Cliff Stiletto (Thereva strigata)
Key Details
| Taxonomic Groups: | Invertebrate > insect - true fly (Diptera) > Soldier fly or ally |
| Red List Status: | Endangered (Not Relevant) [EN(nr)] |
| D5 Status: | Included in the baseline Red List Index for England (Wilkins, Wilson & Brown, 2022) |
| Section 41 Status: | (not listed) |
| Taxa Included Synonym: | (none) |
| UKSI Recommended Name: | Thereva strigata |
| UKSI Recommended Authority: | (Fabricius, 1794) |
| UKSI Recommended Qualifier: | (none specified) |
| Red List Citation: | Drake, 2017 |
| Notes on taxonomy/listing: | (none) |
Criteria
| Question 1: | Does species need conservation or recovery in England? |
| Response: | Yes |
| Justification: | Latest accepted records are from 1991 at Dover, Kent and a single record from Ventner, Isle of Wight from 2007. |
| Question 2: | Does recovery/ conservation depend on species-specific actions? |
| Response: | Yes |
| Justification: | Most records are from or close to chalk cliffs, with no confirmed records on the English mainland for over 30 years and the IOW for 17 years) targeting actions for the species would be problematic without further exploratory work. The ecology is not readily understood and the fly has always had a limited distribution. The habitat is hugely restricted which means that targeted surveys to establish whether populations still exist should be possible (with safety considerations around cliffs a potential issue) |
| Question 3: | At a landscape scale, would the species benefit from untargeted habitat management to increase habitat mosaics, structural diversity, or particular successional stages? |
| Response: | Yes |
| Justification: | Maintenance, protection and expansion of chalk grassland areas potentially may at least make any populations should they exist more stable. |
Species Assessment
| Current step on the Species Recovery Curve (SRC): | 2. Biological status assessment exists |
| Recovery potential/expectation: | Low - Relict or natural rarity |
| National Monitoring Resource: | Opportunistic - insufficient |
| Species Comments: | It appears that the fly is severely limited by it's rare habitat requirements, populations are assumed to be tiny which may eventually lead to extinction even if the habitat is protected. |
Key Actions
Key Action 1
Proposed Action: Tailored surveys of the required habitat in the adult flight period (late May to July) to be completely sure whether the previous populations are present and where possible in other suitable Southern sites that have little previous history of surveying.
Action targets: 3. National Monitoring Plan agreed and implemented
Action type: Targeted monitoring
Duration: 2 years
Scale of Implementation: ≤ 5 sites
High priority sites: The two most recent sites are in the Isle of Wight, Ventner, Castle Cove, 2007, (SZ552770) East Kent, 1983,St Margaret's-at-Cliffe (TR3847),1974, Shakespeare Cliff, TR33; there is an unconfirmed record from Devon, Branscombe Pastures SSSI (1991) SY206885)
Comments: To determine if older sites are still populated and also to discover if other viable sites have undiscovered populations. This would then hopefully allow for more targeted approaches. Passive water trapping may be considerably safer than active searching. This habitat always poses significant H&S issues. however, what is needed is a decent structural and resource assessment of the habitats sampled so we have some sort of assessment of exactly what is in the area it is found. This should include nectar resources from adults and also some assessment of the beetle community if the larvae are suspected as larval food. Suction sampling would work, hazards aside. Certainly rope access protocols might be very useful there.
Key Action 2
Proposed Action: Following on from initial surveys, if sites with populations are confirmed, research aimed at understanding the exact habitat/ecological requirements to allow more precise actions in relation to larval requirements would be beneficial as little appears to be known at present.
Action targets: 4. Autecology and pressures understood
Action type: Scientific research
Duration: 3-5 years
Scale of Implementation: ≤ 5 sites
High priority sites: The two most recent sites are in the Isle of Wight, Ventner, Castle Cove, 2007, (SZ552770) East Kent, 1983,St Margaret's-at-Cliffe (TR3847),1974, Shakespeare Cliff, TR33; there is an unconfirmed record from Devon, Branscombe Pastures SSSI (1991) SY206885)
Comments:
Key Action 3
Proposed Action: Liaison with local authorities to alleviate the threat of coastal development whether this is housing or agricultural. Previous sites are in small areas and coastal chalk grassland in situ with cliffs is a rare habitat.
Action targets: 4. Autecology and pressures understood
Action type: Advice & support
Duration: >10 years
Scale of Implementation: ≤ 5 sites
High priority sites: The two most recent sites are in the Isle of Wight, Ventner, Castle Cove, 2007, (SZ552770) East Kent, 1983,St Margaret's-at-Cliffe (TR3847),1974, Shakespeare Cliff, TR33; there is an unconfirmed record from Devon, Branscombe Pastures SSSI (1991) SY206885)
Comments: Dependant on whether fixed populations are found at these sites or adjacent areas, with the lack of records over the past 30 years this may be difficult.
Acknowledgment:
Data used on this website are adapted from Threatened species recovery actions 2025 baseline (JP065): Technical report and spreadsheet user guide (Natural England, 2025). Available here.