Household level insecticide deployments (micro-mosaics) for insecticide resistance management: Evaluating deliberate and accidental deployments

Mixtures of two insecticides in a single formulation at full-dose are frequently evaluated as the "best" insecticide resistance (IRM) strategy in public health. However, this requires both insecticides to be mixed together in a single formulation which may not be possible or practical. Deploying different insecticides in different households ("micro-mosaics") may allow for mosquitoes to encounter different insecticides in subsequent gonotrophic cycles obtaining a "temporal mixture". We evaluate micro-mosaics considering their deliberate use and their accidental use using a mathematical model assuming polygenic resistance. Deliberate micro-mosaics are evaluated against rotations and mixtures (full-dose or half-dose) over a range of scenarios allowing for cross resistance and insecticide decay on their ability to slow the development of resistance. Accidental micro-mosaics are evaluated to understand the implication of mixture insecticide-treated nets (ITNs) and standard (pyrethroid only) ITNs being deployed alongside one another on the development of resistance across a range of initial resistance scenarios. Deliberate micro-mosaics are found to not differ substantially in their IRM capability from either rotations or half-dose mixtures. When micro-mosaics do outperform rotations or half-dose mixtures the benefit is often small. Micro-mosaics are found to perform worse than full-dose mixtures. Accidental micro-mosaics are found to reduce the ability of mixtures to slow the development of resistance. The deployment of deliberate micro-mosaics was found to not be beneficial versus rotations or mixtures indicating this strategy should not be pursued. Micro-mosaics occurring accidentally due to multiple distribution channels inhibits the effectiveness of mixture ITNs in slowing the development of resistance. Where mixture ITNs are used keep the coverage of the mixture high relative to standard (pyrethroid-only) ITNs is key.