Many a gardener knows that milkweeds (Asclepias) are host plants for Monarch butterflies. This means that adults lay their eggs in the foliage of these plants, the caterpillars hatch, start feasting on the leaves to grow and fatten, and then at some point spin themselves a chrysalis from which eventually emerges an adult butterfly. What may not be as well known are the factors underlying these instinctual decisions. Each species of butterfly has one or more host plant that they choose over all others on which to lay their eggs and begin this cycle.

These choices have to do with the specific botanical nature of the host plants, their location and when they flower, especially if this involves migration. In the case of Monarchs, an important reason for choosing Asclepias as their host plant is that the leaves contain a milky sap, consisting of a latex containing toxic alkaloids. The caterpillars are unharmed, but any predator eating them gets a nasty surprise. Monarch caterpillars advertise this toxicity by being brightly colored.

Gardeners can do their part in helping the local butterfly population by planting a number of common and commercially available plants in their garden. Though you may choose a particular plant as a host plant, rest assured that other species of butterflies will likely come calling for the nectar these flowers provide.

It is advised to use either of the two native milkweeds (A. fascicularis and A. speciosa) to avoid potential issues with parasites. (See article online here: http://bit.ly/2mtAu6o)

Earl Nickel is an Oakland nurseryman and freelance writer. Email: home@sfchronicle.com

Butterfly summit

Annie’s Annuals & Perennials hosts a Butterfly Summit, 10 a.m.-3 p.m. on Saturday, March 25. Among the guest speakers will be Mia Monroe, a Xerces Society volunteer and coordinator of the Western Monarch Thanksgiving Count; and Tora Rocha, founder of the Pollinator Posse at the Gardens at Lake Merritt in Oakland. For information: anniesannuals.com.

Four more host plants

Eriogonum (California Buckwheat) is one of the best food sources for local pollinators, but it is also an invaluable host plant for several species of butterflies — blues (Polyommatinae), coppers (Lycaeninae) and hairstreaks (Theclinae). Eriogonums are widespread throughout California, found especially in dry hilly regions of the state. The flowers, leaves and seeds are also used by a variety of birds, including finches, juncos, larks, sparrows and towhees.

Passion flower vines play an important role in the local ecology. Species such as Passiflora caerulea and P. loefgrenii x caerulea are active host plants for the pretty Gulf Fritillary (Agraulis vanillae). One advantage of adding this to your garden is that plants typically produce a wealth of foliage, meaning that even a good number of caterpillars will barely make a dent in the leaf mass.

People love snapdragons, but did you know that Antirrhinum majus is one of the host plants for the Common Buckeye (Junonia coenia)? Other host plants include Linaria (Toadflax), Plantago (plantains) and Acanthus mollis (Bear’s Breech).

Lupines are a popular spring and early summer plant, and native species are often found in great abundance in local alpine meadows. Outstanding native bush species such as Lupinus propinquus and Lupinus albifrons are host plants for certain blues (such as the Mission Blue for L. albifrons), hairstreaks and the Northern Cloudy-Wing (Thorybes pylades). They often self-seed vigorously and are drought tolerant.