Grub/ɡrʌb/ Definition 1 of 3Pro Tip 1/3Lifecycle Stage The term 'grub' refers specifically to an immature insect, not fully grown. After the grub matures, it turns into a beetle. Pro Tip 2/3Visual Identification 'Grub' often describes white, worm-like insects found in soil or decaying wood. You'll recognize the grubs by their soft, plump bodies. Pro Tip 3/3Gardening Context Grubs are commonly mentioned in gardening as pests or bait for fishing. He bought grubs at the bait shop for his fishing trip.