Conservatory Collaboration: Teamwork Addresses Slug Situation

The Huntington’s Botanical staff members routinely collaborate with other institutions and agencies to tackle conservation challenges. Most of the time, these are carefully planned projects: propagating rare and endangered species, making gardens more resilient to the changing climate, and teaching such techniques as cryopreservation or culturing plant tissue. But sometimes, the unexpected happens.

In December 2022, Huntington visitor Jake Garcia spotted an unusual slug in The Rose Hills Foundation Conservatory for Botanical Science. He snapped a photo and posted it on the community science platform iNaturalist with his conjecture that it could be a slug in the Veronicellidae family—leatherleaf slugs. Jann Vendetti, associate curator of malacology (mollusks) at Los Angeles’ Natural History Museum and an administrator on iNaturalist, responded: “That would be a very unusual slug for Southern California. Do you have more information?” Once Vendetti confirmed that the slug was indeed Veronicella cubensis, a potentially very problematic invasive species from the Caribbean, she alerted Huntington staff.

In response, The Huntington closed the Conservatory to the public and assembled a containment team comprising Botanical staff, scientists from the University of Hawai‘i (the Cuban slug was discovered in the islands in the 1980s), and representatives from the California Department of Food and Agriculture (CDFA) and the University of California Division of Agriculture and Natural Resources.

Land-dwelling snails and slugs are known as terrestrial mollusks, as distinguished from freshwater- and marine-dwelling groups. Approximately 240 of the world’s estimated 35,000 terrestrial mollusk species are found in California. They are vital parts of healthy ecosystems, helping to break down organic matter, promoting nutrient cycling, and providing an important food source for such small predators as birds. Many species perform these functions at The Huntington, helping to keep plant collections healthy. Some are native; some are introduced.

Not all introduced species are considered invasive. An invasive species is one that aggressively establishes populations outside its natural range, causing ecosystem and agricultural damage. In some cases, the invaders are pets or garden plants that have escaped into the wild. Others are hitchhikers that have been unintentionally transported to distant regions, thanks to global trade and international travel. Many are unable to become established in new locations because of unfavorable climate conditions. But in a hospitable setting, invasive species can wreak havoc, destroying agricultural crops, displacing native species, and potentially spreading new pathogens. For example, Diaphorina citri is a sap-sucking, aphid-like insect native to Asia that was accidentally introduced in 1998 to Florida, where it continues to ravage the citrus industry. The insect causes only moderate damage to a plant when it feeds, but it carries a bacterium that causes the devastating disease huanglongbing, also known as citrus greening. When the insect was documented in California in 2008, intensive control measures were enforced to slow its spread.

V. cubensis became a pest of both ornamental garden plants and agricultural crops such as papaya, banana, and mango after its 1985 arrival in Hawai‘i. Since the slug thrives in the tropics, it probably couldn’t establish a population in California outside of greenhouse conditions. Nevertheless, the slugs in the Conservatory needed to be contained as quickly as possible.

Fortunately, the Conservatory’s controlled environment made that process easier.

Strategies for containing V. cubensis at The Huntington included removing mulch produced in-house and replacing it with commercial mulch, which is routinely sterilized. Heating mulch to 122 degrees Fahrenheit is 100% effective at killing slugs and their eggs. The Huntington acquired a low-pressure steam generator to sterilize any future mulch produced on the grounds. Routine monitoring continues, with Vendetti assisting in specimen identification.

Research into effective control methods revealed that a variety of essential oils—particularly clove bud oil—are highly effective at killing the slugs and their eggs. These oils break down relatively quickly, thus avoiding long-term toxicity. Also effective are standard commercial slug and snail baits as well as salt, and these are applied regularly. Early intervention is essential to break the life cycle before a potentially invasive species can become established.

Before the Conservatory can fully reopen, the CDFA requires assurance that the risk of a wider infestation is negligible. Thanks to diligent monitoring and control measures, the V. cubensis population has shown a steady and significant decline, with only stray individuals remaining. Although the Conservatory remains closed, CDFA officials are confident that Huntington staff has the situation well in hand and approved a plan to open a restricted section of the building to display this season’s Corpse Flower (Amorphophallus titanum). Updates regarding the Conservatory, including a reopening date, will be posted online.

Sandy Masuo is the senior writer in the Office of Communications and Marketing at The Huntington.