The species name was spelled dencyanna originally in Koss and Edmunds (1970) but was subsequently incorrectly emended to dencyannae by McCafferty et al. (1997) because Koss named the species after his wife and he should have given it the appropriate genitive ending. The original name is used here, unlike on the World Checklist of Freshwater Ephemeroptera Species (Barber-James et al. 2013).

The Gila Mayfly is thought to be restricted to a 84 kilometre stretch of the Gila River in New Mexico. The extent of occurrence is estimated to be 548 km² and the area of occupancy is 180 km². There are two locations. Current population information is not available for this species, however, the species is thought to occur in the relatively pristine wilderness between the two known localities, although the number and abundance of populations are expected to have declined in recent years due to land use changes in the watershed and water-quality impairment in the streams. Based on this information the species is categorised as Endangered using criteria B1ab(ii,iii)+2ab(ii,iii).

The species is considered to be endemic to New Mexico in the United States of America. The first specimen of this species was collected from the Lobo Creek, one mile south of Cliff in Grant County, New Mexico in July 1967. Two hundred and twenty one additional specimens were subsequently collected from the East Fork of the Gila River at the junction with the Gila River. Additional specimens were collected from the type locality in 1969 (Koss and Edmunds 1970). In 1998 and 1999 monitoring at the type locality revealed 29 different macroinvertebrate taxa, four of which were mayfly species. Lachlania dencyanna was not collected in these surveys despite targeted collections of mayflies at a suitable time of year for the larvae of the species. Likewise, extensive monitoring in other parts of the watershed has not revealed this species (Jordan et al. 2010). The Gila Mayfly is not known to have been observed or collected since 1969, and appears to have declined at the type locality. It is, however, likely that the species still persists at the type locality at low numbers, although it has not been detected during recent surveys. The species is also thought to occur in the relatively pristine wilderness between the two known localities, although the number and abundance of populations are expected to have declined in recent years due to land use changes in the watershed and documented water quality impairment in the streams.

Current population information is not available for this species. Over 200 larvae were collected in July 1967 and further specimens were collected in 1969. Subsequent monitoring between 1987 and 1997 did not find any specimens of the Gila Mayfly (Koss and Edmunds 1970). This species is not known to have been observed since 1969, however, it is likely that this difficult to capture species still persists at the type locality at low numbers, although it has not been detected during recent surveys. The species is also thought to occur in the relatively pristine wilderness between the two known localities, although the number and abundance of populations are expected to have declined in recent years due to land use changes in the watershed and water-quality impairment in the streams.

The type locality was described as a warm, turbid and rapid stream, mostly 0.15 m to 0.61 m deep, 1.8 to 3 metres wide, and unshaded for most of the day (Koss and Edmunds 1970). A detailed assessment of habitat use found that the larvae of this species utilise sticks and other vegetation caught in crevices among the rocks, unlike the closely related Lachlania saskatchewanensis larvae, which were collected clinging to rocks (Koss and Edmunds 1970).

Specific feeding behaviours of L. dencyanna larvae have not been observed, but mayfly larvae are mostly collectors or scrapers and feed on a variety of detritus and algae, as well as some macrophyte and animal material (Waltz and Burian 2008). Adult mayflies have non-functional mouthparts and do not feed (Waltz and Burian 2008). The behaviour of subimagos of the Gila Mayfly is highly unusual, even with regard to others in its genus, in that the subimago sheds the exoskeleton and moults into an adult without alighting, in contrast with typical mayfly subimagos which perch on shoreline vegetation during the moulting process (W.P. McCafferty pers. comm. in Jordan et al. 2010).

