By Lorelle Sherman, OSU Extension Forester

Imagine driving in the Cascades through a sea of orange butterfly wings. Hikers, backpackers, and folks living in Central Oregon have experienced an exceptionally large emergence – or irruption – of California tortoiseshell (Nymphalis californica) butterflies this summer. Although they look similar to the monarch (Danaus plexippus) butterfly, California tortoiseshells lack the famous ‘stained glass’ wing pattern and spotted body of monarchs.

Life Cycle on Ceanothus
All butterflies share similar life cycles where the young (also called larva or caterpillar) look very different from the adults. Butterflies go through complete metamorphosis through four stages starting with an egg, then larva, pupa, and adult. The illustration below starts with the very small eggs laid by the adult female butterfly. These eggs hatch into larva, which are called caterpillars when pertaining to butterflies and moths. Number one on the to do list of a caterpillar is to eat, eat, eat! Caterpillars grow rapidly, shedding their skin as they fill out their exoskeleton. Once they are fully grown, they attach themselves to and hang from the underside of a leaf or branch, eventually turning into a pupa. Inside the pupa is where the magic happens. Special cells in the larva allow it to transform into an adult butterfly, whose job is to mate and lay eggs, restarting the cycle.

The California tortoiseshell is strongly associated with plants in the Ceanothus genus, also known by many common names including: buckbrush, snowbrush, California lilac, and blue blossom. The adult lays its eggs on Ceanothus species, because this is the preferred food source of its caterpillars. Once hatched, the caterpillars can and will entirely defoliate large swaths of Ceanothus, especially in irruption years. If you’ve ever hiked in the Central Oregon Cascades, you know there is plenty of Ceanothus to go around. In the case of the California tortoiseshell, caterpillars will attach themselves under small branches of surrounding conifers or really anything that shelters them (Nymphalis chrysalis 2021.jpg).

Parasitoid Pressure
Parasitoid wasps are predatory insects who sometimes lay their eggs inside the chrysalid of a still forming butterfly. When the wasp eggs hatch, they feed on the pupa and eventually hatch out of the chrysalid. While this may seem gruesome, these native wasps are an integral part of the ecosystem. Parasitoid wasps are usually very small, but I happened to catch a larger species injecting its eggs into a California tortoiseshell chrysalid in Central Oregon. Irruption years likely help butterflies overcome predatory pressures like this one.

A Mountain Migration
While Monarch butterflies migrate thousands of miles each year to and from Mexico, the California tortoiseshell remains in the California and Pacific Northwest mountain ranges year round. Some individuals expand northward from California and Southwest Oregon in the spring, returning to warmer climates in the fall. Other individuals only move up in elevation in the spring and down in elevation in the fall to hibernate in the Oregon Cascades. The size and success of migration depends on environmental conditions and can range from a trickle of butterflies throughout the summer to a flood of butterflies which has, in past years, temporarily shut down mountain roads.

Summer adults can emerge in massive numbers during irruption years. Researchers don’t fully understand why irruption years happen when they do. Sometimes irruptions happen several years in a row and sometimes they happen with many years in between. What we do know is that several irruption years in a row leads to complete defoliation of the butterfly’s food source subsequently resulting in a population collapse of the California tortoiseshell. The boom and bust cycle may seem risky, but it’s worked well for this species which continues to be our most common irruption species in the Pacific Northwest.

Butterflies in Decline (and what you can do)
A recent study (Edwards et al. 2025) found an overall decrease in butterfly abundance across the contiguous United States at a rate of 1.3% annually. When they looked at the Pacific Northwest alone they found stability in butterfly populations overall. However, more species are declining than species increasing and measured population increases were likely driven by the irruption years of species like the California tortoiseshell.

Why the decline? The global decline of all flying insects is likely attributed to pesticide use, habitat loss, and climate change. Insecticides are a leading cause of butterfly decline in many regions of the U.S. If you’re looking to create or enhance habitat for butterflies in your backyard, skip the insecticides and use a little elbow grease instead. Plant native plants which serve as the food plants for native butterfly larvae. Butterflies are one of few taxa that respond well to small-scale changes to the landscape… even one backyard can make a difference!

So, the next time you’re driving mountain highways, and the air seems alive with orange wings, slow down and take a moment to witness a dramatic and beautiful butterfly story.

