The Culex, Aedes and Anopheles mosquitoes are the major vectors that transmit life-threatening infectious diseases like dengue fever, filariasis, zika virus infection, chikungunya virus infection, yellow fever, malaria, Japanese encephalitis etc. (Alvarez-Hernández and S-Rivera, 2017). According to the World Health Organization (WHO) report published in March 2020, One or more vector-borne diseases (VBDs) threaten 80% of the world's population (Vector-borne diseases, WHO, 2020). Most of the VBDs are not prescribed with effective medication or vaccines to prevent them, therefore, the control of mosquito breeding may be an essential step towards the reduction of VBDs. A Global Vector Control Response (GVCR) programme – 2017–2030 was approved by World Health Assembly in 2017 to reduce the burden and threat of VBDs through an effective and locally adapted sustainable vector control (WHO, Global vector control response, 2020).

There are different approaches adopted across the world to control VBDs. However, the treatment of water bodies with suitable larvicidal agents is found effective to control mosquito breeding at its immature stage both indoors and outdoors (Dhra et al., 2017). Synthetic chemicals such as organophosphates and organochlorine compounds are found to be effective in controlling mosquito breeding in stagnant water. But the unfavourable impact of synthetic larvicidal agents on humans, animals, and non-target organisms in the ecosystem forced the regulators to withdraw or discourage their use in many countries (Ghosh et al., 2012; Chaudhary et al., 2017; Damalas and Koutroubas, 2016). The larvicidal agents of natural origin are considered an alternative strategy to control mosquito breeding in stagnant water. The potent larvicidal activity of many of the plant extracts and their essential oils like Azadirachta indica L (Linnaeus, 1753), Piper nigrum L (Linnaeus, 1753), Lantana camera L (Linnaeus, 1753), Leucas aspera L (Linnaeus, 1753), Anethum graveolens L (Linnaeus, 1753) etc. were scientifically proven (Ghosh et al., 2012; Kumar et al., 2012; Elumalai et al., 2017). These agents from natural resources produced selective toxicity to the targeted species with minimum off-target effects. They are effective in smaller concentrations, non-toxic, biodegradable, easily available, economical as well as safer for the ecosystem. Among these Neem (A. indica) emerged as a highly potent biopesticide that contains more than 200 allelochemicals in variable concentrations and is proven to be having a variety of pesticide activities (Chaudhary et al., 2017).

The neem-based larvicidal products are available in different forms such as powders, granules, oils and emulsions. The liquid forms are found to be popular in the market but the drawbacks of these liquid larvicidal products include the inconvenience in handling, accidental spillage, run-off, left-over wastage, and upon contact with the skin it may irritate and even permeate through the skin (Pereira et al., 2019). The direct inhalation of aerosols or droplets of larvicides with less than 5 μm in diameter may be readily absorbed through the respiratory system. The dust associated with powder as well as a granular form of larvicidal products may lead to environmental contamination and poisoning (Chaudhary et al., 2017). The existing liquid, powder and granular products face difficulty in measuring the exact quantity or volume and may lead to poor or no larvicidal action along with unnecessary wastage of the product (Kala et al., 2019).

The major challenge faced by the available neem oil-based larvicidal products is the low stability, sustainability and short half-life which may lead to frequent application and inconsistent results from the product (Senthil-Nathan, 2020). In neem oil-based products, azadirachtin (AZA) is the major ingredient responsible for the larvicidal activity. Previous study reports suggested that the stability of azadirachtin was considerably low in liquid formulations due to the presence of moisture and it also has photostability concerns (Pereira et al., 2019). The drawbacks of existing neem oil-based larvicidal formulations reduced their popularity as well as acceptance among consumers and at present, the global market share for natural insecticides is less than 1% (Senthil-Nathan, 2020). The potential of neem as a larvicidal shall be explored as it is considered to be a safe, and effective alternative for most of the available synthetic products. But the unavailability of convenient, safe and stable neem-based products is a concern for most of the users. Therefore, the purpose of this study was to determine whether bilayer tablets containing neem oil might be used to stop mosquito breeding in standing water. Objectives included the characterisation of neem oil bilayer tablets and comparing their bio-efficacy against the existing marketed products of synthetic (Temephos 50% emulsified concentrate) and neem oil.