A detailed account of adult mating behaviour is provided in Koss and Edmunds (1970). Adults first appeared around 11:30 am and remained active until approximately 1:30 pm when the air temperature reached 82^oF. The males flew a distance of 1 to 1.5 metres back and forth across the stream, facing upstream at approximately a 45^o angle to the direction of current, maintaining a height of 2.5 to 5 centimetres above the water, with their tails widespread. The males would occasionally dart up- or downstream a distance of 1.5 metres or more, or fly in one or more circles before continuing the back and forth flight pattern. Presumably exhausted males would occasionally alight on the water surface for one or two seconds and then resume the back and forth flight. Males were also observed sitting and clambering about in the grass along the waters edge. Mated pairs of this species floated on the water when in tandem, the male on top of the female with his head posterior to hers (Koss and Edmunds 1970).

Like most mayflies, the Gila Mayfly requires a narrow set of environmental conditions to survive, including clean, rapidly flowing, well-oxygenated water and a substrate composed of rocks, leaves and other vegetation, and free of heavy siltation (Koss and Edmunds 1970). Impaired water quality and habitat conditions have long been documented in many streams in the upper Gila River drainage, including the East Fork of the Gila River, the type locality for this species. Historically, the watercourse was assessed as having high concentrations of aluminium in the water, which is highly toxic to aquatic insects. Since aluminium is a major component of both basalt and andesite (the main rock types which the East Fork Gila River runs through), it is not surprising that this metal is dissolved in the water column. Activities which lead to increased erosion are likely to result in the release of aluminium into surface waters.

The habitat conditions and water quality requirements of the Gila Mayfly are threatened by intense recreational activities at the type locality and the surrounding area. The type locality is located at Grapevine Campground, a popular and very heavily used Forest Service dispersed campground. Common recreational activities at the campground and surrounding area include hiking, fishing, visiting hot-springs, swimming in the river, driving, horseback riding, and off-road vehicle use (Monzingo pers. comm. in Jordan et al. 2010). Rafting is popular when flows are high enough (March through May), and Grapevine Campground is a common put-in point for rafters. In summertime, when flows are low, campers frequently swim in the river and have been observed re-arranging rocks and building dams in order to collect enough water to swim in (Monzingo pers. comm. in Jordan et al. 2010).

Recreational activities at this site may adversely affect the habitat of the Gila Mayfly in numerous ways, including increased erosion and sedimentation from foot, bike, car, and off-road vehicle traffic; runoff of pollutants from roads and off-road vehicle trails; introduction of bacteria and excess nutrients from dog and horse waste; manipulation and alteration of stream flow by swimmers; and the trampling of streamside riparian habitat by campers, hikers, rafters, and anglers. Overall, intensified human activities in and around the Gila Mayfly type locality is resulting in negative impacts on the aquatic and riparian habitat at this site.

Livestock grazing is a common non-point source of pollution in this region, and both the East Fork Gila River and the Lobo Creek are threatened by grazing-related habitat impairment. The East Fork of the Gila River has suffered extensively from grazing in the past, and although substantial recovery has been made on Forest Service land, the river is still heavily impacted by grazing on private land, and many reaches are characterised by high sediment loads and eroding banks virtually devoid of woody vegetation (Hudak pers. comm. in Jordan et al. 2010).

Livestock grazing can degrade water quality and negatively impact aquatic macroinvertebrate communities in several ways: trampling riparian vegetation; consuming streamside vegetation and downcutting the riparian buffer; defecating and urinating on stream banks or in the channel; and increasing sedimentation due to removal of riparian vegetation and direct damage to banks and the channel from trampling and wallowing. This sedimentation may have resulted in the apparent decline or local extirpations of this species.

There is no known use of, or trade in this species.

No Conservation Actions are in place at present. This species was petitioned for protection under the Endangered Species Act in 2010 (Jordan et al. 2010). In 2012, it received a positive 90 day finding that listing as Endangered may be warranted (USFWS 2012) and it is included in the US Fish and Wildlife Service FY23–27 National Domestic Listing Workplan  (USFWS 2023). The species is listed as Critically Imperilled (G1) by NatureServe (2024).

Conservation action required includes preventing erosion of banks through management of recreational activities in the upper Gila River watershed. In addition, livestock grazing should be controlled to prevent damage to riparian vegetation and erosion of the banks of the stream.