Am growing Orchids outside and just before the flower opens something bites the base of the bud and it falls?

Check around the plants base for holes. I grow several species of orchids and have found that both gophers and moles LOVE to snack on them! If they so much as sink their little teeth into the plant from bulb to bud, booHOO the flower will drop.

If you do find holes, buy some poison peanuts (Home Depot) to put in the holes and they will munch on those instead, much to their demise.

But, there are other insects such as caterpillars and grasshopers that find orchids tasty, too. You might want to protect the plants with insecticide around the base.

If it is an inside plant, make certain that it is growing in orchid mix, also available at the home depot. Although I successfully grow several species of orchids outside in outrageously poor soil, they would perish inside if I don’t use orchid mix and fertilize.

check the tree for bugs and treat the trees

might be a squirrel

This is called "bud blast" and it’s not from an insect. Heres ssome information. I hope this explains it.

http://www.wickfordorchids.com/orchid_bud-blast.htm

The same thing happened to me when I started growing orchids, I sent for an expensive plant, and all the buds started falling off before they ever opened. At first I thought some children were picking them off, but as it turned out it was a fairly common problem.

Must be one hungry bug.