Why not just test this by using a set of completely fresh strips in the
hive? Put in a sticky insert on the floor and see how many mites you
kill. (Of course, you could be in a northern latitude and in the grip of
winter, thus it is of no use at all to test now (other than by the
suggested jar method) and you must wait until the weather starts to open
up again in spring, but before large amounts of brood are present)

Whilst the risk of resistant mites should not be underestimated, as it
is significant and spreading, my understanding of the situation is that
a very high proportion of those resistant mite reported by beekeepers
turn out not to be so when properly tested under the correct conditions.

Many of the cases turn out to be because of deficiencies in how, or
when, the treatment was applied (including incorrect storage of strips).

I would be wary of jumping to a conclusion either way until such time as
you have tried fresh strips under good conditions

Reading between the lines a bit I wondered if you had bought a full pack
of Apistan, opened it, and used some of the strips the previous year.
Then you put the pack back on the shelf unsealed to keep the rest for
this year. If this is indeed the case then whoever told you about the
strips having lost there effectiveness is probably correct as the
fluvalinate will largely have evaporated off during the period of
incorrect storage.

If. however, they were perfectly sealed up, then you could have either
resistant mites, or (more likely in my opinion) a re-infestation
problem. As this appears to have been the colony you both treated first,
and removed treatment first, still in the active season, then I would
hazard a guess at re-infestation from a collapsing colony or colonies
within a reasonably short distance from your hives. In some experiments
re-infestation has been shown to occur at an alarming speed (I have
heard reports of 2000 mites entering a colony in a single day) during
the peak period for collapsing (possibly just about the time you took
the strips out of the colony). Your other colonies would still be
protected as they had full treatment still in progress.

The final fall back is the resistant mite scenario. To ascertain if you
have this trouble we come back to the treatment with fresh strips. Even
better would be to test them with flumethrin (Bayvarol or similar)
rather than fluvalinate. True resistance confers cross resistance to
both pyrethroids, so if nice fresh flumethrin also fails to kill them,
when applied at the correct time, then your resistant mite problem will
probably be confirmed.

As mere e-mail correspondents none of us really seem to have enough
information to jump to any conclusions about your situation. Thus it is
best that you go through a process of elimination to find out what has
happened. My GUESS from the information provided is that your most
likely problem is duff strips, and next best bet is re-infestation, and
finally resistant mites. Seek advice from your beekeeping authorities
locally, and if you don't trust them, try a local beekeeper whose
knowledge and expertise you have faith in.

Good luck,

Murray
--
Murray McGregor