Explaining the explosion of butterflies at Lake Tahoe | TahoeDailyTribune.com

Explaining the explosion of butterflies at Lake Tahoe

Sarah Hockensmith
Special to the Tribune
A California Tortoiseshell rests on a Mountain Pennyoyal Flower.
Dr. Will Richardson / Provided

Have you noticed the swarms of butterflies fluttering around Tahoe?

You are not alone, as locals and tourists alike have been questioning the massive butterfly explosion (especially as they keep running into the windshield of your car).

Over the past few weeks, the Tahoe Institute for Natural Science has been bombarded with questions about this phenomenon. Here is our best explanation of this Tahoe spectacle.

What type of Butterfly is it?

The California Tortoiseshell (Nymphalis californica) is a native orange and black butterfly that migrates up and down in elevation as it searches for the perfect plant to lay its eggs. Adult butterflies lay their eggs on plants in the genus Ceanothus, also known as the California Lilacs.

In the region, there are a few species of plants within this genus, including Mountain Whitethorn, Mahala Mat, Tobacco Brush, and at lower elevations, Deerbrush.

Once the eggs hatch, the caterpillars feed on the host plants, rapidly growing until the larvae are ready to form a chrysalis. During this stage they transform into adults, a butterfly.

Although we see California Tortoishells each year, this summer has experienced the largest population explosion within the past few decades.

What are they doing here?

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. Most of these partake in an elevational migration during this cycle.

The first generation of butterflies were hatched in the previous year at a high elevation (such as Tahoe) in mid- to late summer, migrated to a lower elevation (such as Nevada City), 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 has completed their life cycle.

The next generation of butterflies live 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 around June. Once they find suitable host plants, they lay their eggs and complete their life cycle, all within one summer season.

Now, the butterflies that we are seeing today in massive numbers, are the third cohort of butterflies this year, which have fed on Ceanothus here in Tahoe. These butterflies 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.

Why are there so many?

Population dynamics and migratory patterns of California Tortoiseshells are poorly understood, however, one of the main theories is the butterfly response to healthy host plants.

If a large quantity of food is available, insects respond to the surplus with efficiency and large reproductive output. Over the past few winters, Lake Tahoe has accumulated a lot of precipitation, leading to very happy and healthy Ceanothus plants, which in turn has created a lot of nutritious food for California Tortoiseshell caterpillars.

Additionally, Ceanothus like the Mountain Whitethorn respond well after a fire. Mountain Whitethorn has taken over the entire hillside of the Angora Fire scar, 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 this location as a copious food source and produced millions of offspring from that patch alone.

This may prove beneficial, as caterpillar defoliation should allow space for young trees to take hold and reforest the area.

What now?

The majority of these butterflies will relax for a few weeks before either finding a hibernation 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 and small mammals.

Be sure to get outside and enjoy the spectacle of millions of butterflies over the next few weeks.

Sarah Hockensmith is the outreach director with Tahoe Institute for Natural Science. TINS seek 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.

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