We don't know if you noticed, but there are lots of insects and insect-y images in this novel. When Tom Joad hitches a ride with a truck driver, a grasshopper finds its way into the truck cabin, and "Joad reached forward and crushed its hard skull-like head with his fingers, and he let it into the wind stream out the window" (2.56). This moment certainly gives the phrase, "smooshed like a bug," a new meaning.
Our narrator pays special attention to the insects that populate the farmland, and we are reminded of that Biblical story in the Book of Exodus that describes the swarm of locusts that descended upon Egyptian crops after the Pharaoh refused to free the Hebrew slaves. We also are reminded of July 26, 1931, when a swarm of grasshoppers hit the Midwest region of the United States, destroying crops and devastating farms. The swarm was so thick, that the sun was temporarily blocked (source). Because of these stories, we fear insects and we know them to be capable of ruining a farm.
However, in The Grapes of Wrath, we also notice how easy it is for humans to kill insects, and there's a violence to the way, for instance, Tom crushes the grasshopper. We begin to see similarities between the way humans treat insects and the way landowners treat tenant farmers.