Why does Pharaoh refer to the Plague of Locusts as death?
וְעַתָּה שָׂא נָא חַטָּאתִי אַךְ הַפַּעַם וְהַעְתִּירוּ לַי־הֹוָ־ה אֱלֹהֵיכֶם וְיָסֵר מֵעָלַי רַק אֶת הַמָּוֶת הַזֶּה
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Torah Temimah has an interesting comment on a different point: why does Pharaoh ask for the removal "only of this death"?
He answers based on an episode in the Gemara (Taanis 8b), when a community was threatened by two dangers, plague and famine. People were unsure which one to focus on in their prayers (as is the proper thing to do, as the Gemara goes on to point out, and as is codified in Shulchan Aruch, Orach Chaim 576:15). R' Shmuel bar Nachmani recommended that they pray to Hashem to eradicate the plague, since "when the Merciful One gives plenty, it is to the living that He provides it" - i.e., that the cessation of the famine would necessarily lead to the end of the plague too.
Pharaoh used the same logic. The Plague of Locusts, like all of the other plagues, was accompanied by pestilence (Shemos Rabbah 10:2). He was canny enough to realize that he need ask only for "this death" - the locust infestation that was causing a famine - to go away, and automatically the pestilence would disappear too. (Whereas with the other plagues there wasn't this direct relationship, since they didn't attack Egypt's food supply.)
Rav Shimshon Rafael Hirsch (as well as others) point out that this was the first time locusts had reached Egypt. If they were not eradicated before they managed to lay their eggs, it would become a permanent and recurring plague. This would be the death of Egyypt because it would destroy all future agriculture. This was not a temporary plague that would disappear once things went back to normal. The very existence of locusts, even as a normal event, would be the death of Egypt.