is there any evidence of an introduced species program being successful without

i believe that there are a few.
but which ones i don't remember.

a fungus that infects a pest insect, and
a small (tiny) hornet that parasitises a pest species come to mind.

now that we / scientists understand the problem of unintended consequences better, more testing is done.
 
Whenever an invasive species is introduced, there can be thousands of unintended consequences. It's impossible to know them all. Every new inhabitant effects all of its neighbors in some way. If something takes up space in an environment, it is replacing something else that was in that spot. There is also the issue of "unintended" consequences vs. "unexpected" consequences. And finally, you should specify if you mean harmful consequences or positive consequences. But what i suspect you're referring to is something like sweet corn being brought to Europe. Whether any of its consequences were harmful.... that's up for speculation.
 
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