Butterflies, and moths as well for that matter, are very particular about laying their eggs. Most butterflies leave them on only one sort of plant because this is the plant that the newly hatched larvae, called caterpillars, will feed on. So the female butterfly will spend some time finding the correct plant (the foodplant) before laying her eggs. You might think that for an insect this would be particular enough, but most butterflies even choose a particular place on the food plant for the eggs and will leave them in no other. For example, one species of brimstone butterfly will only lay eggs on the topmost shoots of the buckthorn bush. You would never find them if you searched lower down the plant.