Resources:
Butterfly host plants western Oregon:
Native Plants for Pollinators & Beneficial Insects – Martime Northwest
Native Butterfly Host Plants for the Willamette Valley (a blog post)
Butterfly host plants central/eastern Oregon:
https://www.deschuteslandtrust.org/news/blog/2022-blog-posts/butterflies-and-host-plants
Native Plants for Pollinators & Beneficial Insects – Inland Northwest

Cited Source:
Rapid butterfly declines across the United States during the 21st century

By Lorelle Sherman, OSU Extension Forester

If you’ve noticed dead twigs and small branches scattered throughout the canopy of your Oregon white oak, especially near the tips of branches, you’re seeing what’s known as branch flagging. This phenomenon is noticeably more severe in certain years and in certain locations and is currently causing alarm throughout the mid-Willamette Valley.

This story starts with an insect smaller than a grain of rice and a squirrel with a taste for larval protein.

Gall wasps and their galls

Gall wasps (family Cynipidae) are highly specialized insects associated with gall formation on oak trees. The adult gall wasp lays its eggs into the tissue of an oak twig, leaf, or bud, where the egg will hatch into a hungry larva. Larval feeding on the plant tissue triggers swelling of the plant tissue. The result is a gall — a swelling or growth made from the tree’s own tissue, which forms a protective chamber for the developing larvae. Eventually the larva will pupate into an adult which exits through the gall to start the cycle over again.

Galls can be variable in size, structurally complex, and species-specific, often serving as microhabitats for other insects. Oaks, including our Oregon white oak, are common hosts to gall wasps and other gall-forming organisms. While most galls are harmless, repeated or clustered twig galls can girdle small branches, effectively cutting off the flow of water and nutrients. Over time, this causes localized dieback — branch flagging — that’s particularly noticeable in the summer.

The species at play in our Oregon white oaks is the oak twig gall wasp (Bassettia ligni). The Oregon white oak has evolved with gall-making insects like the oak twig gall wasp and is quite adapted to this cycle of crown thinning. Branch flagging is cosmetic and oaks will rebound from it in subsequent years. However, it is unclear how changing environmental factors like hotter and more frequent droughts will interact with the oak twig gall wasp.

Western gray squirrel and Oregon white oak

Our native Western gray squirrel (Sciurus griseus) is strongly associated with oak woodlands in western Oregon. Western gray squirrel diet includes acorns, catkins, nuts, berries, aphids, fungi, insect larvae, and other protein-rich plant materials. Oregon white oak is the only acorn-producing species across much of the Willamette Valley and it has highly variable acorn production from year to year. Oregon white oaks will produce an incredibly abundant acorn crop in one year – known as a mast year – and hardly produce any for several years after. This is a major limitation on oak-associated wildlife who depend on acorns as a food source. During mast years, squirrel populations will increase due to the abundant protein-rich food source. Failure of Oregon white oak to produce acorns can have serious impacts on squirrel populations.

If you have Oregon white oak on your property, you may have noticed an incredible abundance of acorns two years ago, which means we had a mast year that may have supported an increase in squirrel populations.

All these squirrels gotta eat!

Western gray squirrels discovered long ago that certain galls house protein-rich wasp larvae. In the Willamette Valley, this is the oak twig gall wasp! To access this hidden food source, they strip bark from infested twigs, removing cambium tissue in the process. This debarking behavior can compound damage caused by the galls themselves. It increases branch mortality and may contribute to more severe branch flagging in some trees.

Should we be concerned?

Despite the visible damage, Oregon white oaks (Quercus garryana) are remarkably resilient. Branch flagging due to gall formation and squirrel foraging is a natural and cyclical process that these trees have evolved with over millennia. Whether it’s an increase in squirrel populations, or an increase in gall wasps due to a mild La Nina event last year, we can be sure of two things. First, branch flagging of Oregon White Oaks in the mid-Willamette Valley is more severe than usual this year. Second, branch flagging is an eye sore (not a sign of mortality) that can be dealt with by calling an arborist to remove dead branches if desired.

ODF resource on oak pests: https://www.oregon.gov/odf/documents/forestbenefits/oak-pests.pdf

Sources

• Sciurus griseus
• Oak Gall Wasp – PNW Pest Management Handbook
• Oregon White Oak in the Willamette Valley… some in your face issues