Capability of varroa passing between apiary A and apiary B is transfer potential T_{AB}.
Value of direct transfer potential is 1 : T_{AB} = 1.
Transfer potential is equal in both directions : T_{AB} = T_{BA}.
Varroa can pass directly only between apiaries less than 3 km apart.
Capability of apiary A to pass varroa to other apiaries is apiary invasion potential. I_{A}.
Apiary invasion potential is sum of its transfer potentials.

Varroa can pass indirectly between two apiaries A and C more than 3 km apart if both conditions are met:
There is at least one intermediate apiary B, which is less than 3 km apart from both apiaries A and C.
Varroa in newly infested intermediate apiary B has enough time to develop over critical level.
Value of indirect transfer potential in second step is 0.3: T_{AC} = 0.3