Animals and Insects
The first of the Grapes of Wrath symbols from the wild is the turtle, who appears in the "intercalary" chapters early in the novel -- those that interrupt the Joad narrative with its progress. The turtle is headed somewhere, and won't change direction, even though cars menace it. Eventually, it ends up in Tom Joad's coat.
This turtle represents the persistence of the migrants, who refuse to give up even after their farms have been taken away by the banks, their money has been cheated from them by dirty car salesmen and other business owners, and their family members have sickened and died.
The Joad's dog dies in such a grotesque way that one can't help but see it as an omen for the distortion and destruction of the Joad family through the rest of the novel. Not only will many of the family members simply pass away, but those who do remain will have their lives twisted irrevocably.
The grasshoppers that block out the sun, in the novel's reference, and destroy crops, bring to mind the Old Testament curse called down on the Egyptians, involving the locusts. The idea here, as mentioned above, is that nature can indeed be cruel, especially if there is no human intervention to help the dispossessed.