The answer is <span>Determine why the beetle is eating one tree but not the other.
The botanist should first d</span><span>etermine why the beetle is eating one tree but not the other. By determining that, he will understand what connects beetle to the tree. This will also explain does the tree benefits from this interaction or not.</span>
It allows the scientist to determine the effect of the changed variable , it also allows samples and organisms to be tested under comfortable conditions