Leaf-cutting ants of the genera Atta spp. and Acromyrmex spp. (Hymenoptera: Formicidae) are one of the most severe insect pests in the world (Moreira et al. 2004; Pérezet al. 2011; Furtado et al. 2013; Boulogne et al. 2014). These insects reduce the vegetal mass of various crops at all stages of plant development, resulting in significant production losses. Leaf-cutting ants can reduce plant biomass by up to 15% and cause mortality in young plants (Urbas et al. 2010; Swanson et al. 2019). Losses caused by these species reach approximately $8.26 per eucalyptus tree, and the ant control costs approach 30% of the production costs (Della Lucia 2011). Among the ant species found in Brazil, Atta sexdens L. (Hymenoptera: Formicidae) is one of the most aggressive, with a wide geographic distribution, polyphagous behavior, and difficult to control (Teixeira et al. 2023).
There are several ways to deal with leaf-cutting ants, including mechanical, cultural, biological, and chemical control techniques (Zanetti et al. 2003a; Reis et al. 2015). The most efficient and widely distributed method is the chemical one (Della Lucia 2011). One popular approach is to use ant baits, which are inexpensive and easy to apply locally. This method has been proven highly efficient in the field without requiring expensive equipment (Cruz et al. 1996; Zanetti et al. 2003b).
There are many active products currently used to control leaf-cutting ants. In agriculture are sulfluramid (C10H6F17NO2S), chlorpyrifos (C9H11Cl3NO3PS), and fipronil (C12H4Cl2F6N4OS) (MAPA 2003). Chlorpyrifos is probably the most widely used insecticide in agriculture due to its low cost, easy application, and broad spectrum of action (Lima et al. 2011). This insecticide acts through contact, ingestion, and fumigation (Nakagawa et al. 2016; Wołejko et al. 2022). However, due to their high toxicity and persistence in the environment, they can lead to the contamination of the ecosystems. One biological agent that shows toxicity to ants is the Beauveria bassiana fungus, which is commercially available and widely used in agriculture and forestry (Li et al. 2011). It has been extensively studied, showing prospects for future success in ant management (Cantú-Ruiz et al. 2017; Folgarait et al. 2020; Loureiro et al. 2022). The success of using B. bassiana may be true because it has been observed that there are isolates of B. bassiana that cause mortality of 87% of A. sexdens workers after 4 days of application (Loureiro et al. 2005). On the other hand, some ant control products on the market contain sodium tetraborate (Na2B4O7·10H2O). These products promise to repel and eliminate insects, especially ants, by altering their digestive system and overall behavior by acting as a disruptor, preventing ants from functioning normally and eventually leading to their death. Studies have reported toxicity and repellency to the Argentine ant - Linepithema humile Mayr and the Anoplolepis tenella Santschi (Hymenoptera: Formicidae) ant exposed to compounds containing boron (B), such as boric acid, disodium octaborate tetrahydrate, and borax (Hooper-Bui et al. 2000; Klotz et al. 2000; Sola et al. 2013; Hanna et al. 2015; Suiter et al. 2021). Hence, using materials that can act as matrices for the gradual release of active leaf-cutting ant control products can be an effective alternative to reducing the use of traditional chemical compounds for insect pest management. A promising approach to ant control is to formulate chlorpyrifos, B. bassiana spores, and sodium tetraborate within polymeric organic-inorganic hybrid matrices in bead form. This approach differs from conventional reliance on chemical compounds and can be a more sustainable method for managing ant populations (Vieira et al. 2014).
Notably, in Brazil, sulfluramid (0.2%) baits are the most commonly used to control leaf-cutting ants, as they show good results on several species. These baits come in the form of pellets containing the active ingredient sulfluramid and dehydrated orange pulp dissolved in soybean oil (Forti et al. 2007; Buratto et al. 2017). However, these commercial materials have low humidity resistance, which can cause the degradation of the bait before contacting the insect, resulting in low control efficacy (Peregrine et al. 1976; Zanúncio et al. 1992). Therefore, there is a need to produce more efficient baits with better environmental resistance, sustained release of the active chemical, and good ant-killing efficacy.
In the context of controlling leaf-cutter ants, the use of organic-inorganic hybrids, shows promise in improving the effectiveness of control methods and promoting environmental sustainability (Fernandes et al. 2014). These materials are expected to have promising properties due to the synergistic effect between their components, which are not found in the original materials (Sanchez et al. 2011a; Sanchez et al. 2011b; Abellán et al. 2015). To form improved materials, renewable biopolymers and 2D inorganic materials, such as clays, layered double hydroxides, and graphene, are combined to form organic-inorganic hybrid materials (Darder et al. 2005; Sanchez et al. 2011a). These materials are being applied in various fields, including pharmaceuticals (Takahashi and Yamaguchi 1991), agriculture (Benício et al. 2015), environment (Sanchez et al. 2011b), chemistry (Sanchez et al. 2011a) and others.
Sodium alginate (NaC6H7O6) is a polysaccharide obtained from marine algae, a derivative of alginic acid formed by 1,4-β-d-mannuronic (M) and α-l-guluronic (G) acids. This polymer has interesting properties such as biodegradability, non-immunogenicity, and the ability to form gels with various crosslinking agents under mild conditions (Cunha et al. 2010; Chan et al. 2016). In aqueous solutions, when the sodium alginate comes into contact with divalent metal cations, such as Ca2+, Mg2+, or Ba2+, due to the presence of carboxyl groups (COO-) in its structure, a hydrophobic gel is formed by crosslinking (Haug et al. 1970; Jain and Datta 2016). Thus, this polymer represents a potential carrier for the sustained delivery of chemical actives (Wan et al. 1994).
Inorganic modifiers include carbon nanotubes, active carbon, natural clays, and activated clays. The activated clays have been widely used as fillers due to their abundance and the possibility of structural modification of the layers. Kaolinite is a clay mineral (Al2O3·2SiO2·2H2O) with a chemical composition of 39.5% of Al2O3, 46.5% of SiO2, and 14.0% of H2O. This clay has excellent absorption and stabilization properties that promote the sustained release of active chemical ingredients when they are part of the organic-inorganic hybrid material (Etchepare et al. 2015).
The scientific literature has not yet reported the synthesis of organic-inorganic hybrid materials made from alginate-kaolinite beads containing sodium tetraborate, B. bassiana spores, and chlorpyrifos as baits for leaf-cutting ant control. However, the physicochemical interaction between the pristine parts of the organic-inorganic hybrid material can modify the stability of the baits under environmental conditions and the characteristics of reactivity, dissolution, and the release of the active chemical ingredient.
The present work aimed to synthesize and characterize ant baits prepared from alginate-kaolinite beads containing sodium tetraborate, B. bassiana spores, and chlorpyrifos; evaluate the "in vitro" and leaching release of boron from the produced baits, and compare the results with a commercial source; and evaluate the effectiveness of the produced baits in the control of A. sexdens through bioassays under field and laboratory conditions.