Butterfly Bliss: California Tortoiseshells in Tahoe
LAKE TAHOE, Nev./Calif. – Have you noticed the butterflies fluttering across Tahoe this summer?
You’re not alone. Locals and visitors alike have been captivated by this noticeable butterfly bloom. From hiking trails and roadsides to mountain peaks, these vibrant insects seem to be fluttering everywhere this time of year. While this summer’s display is not quite the massive explosion we experienced in 2019, there are certainly plenty of California Tortoiseshells filling the landscape and catching people’s attention.
These orange and brown California Tortoiseshells (Nymphalis californica) are often mistaken for Monarchs or Painted Ladies, but this year these “torties” are once again taking center stage. This fascinating, multigenerational, Ceanothus-eating insect species has an interesting life cycle, and researchers are still learning what drives these dramatic population surges.
Meet the Torties
The California Tortoiseshell is a native butterfly and a familiar sight in the Sierra Nevada and throughout much of the western United States. The upper side of its wings displays bright orange and brown patterns that are especially noticeable in flight. In contrast, the underside is mostly dark, decorated with blotchy browns that resemble a dead leaf when the butterfly folds its wings, providing excellent camouflage while resting.
Observers may find adult butterflies pollinating and drinking nectar from a diversity of flowers and shrubs, but can also be found gathering around mud or nutrient rich soils near a water’s edge. These gatherings, known as puddling, occur when butterflies unroll their straw-like mouthparts (proboscis) to extract salts and other minerals that are less abundant in their primary adult food source: flower nectar. This behavior is thought to help compensate for limited access to micronutrients in the adult diet, which are important for nervous system function, growth, and reproduction.
This species often migrates topographically, moving up and down in elevation throughout its life cycle. Local individuals begin their lives in valleys or foothills and travel upslope into Tahoe and other areas of the Sierra Nevada to lay their eggs. To complete this journey, adults seek out specific host plants within the genus Ceanothus, also known as California Lilacs to lay their eggs.
At lower elevations, California Tortoiseshell caterpillar (larvae) feed on the Ceanothus species found in those habitats. Here in Tahoe, however, they rely on our high-elevation Ceanothus plants, including Mountain Whitethorn (Ceanothus cordulatus), Mahala Mat (Ceanothus prostratus), and Tobacco Brush (Ceanothus velutinus). These native shrubs provide the essential food source for developing caterpillars and support the next generation of butterflies.
When the eggs hatch, the caterpillars feed on the host plants, rapidly growing until the larvae form a chrysalis (pupa). Once they have undergone a complete metamorphosis in their pupa stage, they emerge as adults. As late summer occurs, these winged insects then head up to mountain peaks, often above the treeline to estivate, or enter a state of inactivity where they live off their fat reserves. High-elevation estivation allows adults to avoid harsh, hot summer conditions while conserving energy before seasonal movements resume.
A Mountain Traveler With a Complex Life Cycle
The yearly cycle of Tahoe’s California Tortoiseshells is complex and is broken into three generations of butterflies, comprising two different flights, over the span of one calendar year.
The first generation of butterflies was hatched in the previous year at a high elevation, such as Tahoe, in mid to late summer. They migrated to a lower elevation in the foothills or valleys, hibernated as adults over the winter, awoke from their slumber, found mates, and laid eggs on larval host plants. Once that duty was accomplished, this generation of adults had completed its life cycle.
The next generation lives a much shorter life cycle than the first. These butterflies hatch at lower elevations, but many drift upslope and are the ones we first observed in Tahoe after snowmelt. Once they find suitable host plants, they lay their eggs and complete their life cycle, all within one summer season.
Now, the butterflies we are seeing in such numbers today are the third cohort of the year. These individuals have fed on Ceanothus here in Tahoe. They will not lay eggs for months, and many will eventually head back down in elevation to hibernate over the winter months until the cycle begins again once they emerge from hibernation in spring.
But Why So Many Tortoiseshells?
The population dynamics and explosions of torties are still being researched and one of the main theories is the butterfly’s response to healthy host plants.
We have seen true explosions in the past, most recently in 2019. This year is just a bit above average. If a large quantity of food is available, insects respond to the surplus with efficiency and large reproductive output. If you find yourself on trails throughout the Basin and Truckee, you’ll discover that Ceanothus plants are everywhere. Despite the last few years of irregular winter and spring precipitation, it appears we have healthy and prevalent Ceanothus plants, which in turn have created a lot of nutritious food for tortoiseshell caterpillars.
Additionally, Ceanothus, like Mountain Whitethorn, responds well after a fire and in disturbed areas, including landscapes affected by vegetation management and mastication, providing an important food source for these caterpillars. Mountain Whitethorn has taken over the entire hillside after the Angora Fire scar and other compromised areas in Tahoe, growing chest high in spots and leaving very little space for other plants to grow. The summer generation of butterflies that came to Tahoe found these areas as a copious food source and produced millions of offspring from those patches alone.
These butterflies can produce a lot of offspring which in turn, may defoliate leaves at alarming rates. This may prove beneficial, as caterpillar defoliation should allow space for young trees to take hold and reforest the burn areas mentioned earlier. Although dramatic, temporary defoliation events are a natural part of forest ecosystems and do not typically kill healthy established Ceanothus plants. Instead, they can contribute to nutrient cycling and influence plant competition.
Enjoy the Show While it Lasts
The majority of these butterflies will linger for a few weeks before either finding a summer estivation spot locally or heading back down to lower elevations to overwinter and hibernate as adults.
Come spring, the California Tortoiseshell cycle will start once again. In the meantime, these butterflies are an important food source for hungry migrating and local birds as well as small mammals.
Be sure to get outside and enjoy the spectacle of butterflies over the next few weeks!
Sarah Hockensmith is the outreach director with Tahoe Institute for Natural Science. TINS seeks to advance the natural history, conservation, and ecosystem knowledge of the Tahoe region through science, education, and outreach. If you would like to learn more about Tahoe’s natural history, join TINS on one of its many free nature tours. To learn more, visit “http://www.tinsweb.org.
