Bespoke (‘X-tec’) silica particles for IRS applications

The efficacy of indoor residual spray (IRS) products is affected by various factors, such as the substrate on which they are sprayed, and the surface concentration and bioavailability of the insecticide. Insecticide bioavailability encompasses several factors in the target organism, in-cluding insecticide pickup, absorption through the cuticle, distribution, metabolism and excre-tion. Understanding what influences the bioavailability of an insecticide is complex, and for IRS, this can significantly impact the insecticide dosage, application frequency, cost, and environ-mental impact of a product. A factor that can substantially influence bioavailability, specifically insecticide pick up and absorption, is the physical state of the insecticide on a surface. For IRS products, this poses a significant challenge, as due to the variety of surfaces (e.g., mud/concrete/wood) on which insecticides are sprayed, it can be difficult to control the physical state of an insecticide. This study investigates the potential of bespoke silica particles (hereafter referred to as ‘X-tec silica’) as a unique carrier for insecticides, to reduce the insecticide content of an IRS formulation, by improving pickup by mosquitoes and optimising the physical state of the insecticide, while maintaining its residual biological activity on a surface. Molecular computer modelling was used to define the critical crystallisation size of clothianidin, and silica particles were manufactured with pore diameters smaller than this length to maintain the insecticide in an amorphous state. The silica carriers were then formulated to incorporate clothianidin inside their pores, and a full material characterisation was conducted to assess the clothianidin coating position on/in the silica particles, their concentration, and their physical form. The clothianidin-formulated silica (10%) was sprayed at three different application rates (30, 60 and 90 mg active ingredient (a.i.)/m2) onto two surfaces: glazed and unglazed tiles. The tiles were tested for bioefficacy against the insecti-cide-susceptible Anopheles gambiae s.s. Kisumu mosquito strain at 1 week and 8 months post-spray application. After 1-week post-spray application, at 60 and 90 mg a.i. /m2 application rates, 100% mortality was observed on both surfaces within 48 hours. For the lowest concentration (30 mg a.i./m2) 100% mortality was reached within 72 hours on glazed tiles, however, for unglazed tiles, due to the surface irregularity and porosity, it remained below 60%. At 8 months post-spray application, on glazed tiles, 100% mortality was reached within 24-hour hours at 60 and 90 mg a.i./m2 application rates, and within 48-hours at 30 mg a.i./m2. On unglazed tiles, 96-hour mortality was not meas-ured, however, 100% mortality was reached within 72 hours (90 mg a.i./m2) and 120 hours (60 mg a.i./m2) at higher concentrations. At the lowest concentration (30 mg a.i./m2) at 120 hours mor-tality only reached 25%. The lowest application rate tested (30 mg a.i./m2) is ten times less than current products on the market and demonstrates the potential of this approach.