Not necessary, because the argument concerns efforts to control a certain whitefly that was thought to be one variety of sweet potato whitefly. This doesn’t require an opinion about “all” kinds of sweet potato whiteflies.

Not necessary, because the author never argued that any crop pest that has a parasite can always be controlled by that parasite. The argument concerns the specific efforts surrounding one particular pest and a search for parasites of that pest. Whether other pests with parasites can successfully be controlled by those parasites is not relevant.

Not necessary, because the author doesn’t argue that there’s been a change in our chances of identifying a parasite of the newly identified pest. The conclusion is just a comment about our efforts to search for a parasite to date, and does not encompass an opinion about future chances to identify a parasite.

Necessary, because if this were not true — if there are SOME parasites of the sweet potato whitefly that are also parasites of the silverleaf whitefly — then the fact we misidentified the pest as a sweet potato whitefly does not prove that our efforts have been wasted. Our efforts might still have been useful, because some parasites we might have discovered during the search could be among the parasites of silverleaf whiteflies, too.

Not necessary, because the reason the author thinks our efforts have been wasted is that we misidentified the pest. The author did not say that our efforts were wasted because we didn’t find any parasites. The issue is that whatever parasites we did find, in the author’s mind, are not going to work for the silverleaf whitefly.