By Kate Samra

Apr 30 2020

While working as a digitization intern, one of the most pleasing features of working in the herbarium’s Cupressaceae collection is the waft of cedar, incense, and woodiness that envelops you when you open each cabinet. Warm and comforting, it can transport you deep into the understory of an ancient evergreen forest.

So, it was with alarm when we recently came across what looked to be mold on several of the dear Cupressaceae specimens. The ‘mold’ was fuzzy, white, and webbed across several plants of a particular species.

Mold and insects can be significant threats to historical archives and collections, but they are particularly difficult to manage within herbarium collections. Insect infestations particularly can spread quickly and destroy invaluable plant material. Perhaps not surprisingly, the herbarium team decided to investigate the suspicious specimens immediately.

Laura Briscoe, collections manager of the Cryptogamic Herbarium, took a specimen for a closer look and to confer with a few curators. Under the microscope, it was determined that the white webbing was not fungal hyphae, but arthropod silk. The cabinets in the herbarium that housed the specimens were checked for any sign of live insects and frass, but no evidence of either was found. So, how did this webbing get here, and why was it there?

The only species of cypress the webbing was found on was the Tecate cypress, or Hesperocyparis forbesii (a synonym of Cupressus forbesii). The Tecate cypress is native to small pockets of Southern California and Northwestern Mexico¹. This tree can grow up to 30 feet high, has fragrant, peeling bark, and scale-like evergreen foliage. It is classified as a rare and endangered tree by the IUCN red list, with only fifteen known populations².

Laura reached out to one of the collectors of the specimens, Dr. Damon Little, a gymnosperm specialist and curator of bioinformatics at NYBG. He inspected the webbing himself, and determined that it was webbing spun in the field (or even within the plant press!) from the caterpillars of the endangered butterfly Thorne's hairstreak (Callophrys gryneus thornei). Thorne’s hairstreak has a limited range that overlaps with the restricted range of the Tecate cypress. In fact, the endangered butterfly’s larvae rely on the young leaves of the cypress as their sole food source. There are currently only five documented remaining populations of Thorne’s hairstreak, all of which are in the southern California³.

Both the Tecate cypress and Thorne’s hairstreak are threatened by too-frequent wildfires, climate change, and habitat loss and fragmentation³. Without the cypress, the butterfly would not survive. Not only do these webbed specimens serve as a reminder of the fragility of our ecosystems and responsibility to protect them, they tell the story of a unique relationship between two highly threatened and endemic species.