Benzyl alcohol synergistic effect with deltamethrin against <i>Musca domestica</i> with molecular docking of potential modes of action

The house fly (Musca domestica Linnaeus, 1758) has developed resistance to several pesticides, necessitating innovative approaches for effective control. This study explores the synergistic effects of benzyl alcohol (BA) on the toxicity of deltamethrin (D) against various life stages of M. domestica larvae, pupae and adults. Additionally, molecular docking analyses were conducted to investigate interactions with the acetylcholinesterase enzyme (AChE) and voltage-sensitive sodium channel (Vssc). Two formulations of deltamethrin were tested: deltamethrin + benzyl alcohol (DBA) mix (D 5% dissolved in ethyl alcohol, then mixed with BA at its LC₅₀ concentration) and DBA combination (D 5% dissolved directly in pure BA). In vitro testing revealed significant synergistic effects. For larvae, the LC₅₀ of deltamethrin alone was 0.95 mg/mL, while the DBA mix reduced it to 0.09 mg/mL, yielding a synergism factor of 10.56. For pupae, deltamethrin's LC₅₀ was 1.32 mg/mL, compared with 0.104 mg/mL for the DBA mix, with a synergism factor of 12.69. The DBA combination also exhibited notable toxicity, with LC₅₀ values of 0.14 mg/mL for larvae and 0.16 mg/mL for pupae, corresponding to synergism factors of 6.79 and 8.80, respectively. Against adult flies, the DBA mix and DBA combination demonstrated high fumigant toxicity, with LC₅₀ values of 0.08 mg/L and 0.09 mg/L air, respectively, and synergism factors of 9.31 and 8.11. Docking analyses showed that deltamethrin exhibited a strong binding affinity to AChE, forming two hydrogen bonds with Lys480 and a pi-sigma interaction with Met351. BA occupied a separate binding pocket in AChE, forming two hydrogen bonds with Met309 and Lys335. Both compounds demonstrated strong, distinct binding affinities, confirming their synergistic inhibition of AChE. Against Vssc, deltamethrin (ΔG = -5.90 kcal/mol) formed a pi-pi interaction with Phe78 and a hydrogen bond with Lys79, while BA (ΔG = -3.79 kcal/mol) interacted with a different pocket via hydrophobic interactions. These findings highlight the potential of combining BA with deltamethrin to overcome insecticide resistance in house fly populations, offering a promising strategy to enhance pest control effectiveness.