I'm sure this is a pretty common topic for discussion and I've checked
the archives for the answer I'm looking for without success.

I used up the last of my stock of granules last year (Bromothos wasn't
it?) and as I gather there isn't a chemical replacement, decided to
got the mechanical root and make collars from 4" squares of roofing
felt.

Can someone tell me at what point of the season I can retrieve these
bits of felt off the plants for re-use next year. ie when does the
risk from the flies cease?

Thanks

Rob