Host Plant Specificity: Why Most Caterpillars Starve on the Wrong Leaves
The single biggest myth in casual caterpillar-keeping is the 'general leaf-eater' — most species are obligate specialists on one plant family and will literally starve in front of the wrong food.
Roughly 95% of caterpillar species are host plant specialists, evolved to digest one plant family and often unable to recognize or process anything else. A caterpillar on the wrong host plant wanders, refuses food, thins out, and dies within 1-3 days. This is the most common cause of death among well-meaning hobbyist rearers and the central reason iNaturalist and the Amateur Entomologists' Society both lead with "identify the host plant first." Specialist examples are diagnostic: milkweed hosts Monarch larvae; parsley, dill, fennel, and carrot tops host the Black Swallowtail; nettles host Red Admiral, Peacock, and Small Tortoiseshell across Europe; cabbage, kale, and nasturtium host the Cabbage White; spicebush and sassafras host the Spicebush Swallowtail. Grass-feeders exist — some skippers and a few moth families including Noctua pronuba (Large Yellow Underwing) — but they are the minority of what people find. Polyphagous species are the exception. Most noctuid moths (Noctuidae) eat a wide range of low herbaceous plants: grasses, dandelion, plantain, dock, clover, brassicas, and beets. For a noctuid larva, a buffet approach works — offer several candidate plants and watch which one shows bite marks the next morning. The practical workflow: note the exact plant the caterpillar was on when found, cut several stems of that plant, refresh every 1-2 days, and wash leaves to remove possible pesticide residue. Supermarket parsley and dill are routinely sprayed; even "organic" labels are not always reliable. Caterpillars killed by pesticide residue on store-bought greens is a documented hobbyist failure mode.