San Bruno Mountain Elfin Monitoring

Posted on July 29^TH, 2020 by Olivia Kurz & Johanna Harrison

Spending many hours crouched on all fours, face close to the ground, and gently examining every leaf of a tiny succulent ground cover was worth it for this amazing creature! Every other year the Natural Resource Management team does a survey of San Bruno Elfin, Callophrys mossii bayensis, an endangered species on San Bruno Mountain. Their larva, or caterpillars, feed on Sedum spathulifolium, an attractive native succulent that grows on rocky outcrops at San Bruno. The caterpillars range in color from brilliant red, to bright orange, to soft yellow with sunny chevron patterns. We look for these bright larva, reminiscent of small chubby gummy worms, in May. After that, they create their mysterious pupa, which are dormant the rest of the year until early spring. The rich brown-red adult butterflies then emerge in early spring, and only live and fly for about a week!

We survey the population of San Bruno Elfins by returning to the same designated 25-foot plots year after year to comb through all the sedum growing within that circle. We checked the plots three times over the course of May, counting each little larva that we saw. This gives us an idea of how the population is doing, and how we can further support this species surviving and thriving. The larva particularly enjoy munching on the newly emerged flowers of the Sedum, and damaged flowers as well as frass (caterpillar poop) are a sign the larva may be tucked nearby. 

An aspect that we found quite interesting was the mutualistic interaction between the larva and ants. The ants are attracted to an excretion the larva produce known as honeydew. The ants swarm the dorsal gland region on the larva to “drink” the liquid. In return for the honeydew, ants act as a body guard for the defenseless larva, protecting them from predators and parasites. Larva are considered prey to a large array of insect-eaters; ants, on the other hand, can taste bitter when eaten, so they act like a shield. 

It’s a delight to find a healthy, vibrant San Bruno Elfin larva chewing away on a beautiful sedum blossom! You can help care for this special and sensitive species by staying on paths at San Bruno Mountain, and avoiding stepping on Sedum. We are so lucky to have such rich biodiversity in our parks!