>We are using screened bottom boards and have seen no mites so far.
>Is it certain that they will appear this season
> and if they do at what level
of infestation should we consider treatment??
I'd watch the trend more than the actual number. If the numbers are doubling every few days you need to step in. If the numbers are stable you don't have a problem. If the numbers stay fairly stable and spike a little in the fall (as all the brood emerges) and then drop back off you still might be ok.
If I were picking a wild number out of the air, and I am, I'd say at about 30 mites I'd be planning what I'm going to do with it hits 50 mites per day natural drop. But 50 mites with five frames of bees is terrible. 50 mites with 40 frames of bees isn't very bad. 50 mites in the spring is worse than 50 mites in the fall. It's very difficult to pick a simple number.