Considering that the locusts had already devoured all the fruit of the trees and whatever grew in the field, why did Pharaoh hurry to call Moshe and Aharon?
Seforno: Before the locusts would devour the roots of the wheat and spelt and any other vegetation that remained from the hail.
Oznayim la'Torah #1: Pharaoh hurried to stop them from destroying the clothes and vessels which they had already begun to attack. 1
Oznayim la'Torah #2: Pharaoh hurried to call Moshe because he knew that the nature of locusts that remain a long time in one location lay myriads of eggs which would continue to plague them long after the original locusts had gone.
Oznayim la'Torah #3: It is possible that, due to the fact that the ground - and therefore how much did, or did not, remain - was darkened, Pharaoh was unaware that nothing remained; and he was asking Moshe to Daven that the locusts should not devour everything. 2
Because Pharaoh realized that Egypt was heading towards total ruin - seeing as they had lost all their sources of sustenance - first the fish and animals, and now fruit and vegetables; so it is not surprising that he quickly pleaded with Moshe and Aharon to Daven to Hashem before nothing remained.
Oznayim la'Torah: Refer to 10:14:0.1:1. This also explains why he referred to plague as 'this death' even though there was nothing green left for them to eat; and why Moshe too, hurried to Daven for their removal.
Oznayim la'Torah: And they would all die of hunger - the worst of deaths - See Eichah 4:9.
What was Pharaoh's sin against Moshe and Aharon?
Hadar Zekenim: He had expelled them from in front of him.
