I have been using the Tamron 90mm on the 10D for the last year and find it too short for insect work.
Extension tubes won't give you that longer focal disatnce, they will shorten the focal distance to enable you to get closer to your subject.
I would opt for the
Tamron 180 (reported to be sharper than the canon version)
Warehouseexpress review,
or the new
Sigma 150mm.
I have seen images produced from both lenses and the results are outstanding.