Blue-grey Taildropper

General information

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The Blue-grey Taildropper is a small (2 -3 cm long) native forest slug found from southern British Columbia through Washington to northwestern Oregon. An isolated population occurs in Idaho. In British Columbia, the slug's distribution is restricted to southern Vancouver Island. Until recently, all records were from the Capital Regional District, but in autumn 2013, two slugs were found at the base of Mt Tzouhalem in the Cowichan Valley, extending the known distribution of the species northward. Another observation from the Cowichan Regional District was made in November 2015, approximately 11 km to the southeast of the 2013 record. Additional undocumented sites may exist on Vancouver Island and the Gulf Islands.

Blue-grey Taildroppers occupy a variety of moist mixed-wood forest habitats, including Garry Oak – Arbutus woodlands, Bigleaf Maple-dominated stands, and older Douglas-fir – Grand Fir stands. The slugs seem to prefer sites with abundant leaf litter, moss and downed wood, which provide hiding places and shelter from adverse conditions.

The Blue-grey Taildropper is the rarest of four species of taildropper slugs (genus Prophysaon) that occur in British Columbia. The other species are the Scarletback Taildropper (P. vanattae), Yellow-bordered Taildropper (P. foliolatum), and Reticulate Taildropper (P. andersonii). The common name of the slugs reflects their ability to detach their tail if seized by a predator, a behaviour also shown by some lizards as a means of escaping predation. Careful examination of a taildropper slug with an intact tail reveals a thin line where the breakage would occur; the tail may start visibly constricting at this point, if the slug is stressed.

How to Identify the Blue-grey Taildropper

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Please consult HAT's identification guide: 

Status in Canada

Canada (COSEWIC 2006): Endangered; status reassessed as Threatened in April 2016

Schedule 1 (official list of species at risk in Canada) under federal Species At Risk Act

BC (Conservation Data Centre): Red list; S1 (critically imperilled)

Global ranking (NatureServe): G3G4 (vulnerable to extinction/apparently secure)

Distribution on Vancouver Island

In Canada, the Blue-grey Taildropper has been found only on southern Vancouver Island. Potentially suitable habitat also occurs on southern Gulf Islands. The map below is current up to March 2016.

Fig 3 BGT sites 2015 16


The distribution of the Blue-grey Taildropper overlaps almost entirely with populated areas on southern Vancouver Island, where urban development continues to fragment habitats. However, many known sites are within parks or other protected areas, which provide refuges for the slugs and other wildlife within developed and modified landscapes. Although protected from development, these areas are not necessarily safe for the Blue-grey Taildropper. In high use recreational areas, excessive trail networks and recreational activities can result in compaction of soils and loss of cover required by the slugs. Human access also facilitates the spread of invasive plants and animals. Invasive plants, such as the Laurel-leaved Daphne, English Ivy and Scotch Broom often form monocultures at the expense of native plants, thus contributing to habitat deterioration. Invasive introduced invertebrates, such as ground beetles and other slugs and snails, can either prey on native slugs or compete with them for shelter or food. Increased frequency and severity of summer droughts, predicted under climate change, have the potential to exacerbate impacts from human activities and introduced species on Blue-grey Taildropper populations.

Why Survey Efforts are Needed

The first step in conserving species at risk is to obtain adequate knowledge of their distributions, so that appropriate protection or management measures can be implemented. Similar to many invertebrates in British Columbia, the distribution of the Blue-grey Taildropper is incompletely understood, and new sites continue to be found with increased survey effort. Efforts by landowners and the public and by "citizen science" initiatives can significantly increase survey coverage and contribute to the conservation of the Blue-grey Taildropper and other species at risk. Since 2010, HAT biologists have conducted annual surveys on public and private lands within the Capital Regional District, in collaboration with landowners and managers, in an attempt to detect the species and clarify its distribution and habitat needs. 

Survey Methods

Blue-grey Taildroppers are small and inconspicuous and rarely seen on the surface. They are most readily detected in late autumn (October – November), right up to freeze-up. One can search for them in their hiding places on the forest floor under sloughed-off bark or other woody debris. Searches should be carried out with special care, replacing any cover-objects that were disturbed into their original positions and avoiding excessive trampling of the forest floor. It is best to refrain from taking decaying logs apart, as such coarse woody debris provides important shelter for a wide variety of wildlife and would be destroyed in the process.

An effective method to locate Blue-grey Taildroppers and other gastropods is to use artificial cover-objects. This method allows one to survey an area repeatedly and with minimal disturbance to the natural habitat. Small (30 x 30 cm) pieces of corrugated cardboard work well. The cover-objects are placed flush with the ground and allowed to absorb moisture from the ground and from rain. The underside of the cover-objects and the soil surface underneath them can then be examined for slugs. It is best to let the cover-objects weather for at least two weeks before the first inspection.

If a slug that might be a Blue-grey Taildropper is found, the date and location should be recorded and a photograph of the slug taken to allow the identification to be verified.


Reporting observations

Please report your observations to HAT:

email: This email address is being protected from spambots. You need JavaScript enabled to view it. 
phone: 250-995-2428.

HAT welcomes reports of possible observations and can help with identification. Your observations should include a photograph, as it is impossible to confirm identification of the Blue-grey Taildropper based solely on description.

Monitoring and Threat Mitigation

Since 2010, HAT has worked with CRD Regional Parks and private landowners within the Capital Regional District to survey and monitor Blue-grey Taildroppers and other gastropod species at risk. There are several ways landowners and community members can help conserve Blue-grey Taildropper populations and habitats, ranging from reporting serendipitous observations to actively participating in HAT's monitoring efforts (see HAT's Blue-grey Taildropper Habitat Monitoring Guide for details).

Actions that landowners can take to protect and improve habitat for the Blue-grey Taildropper include the following:

• Leave a portion of the property in a natural state and keep foot traffic there to well-defined trails to avoid soil compaction and fragmentation.

• Control invasive plants, such as broom and laurel-leaved Daphne that tend to take over from native vegetation and reduce habitat quality. When removing invasive plants, try to minimize disturbance to native vegetation and the soil/litter layer.

• Retain downed logs and bark on the forest floor, as they provide cover for the Blue-grey Taildropper and a variety of other organisms. Large logs and piles of bark, which may be found at the base of snags and old stumps, are particularly productive. Well-decayed wood is best, but remember to leave some larger pieces of newly fallen trees in place to provide future habitat.

• In woodland habitats, maintain a mosaic of forest and small openings. It may be necessarily to control conifer encroachment, especially if your property contains Garry oak meadows, to maintain the open nature of these habitats over the long term.

• Where possible, maintain or restore connectivity with surrounding forest habitat to facilitate movements and to maintain viable populations of the Blue-grey Taildropper over the long term.

More Information about the Blue-grey Taildropper

COSEWIC. 2016. COSEWIC Status Report on the Blue-grey Taildropper slug Prophysaon coeruleum. Committee on the Status of Endangered Wildlife in Canada, Ottawa, Ontario.

Forsyth, R.G. 2004. Land Snails of British Columbia. Royal British Columbia Museum handbook. Royal BC Museum, Victoria, BC.

Habitat Acquisition Trust. Blue-grey Taildropper Habitat Monitoring Guide. 

Habitat Acquisition Trust. Blue-grey Taildropper Identification Guide. 

Ovaska, K. and L. Sopuck. Annual reports prepared for HAT. See


Blue-grey Taildropper Conservation