First instar caterpillars are very small! | Staff archive

On Friday, June 10, the Preservation Trust’s Conservation Project Manager, Kathleen Foley Lewis, was out at the Island Marble Butterfly Habitat Expansion patch on Frazer Homestead Preserve on San Juan Island for a routine check for signs of this rarest of all butterflies. Examining each field-mustard plant closely with a magnifying loupe, she spotted a tiny orange butterfly egg. Continuing her painstaking search, she eventually found a total of 16 butterfly eggs hatched on some of the mustard plants that were planted at the Frazer Preserve (owned by the San Juan County Conservation Land Bank) this past spring.

Kathleen on the hunt

What great news!

As of Monday the following week, five of these 16 eggs had hatched into their first instars (larval stages). While no adult butterflies were seen flitting about the patch, the presence of eggs, then caterpillars, indicates that a female had visited—furthering our excitement about the expansion of this rare butterfly’s range northward and out of their last stronghold at American Camp.

Stay tuned for further news as these very hungry little caterpillars munch their way—almost totally consuming the Brassica rapa plant they were born on—to becoming bigger caterpillars, then eventually overwintering in chrysalis form and emerging next spring as adult butterflies. We’re doing our best to help perpetuate this endangered species, found only on San Juan Island.

Two weeks later …

Island Marble larva in their fourth (or early fifth) instar stage; staff photo taken on June 30

Oh, how these Island Marble Butterfly caterpillars have grown!

As of the end of June at the Frazer Homestead Preserve, nine IMB caterpillars were counted in their late fourth (or early fifth) instar stage, which means they will be traveling soon to pupate—that is, form a chrysalis from which they will (we hope!) emerge next spring as adult butterflies.

Only a few weeks ago, they were hatching from their eggs and emerging into the world in their first instar (see above).

We are so proud and happy about their progress. This is remarkable: a 60-percent survival rate to this stage is very high, and shows how the simple act of fencing an area off (and careful management around pupation sites) can increase survivability of these larvae.

Many thanks to our partners, the San Juan County Conservation Land Bank, for providing the space at the Frazer Homestead Preserve on San Juan Island for us to create this safe place for these extremely rare butterflies to complete their life cycle. And kudos to our own Kathleen Foley Lewis, who heads the project.