Fruit flies can, on occasion, be a very annoying problem if you keep
the worm bins in your house. They will not be present all the time
nor in every house at any time but when they are present they are a
nuisance. Fruit flies aren't unsanitary, they don't bite or seek out
people to bother. They seek out over-ripe fruit and fruit pulp.
Usually, fruit flies will hover around the food source that
interests them. In high summer we have accepted having a few share
our kitchen along with the enormous spread of ripe and ripening
tomatoes atop the kitchen counter. When we're making fresh "V-7"
juice on demand throughout the day, they tend to congregate over the
juicer's discharge pail that holds a mixture of vegetable pulps. If
your worm bin contains these types of materials, fruit flies may
find it attractive.
Appelhof suggests sucking them up with a vacuum cleaner hose if
their numbers become annoying. Fruit flies are a good reason for
those of Teutonic tidiness to vermicompost in the basement or
outside the house if possible.