Finding a few small larvae in a mushroom doesn't mean you have
to leave it behind. A firm, young mushroom with only a few worms
is still worth taking — they can be removed at home.

The line to watch for is quantity and firmness together.
Heavily infested, soft, or visibly breaking-down mushrooms
are usually past their best anyway — older mushrooms attract
more larvae and taste worse. If it's riddled with worms
and going soft, it's not worth the cleaning effort. Leave it.

Soak cleaned mushrooms in cold, salted water for a short while.
The salt encourages any remaining larvae to leave the flesh
and float out into the water.

If you have a large batch, the same salt water can be reused
for a few rounds before it's too clouded to be useful —
no need to make a fresh bath for every handful.

After the salt bath, a second soak in boiled (not boiling) water from the kettle
finishes the job — this isn't cooking, just a gentle sterilising
step, and it tends to release a few last stray larvae too.

This second water, once strained through a fine sieve, carries
a real mushroom flavour and can be kept and used as a light
stock base, since it's already been through the cleaner
salt-bath stage and has fewer worms in it to begin with.

Some foragers worry about birds having pecked at mushrooms,
particularly with concerns about avian influenza in recent years.
Thorough cooking addresses this — heat is effective against
the virus, as it is against most bacteria of concern in foraged food.

A cautious approach: blanch mushrooms briefly in hot water first,
then finish cooking properly — in a soup, stew, stir-fry, or
roasted in the oven. The blanch-then-cook sequence gives
extra peace of mind without much extra effort.

A mushroom that still feels firm and resists slightly when pressed
is in better condition than one that feels squishy or damp
in a way that isn't just from rain. Softness, sliminess beyond
what's normal for the species, or a strong off smell
are all signs to leave it where it is.