What a the tiny worms clustering on the underside of my ripe strawberries. Only some plants seem to be affected. I grow them in a raised bed and have mulched with pine needles.

Without seeing it, it sounds like the larvae of a fruit fly or nematodes. Pick off infected fruit and dispose of, remove the pine needles and put fresh barley straw around the berries. Talk to your local garden centre about a possible controls they may have available.