In the Great Basin and Rocky Mountains, the key to finding monarch caterpillars on showy milkweed in their habitat is to look on younger, healthier milkweeds growing either in A) an agricultural canal B) on milkweeds taking refuge under Russian Olive Trees in a wet meadow agricultural situation or C) on small pockets of showy milkweed growing in canyons.  Younger monarch caterpillar feed on the ventral side of the milkweed where their feeding damage is unique as compared to milkweed bugs in that they burrow a hole in the center of the leaf.