When to apply tomato dust
 

I've just bought Yates tomato dust so that I can at least save some of
my tomatoes from the grubs. I would prefer not have to use it at all but
at this stage I don't have netting setup.

The tomato dust protects against a whole range of things, but I'm only
interested in protecting against the grubs that get into the fruit. Does
anyone know when I should start applying the dust and how frequently?
The directions say every 7-10 days from shortly after they are
seedlings, but this is clearly too early for what I need.

Thanks

Lionel.