsp. israelensis persistence over time (75 days). B. thuringiensis subsp. israelensis spores tended to persist longer in substrates containing organic matter compared to sand-only substrates. B. thuringiensis subsp. israelensis recycling only occurred in presence of mosquito larvae but was unrelated to the presence of organic matter.

Bacillus thuringiensis subsp. israelensis and its dipteran-specific toxins

Bacillus thuringiensis subsp. israelensis (Bti) is the first Bacillus thuringiensis to be found and used as an effective biological control agent against larvae of many mosquito and black fly species around the world. Its larvicidal activity resides in four major (of 134, 128, 72 and 27 kDa) and at least two minor (of 78 and 29 kDa) polypeptides encoded respectively by cry4Aa, cry4Ba, cry11Aa, cyt1Aa, cry10Aa and cyt2Ba, all mapped on the 128 kb plasmid known as pBtoxis. These six δ-endotoxins form a complex parasporal crystalline body with remarkably high, specific and different toxicities to Aedes, Culex and Anopheles larvae. Cry toxins are composed of three domains (perforating domain I and receptor binding II and III) and create cation-selective channels, whereas Cyts are composed of one domain that acts as well as a detergent-like membrane perforator. Despite the low toxicities of Cyt1Aa and Cyt2Ba alone against exposed larvae, they are highly synergistic with the Cry toxins and hence their combinations prevent emergence of resistance in the targets. The lack of significant levels of resistance in field mosquito populations treated for decades with Bti-bioinsecticide suggests that this bacterium will be an effective biocontrol agent for years to come.

Persistence of the spores of B. thuringiensis subsp. kurstaki from Foray bioinsecticide in gleysol soil and on leaves

The aim of this study was to determine how long the spores of Bacillus thuringiensis subsp. kurstaki HD-1 from Foray bioinsecticide persist in soil and on leaf surface after application of the bioinsecticide in an oak forest. Foray 04 UL was sprayed over a 195-hectare oak forest on the Krotoszyn Plateau in Poland. B. thuringiensis was isolated from soil samples and tree leaves taken from randomly chosen sites. B. thuringiensis subsp. kurstaki HD-1 in the samples was identified upon clonal analysis of the cultured isolates by using the RAPD method. One month after Foray spraying, the number of B. thuringiensis increased in soil and decreased on leaf surface comparing to the number estimated two days after the application. The reduction in the number of B. thuringiensis was noted six months after the pesticide application and the number was decreasing during the following months. No B. thuringiensis was noted on leaf surface one year after Foray spraying and in soil after one and a half years. The study showed that B. thuringiensis spores from biopesticide can survive in the forest environment; however, relatively short persistence time does not pose environmental risk.