Do we know what the relative moral worth of cows and insects is? No. But we can make a best guess based on the available evidence, the same way we do with any other kind of uncertainty. It seems to me like this argument is just "we can't be certain about anything, therefore we have no basis on which to choose one action over another", dressed up a little.
Do we know what the relative moral worth of cows and insects is?
You assume that this moral worth objectively exists waiting to be discovered and known.
I think it's past time for another Stupid Questions thread, so here we go.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Please respect people trying to fix any ignorance they might have, rather than mocking that ignorance.