Rashi on Exodus
בארבה means on account of the plague of locusts (the prefix ב signifies “on account of”).
Sforno on Exodus
על ארץ מצרים בארבה. A reference to the region from which the locusts generally came, i.e from the southern part of the continent. We have references of this kind in Isaiah 5,26 describing that these locusts traveled long distances before arriving in Egypt.
Or HaChaim on Exodus
בארבה, with locusts. The word may either refer to a single locust which stuck to Moses' staff, or he may have articulated the word ארבה as he inclined his staff as a sign that the reason he inclined his staff was in order to initiate the plague of locusts.