Beetles in the family Chrysomelidae are commonly called leaf beetles. It is the largest of beetle families among the phytophagous (plant-eating) beetles; chrysomelids are second in number of species only to the weevil, family Curculionidae. There are as many as 35,000 described species and perhaps up to 60,000 total species. Presently, the Chrysomelidae are classified in 195 genera and approximately 1,720 valid species and subspecies (plus 149 Bruchinae species) accepted as occurring in North America north of Mexico. [1]

Leaf beetles feed strictly on plant materials. The adults usually consume leaves, stems, flowers, and pollen. Most larvae are subterranean in habit, feeding on roots and rootlets, but others will consume foliage as well. Many chrysomelids are very specific to particular host plants, but most are able to live on a variety of plants; i.e. the so-called dogbane leaf beetle, Chrysochus auratus, which feeds on prairie plants such as milkweed (Asclepias sp.) and plants in the dogbane genus Apocynum. [2]

"Plant chemical defenses can be eaten by herbivores, stored, and used in defense against predators. To be effective defensive agents, the sequestered chemicals cannot be metabolized into inactive products. Utilizing plant chemicals can be costly to herbivores because it often requires specialized handling, storage, and modification (Bowers 1992). This cost can be seen when plants that utilize plant chemicals are compared to those plants that do not in a situation where herbivores are excluded.

Caterpillar and adult monarch butterflies store cardiac glycosides from milkweed, making these organisms distasteful. After eating a monarch caterpillar or butterfly, its bird predator will vomit and will avoid eating similar individuals in the future (Huheey 1984). Species that feed on milkweeds are usually aposematically colored. Aposematic species are those that advertise their distastefulness by being brightly colored (see Guilford 1990). Two different species of milkweed bug in the family Hemiptera, Lygaeus kalmii and Oncopeltus fasciatus, are thus colored, with bright orange and black markings."
From The Wikipedia "Herbivore adaptations to plant defense."

The orange and black aposematic colors are part of the "milkweed mimicry" complex which includes Monarch, Viceroy and soldier butterflies, and the large and small milkweed bugs among many others. All of these creatures are intimately associated with the milkweed plant, in that adults and larvae feed on the plant, or they mimic insects which do.

Small Milkweed Bug Lygaeus kalmii

Large Milkweed Bug Oncopeltus fasciatus

Viceroy Butterfly Limenitis archippus

Monarch Butterfly Danaus plexippus

Soldier Butterfly Danaus eresimus