Shrews, not mice — what mesh actually keeps them out, and does a 9 mm mouse guard just let them stroll in?

Quebec, fifth year, eight hives at the edge of a woodlot. Two winters ago I opened a colony in March to find it dead, no mouse nest, no chewed comb, but a heap of dismembered bees with the thoraxes eaten out and the abdomens left. A retired beekeeper at my club took one look at the photos and said shrew. The frustrating part: both hives had standard 9 mm round-hole mouse guards on all winter. A shrew is tiny and will go through a hole a mouse can't, and once inside a clustered colony it apparently works through the outer bees all winter. The colony shrinks and gets edgy and eventually fails, and it looks like "winter loss". So — what do people actually use where shrews are a known problem? I've read about excluder-type guards with narrow slots, which stop anything bigger than a bee but knock the pollen off returning foragers. That doesn't matter much in December here, but it would in April. Is there a mesh size that keeps shrews out and still lets a loaded forager through, or do you fit slotted guards for the deep winter only and swap them out?