Synergism of commercialized miticides by trans-chrysanthemic acid (TCA) to the parasitic mite, Varroa destructor

Varroa destructor is a major threat to honey bee health, and resistance to the limited number of approved synthetic miticides continues to increase worldwide. Identifying safe synergists that enhance the efficacy of existing acaricides is therefore a critical priority. We evaluated the toxicity, repellency, and synergistic potential of trans-chrysanthemic acid (TCA), a natural product terpenoid component of pyrethrins, against V. destructor and honey bees. To V. destructor, TCA had low contact toxicity (24 h LC₅₀ = 155 mg/L) but coapplication of TCA with τ-fluvalinate significantly (P < 0.05) increased toxicity by 5.4-fold when compared to τ-fluvalinate alone. TCA also exhibited significant (P < 0.05) repellency in laboratory assays, though repellency did not translate to field conditions. To honey bees, TCA was found to have low toxicity via oral and topical exposure and co-treatment with τ-fluvalinate did not induce mortality or upregulate detoxification or stress marker genes. In a simulated hive assay using mites attached to bees, the TCA/fluvalinate combination resulted in 85% corrected mortality at 48 h, exceeding the sum of individual treatments and confirming synergism. Electrophysiological recordings of the V. destructor synganglion demonstrated TCA significantly increased fluvalinate-induced neuronal firing when compared to τ-fluvalinate alone, providing a mechanistic basis for the observed synergy. Together, these findings indicate that TCA is a safe and effective synergist that enhances τ-fluvalinate toxicity to V. destructor minimal acute toxicity to honey bees under the conditions tested, supporting its potential as a supplemental tool for V. destructor management.