Nature is a constant source of surprising partnerships, and this animal and plant pairing is a great example. Throughout its life cycle in San Diego’s riparian habitats, the western tiger swallowtail (Papilio rutulus) is intricately linked to the arroyo willow (Salix lasiolepis).

Because they only live for 10 to 14 days, western tiger swallowtails have much to do as adults! After feasting on nectar from a variety of sources, the swallowtails mate, after which the females lay shiny dark green eggs on arroyo willow leaves in the canopy. Being up high in the dangling reaches of a willow tree provides a safe spot to hatch as a caterpillar, which feeds on leaves during the night. During the day, a silken hammock anchored to a leaf provides a safe and much needed resting spot. 

Before the transition into a butterfly, the larvae molt four times. Larvae start out life looking like a bird dropping, but eventually transform into a plump green caterpillar with two faux eyes and a long yellow line along its back. In the spring, when the arroyo willow has yellow catkins, butterflies emerge with flamboyance. Their four-inch wingspan with black and yellow tiger-esque stripes easily grabs the attention of anyone nearby. One way to find the males is to look for mud puddles where they gather, sipping salts and minerals. 

Riparian areas are resource-filled habitats for birds, insects, and butterflies, like the western tiger swallowtail, whose life cycle begins and ends near water. Support and grow San Diego’s riparian habitat if possible in your garden, as this habitat has experienced much loss and increased sensitivity with urban development. 

Additional Animals

Lorquin’s Admiral (Limenitis lorquini)
The colorful Lorquin’s admiral butterfly also uses the arroyo willow throughout its lifecycle, including the overwintering phase when the caterpillar shelters in a rolled-up leaf. 

Southwestern Willow Flycatcher (Empidonax traillii extimus)
A state and federally listed endangered bird, the southwestern willow flycatcher requires the dense riparian thickets where arroyo willows reside for both nesting and foraging needs.

Arroyo Southwestern Toad (Bufo californicus)
This tiny toad requires shallow slow-moving water in riparian areas to breed, which arroyo willows aid in creating by slowing down the rate of flow due to their thirsty and rapidly growing root system.

Additional Plants