WBA members visited San Juan Island for a two day field trip and were treated to educational, observational and experiential adventures on May 31 & June 1. The Island Marble Butterfly, Euchloe ausonides insulanus, is currently known to live only on San Juan Island. It was placed on the Endangered Species List in 2020. Known host plants include the common Field Mustard, Brassica rapa, Tumble Mustard, Sisymbrium altissimum, and the native Menzies’ Pepperweed, Lepidium virginicum menziesii. All three grow on San Juan Island.

The intention of our visit was to learn more about the Island Marble Butterfly (IMB) and the concerted effort being made for its conservation. We met with Claire Crawbuck, biologist at San Juan Island National Historical Park, on the first day of our visit for a tour of the small Island Marble Butterfly Rearing Lab at American Camp. To prepare us for the tour, she first discussed in detail the life history of the IMB and shared photos. Then in the tour of the lab we were able to observe the five instars, chrysalises and the minuscule eggs which turn orangish-red color prior to hatching. Initially it’s coloring blends perfectly with the mustard flower buds, and many of us were unable to see it even when she pointed it out! The egg is laid on the flower buds where the first instar with its small mouth is able to eat the tender flower petals. Through the progression of instars the larva moves down the plant, eating the leaves and stems. Claire noted that the IMB remains in the chrysalis stage for about 10 months. It is called the Island Marble Chrysalis by some of those working for its conservation. She then took us to a location where the IMB was flying in good numbers. The small, sandy, south facing hillside by the beach was covered in mustard. Initially we saw very few butterflies flying. As time passed more were seen and our excitement grew.

Cabbage Whites were also flying which made ID a little tricky. We also saw Western Tailed Blue and Milbert’s Tortoiseshell. Young Hill in English Camp was our last stop of the day. A probable Propertius Duskywing was seen in the Garry Oaks. A few butterflies were seen along the trail in addition to the larva of the Garden Tiger Moth, Arctia caja…the first moth raised by David James at age 8! Of note was an apparent Tent Caterpillar irruption resulting in thousands around the island and particularly abundant in this area.

Day 2 began with walks on trails around the Cattle Point Lighthouse. A few butterflies were flying, but the big excitement were two foxes. A black one with white frosting and a white tipped tail seemed pretty comfortable with all of us, at a respectable distance, taking photos and viewing with binoculars. As we watched, he then curled up and slept for a bit. The second was light orange walking along a fence hunting for lunch in the grasses below. Apparently the fox population has become quite large and attracts photographers from all around. We ventured next to Fourth of July Beach where we had been told the IMB was flying. This was a bonus for everyone after seeing them for the first time the day before. We were also surprised to have Claire and the intern Sauni arrive to release two IMB. They had hoped an eclosure would occur the prior day for us to observe, but this was even better. Menzies’ Peppergrass, Lepidium virginicum menziesii, was the native host plant on which they were released at this location.

After lunching on the beach we left for our planned volunteer time, bringing along gardening gloves as directed. We were met by Kathleen Foley, Special Projects Director for the San Juan Preservation Trust (SJPT), and Sarah Hanson and Walt Andrews of On Sacred Ground Land Trust. Sarah and Walt gave a zoom presentation to our group earlier this year. We were led to SJPT’s Phelps Preserve by Kathleen and Walt to weed an area fenced off from deer for future IMB habitat. Kathleen shared the good news that on an inland preserve closer to American Camp, two of the butterflies found their way to the habitat that had been prepared for the species earlier. Our group did a bit of sweating as we worked for several hours clearing ground around the annual mustard plants to allow for next year’s seeds to germinate. Some of us kneeled at plant bases, some used hoes and shovels, and some sat to weed…a great crew. After all we had been given over our two day visit, performing service for the conservation of the IMB felt rewarding. One anecdote by Kathleen was that the butterfly seems to use Field Chickweed, Cerastium arvense, for mating which she humorously called the ‘bedroom’ plant, and Tumble Mustard for egg-laying which she called the ‘nursery’.

After our weeding, we all dispersed for our final evening on the island. Ferry rides awaited us the next day. In addition to beautiful scenery and butterflies, we had been delighted with sightings of foxes, seals, sea lions, deer, orcas, birds, crabs and sea life during our stay. We departed feeling fortunate for the excellent weather, and the rich experiences we shared in learning about the Island Marble Butterfly.

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