The plague of locusts acts as the final blow to an already devastated Egypt, finishing the ruin started by the hail. A massive, unprecedented swarm of insects is set to descend, entirely transforming the landscape and crushing whatever fragile hope the Egyptians still hold.
The sheer scale of the invasion will alter the very appearance of the ground itself. The primary approach among commentators is that the landscape will be physically transformed by the crawling masses [רש״י, קאסוטו, ביאור יש״ר]. Others suggest a more atmospheric change, where the swarm arrives as a heavy, suffocating cloud that blocks the sun, severing the light from the ground and plunging the country into darkness [מזרחי, שפתי חכמים, מלבי״ם, גור אריה]. Taking a metaphorical view, some suggest the impact targets the glory and wealth of Egypt, or even the ability of Pharaoh's officials to govern, as they will be unable to oversee the kingdom through the living wall of insects [העמק דבר]. A unique perspective suggests that the true target is the human eye itself. The locusts will act as a barrier, preventing the outside world from reaching and being absorbed by the human soul [רש ר הירש].
The density of the swarm raises the question of who exactly will be blinded to the ground. Most commentators agree that this simply refers to any human observer. The insects will blanket the earth so thickly that no person will be able to catch even a glimpse of the soil below [אבן עזרא, רשב״ם, רש״י, דברי דוד]. However, a surprising alternative suggests that the locusts themselves are the ones who will be unable to see. Plunged into darkness or pressed tightly together in the massive swarm, the insects will eat blindly. Much like a blind person who eats without ever feeling satisfied, the locusts will consume the crops in a never ending frenzy, driven by an insatiable hunger that will push them directly into people's homes [כלי יקר, אלשיך, חומת אנך].
This relentless swarm is destined to ruin and consume whatever meager vegetation survived the previous disasters [ספורנו, שד״ל]. This includes the late ripening crops, such as wheat and spelt [מלבי״ם, ביאור יש״ר]. The survival of these specific plants during the hail was no accident. God intentionally spared them solely to provide food for the locusts. By allowing the Egyptians to briefly place their hope in this remaining property, God ensures that their pride will be utterly broken when it is finally snatched away [מלבי״ם, אלשיך, רש ר הירש].
Beyond the field crops, the swarm will strip the trees bare. Since the hail had already shattered the trees, it might seem there would be nothing left for the locusts to eat. However, a month separates the two plagues. During that brief window, the broken trees had just begun to heal, sprouting fresh blossoms, tender buds, and thin new branches. It is exactly this soft, young, and defenseless growth that will now be completely annihilated by the jaws of the locusts [רבנו בחיי, העמק דבר, חזקוני].