Fly ash-based Bacillus thuringiensis israelensis formulation: An ecofriendly approach

Vectors play an important role in disease transmission, globally. Despite various advancements in vector management, mosquitoes are still prime vectors of dreadful diseases those influence human health and economy as well. Insecticides have been the most preferred and commonly used tool in the vector management because of their rapid action and visible effects. Although several insecticides in different formulations are in existence, their usage over the years has arisen several problems such as reduced response by vectors in the form of resistance phenomenon and their negative impact on the environment.

Amongst various vector management options, biopesticide is a comparatively safer way. Biopesticides based on microorganisms can play an alternative strategy in crop protection due to safety to human beings and non-target organisms1. Although biopesticides represent only 2.89 per cent of the overall pesticide market in India2, its demand is rising steadily in all parts of the world3. An increased concern of biopesticides as an effective tool for mosquito control has been witnessed4. Amongst the various biopesticides, microbial pesticides represent the major component. Bacterial biopesticides are comparatively cheaper than the other methods of pest bioregulation3. It is evident that Bacillus thuringiensis is one of the most promising biopesticides, representing 70 per cent of the global biopesticide market2. Bacillus species, B. thuringiensis, is known for its property to produce proteins which are structurally distinct and acts as pesticidal toxins against pests of agriculture and medical importance; those are coded for by several cry genes5.

The first commercial Bt product was produced in 1938 in France6. To date, over 100 B. thuringiensis-based bioinsecticides have been developed, which are mostly used against lepidopteran, dipteran and coleopteran larvae3. Bt is effective against more than 150 insect pests. Because of their high specificity and safety in the environment, B. thuringiensis and Cry proteins are efficient, safe and sustainable alternatives to chemical pesticides for the control of insect pests7,8. Amongst 34 recognized subspecies of B. thuringiensis, subspecies kurstaki (lepidopteran specific), israelensis (dipteran specific, mainly mosquitoes and blackflies) and tenebrionis (targeted to Colorado potato beetle) are most widely explored9.

Biopesticides in several formulations such as dusts, powders for seed dressing, granules (GR), micro-GR, water-dispersible GR and wettable powders (WP); suspension concentrates, oil dispersions, suspo-emulsions, capsule suspensions; ultra-low volume formulations are in existence10,11. In India, only 12 types of biopesticides have been registered under the Insecticide Act, 19683. Bacillus sphaericus Neide (Bs) and B. thuringiensis serovar israelensis (Bti) are proven potential mosquito larvicides that stand effective in many operational levels under field conditions. These have numerous advantages such as safety for humans and other non-target organisms, minimal residues in the treated habitats, safe to most other natural enemies and increased biodiversity in aquatic ecosystems4. Bti has been successfully advocated for the management of mosquitoes since the last two decades, and its formulations are highly efficient against all three mosquito genera Anopheles, Aedes and Culex12. It has also been documented that B. sphaericus recycles in the field conditions and exhibits prolonged larvicidal activity, mainly against Culex and a few Aedes species, whereas Bti shows a wide spectrum of activities against all three species4. However, repeated application of B. sphaericus in the same habitat has evolved resistance in larvae13. During field experimentation, efficacy of different Bti formulations lasted for 2-7 days against An. culicifacies in freshwater pools and Cx. quinquefasciatus in polluted pools, 2-14 days against An. stephensi in tanks and drains and 7-28 days against Ae. aegypti in desert coolers and industrial scrapes13.

Both the bacterial formulations have been extensively studied under different geographical regions under variety of habitats. The efficacy of the of Bt depends on various bioenvironmental factors and the type of formulation. Formulation can affect the persistence of toxicity, site of contamination and choice of application method, as well as ultraviolet radiation, agitation, sedimentation, water quality, pollutants, pH, temperature, target host and microbial competition14. Since these bacteria are safe for animals and environment and cause no health risk to humans, several formulations have been produced to control many species of mosquitoes4. Bti formulation such as aqueous suspension, WP and GR have been tested for efficacy in the field13. Bti in briquettes or pellets form can address the problem of persistence14, whereas floating GR are better options for surface feeders. Effective formulations for control of different mosquitoes have been developed. Some formulations are slow release controlled formulation, mixture of chemical and biological agents, sprayed-dried powder as tablet and floating bait14. With the advent of nanotechnology in biological sciences, optimized controlled release formulations of Bti in the form of nanoemulsion, nanosuspension, nanocapsule suspension, etc. are new hope in bringing better biological effects15,16,17.

An effective, broad-spectrum, low-cost, user-friendly and readily available Bti formulation is needed. The cost to grow and produce Bti in highly refined laboratory bacterial culture medium is high18. For economic Bt production, cheaper raw material is essential. Several low-cost raw materials have been utilized for production of Bt biopesticides in India. Agro-industrial remains are simple, cheaper and effective bioresource for fermentation producing bacterial toxins targeting mosquito vectors4. Several agro byproducts and waste materials such as bird feathers, dried animal blood, fish meal and coconut cake soybean have been documented as alternate source for culture medium for Bt production18,19,20. All these raw materials are rich in nutrient sources (carbohydrate and proteins) and lead to the production of bacterial biopesticides Bs and Bti21,22.

Charcoal and plaster of Paris are some of the common carrier material for Bt formulations. Tamilselvam et al23 in this issue have reported a new carrier material fly ash as an alternative to the common carrier powder-based Bti formulation. Use of fly ash as a carrier in insecticides is generally known. Enormous amount of fly ash which is generated as solid waste in thermal power plants is available in India. Coal-based thermal power plants are the chief source of power generation24. During 2014-2015, 184.14 million tons of fly ash was generated25. The disposal of the substantial amount of solid waste from thermal plants is becoming a prime concern to human health. Fine particles of fly ash accumulated in the lungs for long period act as cumulative poisons and residual silica (40-73%) may cause silicosis, whereas heavy metals can exhibit toxicity26. Prolonged exposure to toxic metals in coal ash can cause several types of diseases27.

There is a vast gap between generation and utilization of fly ash among the countries. In developed countries such as Germany, 80 per cent of the fly ash generated is being utilized, whereas it is only three per cent in India26. Utilization of fly ash in the biopesticide formulations can contribute to its proper utilization thereby decreasing environmental pollution. Fly ash-based Bti formulation has been evaluated in field against Culex mosquito in natural ecosystem28 and found effective in reducing larval population, regardless of habitats.

The Indian fly ash is alkaline, facilitates to improve soil quality, maintains porous structure of soil, provides micronutrients and improves the fertility24. Fly ash has earlier been effectively used as carrier in production of biofertilizers and biopesticides29. A range of fly ash-based Bti formulations have been evaluated against all three vectors23, to arrive at the most effective one. Indian fly ash has very low radioactivity and heavy metal count30. Analysis of the shortlisted formulation by Tamilselvam et al23 also revealed the presence of only micro- and macronutrients and absence of any kind of heavy metals. This formulation has also shown safety against non-target organisms and mammalian systems. Further investigation on fly ash from different sources also needs to be done in detail. Although this formulation has potential to replace existing carrier material in biopesticides, an in-depth study on several aspects such as long-term toxicological analysis, its persistence in different habitats, storage stability and its economic needs is to be conducted.