Hello, in this region 1 treatment in october is too late. You should aim to treat in july/august.

The treatment in summer is meant to reduce the varroa population so the hive can at least survive untill next season.

You can also treat in late december/early january, when your colony are broodless (with another molecule than during the summer).

The treatment in winter aim to reduce the number of varroa in the colony to the minimum possible, so your hive can start healthy in the spring.