The icon for "Butterfly Plants" includes two kinds, both those that are host plants for specific butterfly species and those that are attrctive to a wide range of butterflies.
In the first case, the icon indicates those species that are required, or obligate, host plants, or nurseries, for certain species of butterflies. This ancient relationship between butterflies and plants goes like this: the adult butterfly lays eggs on the host plant. When the eggs hatch into caterpillars, they find on this plant the right nutrients or other chemical substances that they need to go through their different larval stages and eventually turn into a chrysalis. The chrysalis eventually becomes the adult butterfly that we all enjoy in our gardens and in the wild.
This plant may provide them with certain toxins that help protect them from predators, as is the case with the monarch and the milkweed. It may protect them or shade the caterpillars in just the right way. Without these host plants, that particular butterfly cannot reproduce and survive. This is a very specific kind of co-evolution. Buckwheats (Eriogonum species) for example, often are host plants for certain butterflies. In LA, the El Segundo Dunes contain coast buckwheat, Eriogonum parvifolium, which is the host plant for the El Segundo Blue Butterfly. When Eriogonum fasciculatum was mistakenly planted, it fostered other species that outcompeted or parasitizd the El Segundo Blue.
Many LA beaches were home to a specific butterfly/host plant relationship that disappeared when invasive species, development, or other factors eliminated the host plant.
This icon refers to plants that are part of this particular insect/plant relationship enabling reproduction. Many of them also provide nutritious nectar or places to lay eggs for a number of species as well.