Synergistic transition from type-I-like to type-II-like symptomology in a pyrethroid mixture and its impact on knockdown-mortality relationships in Musca domestica L.

Abstract

Background: Knockdown effect, especially in pyrethroid-based household and public insecticides, is often considered a practical indicator of insecticidal efficacy. However, whether faster knockdown implies lethality remains uncertain because the relationship between the two effects has not been systematically explored. This study examined whether knockdown speed is associated with mortality across type-I pyrethroids, type-II pyrethroids, and organophosphate insecticides in the house fly.

Results: For type-I pyrethroids, faster knockdown did not predict higher mortality. Early knockdown did not necessarily lead to lethality, and no significant differences were observed among LD₂₀, LD₅₀, and LD₈₀ treatments. In contrast, type-II pyrethroid and organophosphate insecticides showed a positive knockdown-mortality relationship, with most lethal individuals being knocked-down early. In the mixture study, d-phenothrin showed an additive interaction with prallethrin but a synergistic interaction with tetramethrin, although all three compounds are type-I pyrethroids. Computer vision-based analyses using a convolutional neural network (CNN) of symptomology revealed a synergistic shift in intoxication phenotype from type-I-like to type-II-like symptoms in the synergistic d-phenothrin and tetramethrin mixture.

Conclusion: Knockdown speed should not be used as a proxy for lethality for type-I pyrethroids, highlighting class- and mixture-dependent dynamics between knockdown and mortality. These findings support regulatory and operational decisions that consider knockdown and mortality as distinct performance endpoints, particularly when substantiating claims such as 'quick kill' or 'kills on contact' for pesticide registration. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.