Fluvalinate is a pyrethroid insecticide that is widely used in the control of the varroa mite (Varroa destructor), an ecto-parasite of the honeybee. Previously we identified four fluvalinate-resistance-associated mutations in the sodium channel gene of the varroa mite. One of the mutations caused a leucine (L) to proline (P) change at 1770 in the linker connecting domains III and IV of the sodium channel. Interestingly, at the position corresponding to the L to P mutation, all known insect (including honeybee) sodium channel proteins already naturally contain a P residue (e.g., P1577 in the cockroach sodium channel BgNa(v)). To determine whether insect sodium channels are less sensitive to fluvalinate than arachnid sodium channels, we replaced P1577 with an L in a BgNa(v) variant (BgNa(v)1-1) and examined the sensitivity of the recombinant channel to fluvalinate. The P1577L substitution did not alter the gating properties of the BgNa(v)1-1 channel expressed in Xenopus oocytes. However, the BgNa(v)1-1(P1577L) channel was five-fold more sensitive to fluvalinate compared with the BgNa(v)1-1 channel. These results not only implicate the L to P mutation in fluvalinate resistance in varroa mites, but also suggest a possible contribution of L1770 to the higher sensitivity of varroa mites to fluvalinate than their insect hosts.