The Brazilian Amazon has high temperatures and high levels of insolation, climate conditions that contribute to the proliferation of the mosquito Aedes aegypti (L. 1762), which is the vector of the etiological agents of dengue, Zika, and chikungunya. These diseases constitute a group of arboviruses that presents a high risk to the health of many people who live mainly in tropical regions, such as in Brazil (Brasil Ministério da saúde, 2022).

In this context, the biological control of this vector is an efficient and more sustainable alternative compared to chemical routes, and presents some environmental advantages, such as biodegradability and low or no toxicity to the environment. Among these larvicides, the bacterium Bacillus thuringiensis can be highlighted (Bravo et al., 2007; Van Frankenhuyzen, 2009; Ben-Dov, 2014; Vargas et al., 2022). This bacterium produces spores and Cry and Cyt proteins in the form of protoxins, which exhibit larvicidal action for certain orders of insects, including Diptera, to which Aedes aegypti belongs (Crickmore et al., 1998; Crickmore et al., 2023). Cry proteins act by binding to specific receptors that are present in the midgut membrane of insects, causing septicemia and larval death; in contrast, Cyt toxins do not bind to receptors, but insert themselves directly into the cell membrane, enhancing the insecticidal action of Cry (Bravo et al., 2007; Van Frankenhuyzen, 2009; Ben-Dov, 2014).

The long-term persistence of viable bacterial spores in the environment is important for greater duration of larvicidal activity in the field (Viana et al., 2021a). However, it is worth noting that direct exposure of the spores to ultraviolet (UV) rays and high levels of insolation can deactivate the action of these microorganisms, thus reducing their effectiveness and making it difficult to develop efficient formulations based on free B. thuringiensis. The studies conducted by Batra et al. (2000) and Viana et al. (2021a) demonstrated that, when the spores are exposed to such environmental conditions in the field, the duration of the larvicide action is only two to three weeks.

It is important to emphasize that insecticidal formulations based on B. thuringiensis should maintain the characteristics of the microorganisms that are necessary to guarantee their larvicide action, should present low cost and be of easy use. In this way, microencapsulation is a technique that may protect the structure of some active ingredients while also enabling their controlled release under specific conditions (Suave et al., 2006; Favaro-Trindade et al., 2008; Way et al., 2018; De Souza e Castro et al., 2020). Thus, this technique can be explored in order to prolong the activity of B thuringiensis even under adverse environmental conditions in the field (Canevarolo, 2006; Angelo et al., 2010; Hernández-Suárez et al., 2011; De Souza e Castro et al., 2020). Among the polymers used in encapsulation processes, starch stands out as a promising biopolymer since it exhibits low cost and has good potential for the replacement of non-biodegradable materials (Abdillahi et al., 2013; Azevêdo et al., 2018). The use of starch as a matrix for the encapsulation of B. thuringiensis may be an excellent strategy for the development of ecologically sustainable products. Interestingly, in the literature, we did not find any studies that have explored the inverse suspension polymerization technique in the encapsulation of this larvicide in starch matrices (Azevêdo et al., 2018; Bilal and Iqbal, 2019; Araújo et al., 2021).

Thus, this study aims to encapsulate two strains of B. thuringiensis (BtMA-750 and BtMA-1114), which have different combinations of Cry and Cyt toxins, in starch microparticles. For this, we explored the inverse suspension polymerization technique and evaluated the effect of the concentration of spores on the encapsulation process and the larvicidal activity of the encapsulated bacteria.