Mark Greaves, a butterfly enthusiast, points out the slope where he first spotted Phillip Cullen. “He and his mate parked in the layby, climbed over that locked gate, and he was down there running around with a little net.”

Greaves asked Cullen what on earth he thought he was doing with a net on one of the most precious butterfly sites in the UK and was doubtful about the explanation.

“He said he was looking for parasitic wasps and green-winged orchids,” said Greaves. “There were no green-winged orchids by that time of summer. Was he really interested in parasitic wasps? I doubt it. It seemed clear to me he was after the large blues.”

Butterfly expert Mark Greaves https://t.co/TJIHSXpvcM — steven morris (@stevenmorris20) April 6, 2017

A police inquiry was launched which resulted in Cullen, 57, being convicted of illegally capturing specimens of the rare and protected large blue and given a six-month suspended prison sentence. At Cullen’s home near Bristol police found dozens of specimens of rare butterflies. Cullen also had some images of parasitic wasps.

Cullen’s case at Bristol magistrates court has focused attention on a fantastic conservation story: the revival of the large blue, and an uglier one: a trade in mounted butterflies that many might have thought vanished decades ago.

The good news story first. The large blue was declared extinct in the UK in the 1970s but two scientists were determined it would fly again in the British countryside.

The large blue’s remarkable life cycle means it can only thrive in very particular habitats. Photograph: Butterfly Conservation/PA

Jeremy Thomas, a professor of ecology at Oxford University, worked out exactly what the large blue needed to survive – well-grazed grassland and the presence of a particular red ant for its caterpillars to feed on.

David Simcox, a conservation consultant for the Centre for Ecology and Hydrology, drove his VW camper van to Sweden, collected eggs from the large blues found there, and released caterpillars at carefully selected sites in the south-west of England.

The remarkable life cycle of the large blue means it can only thrive in very particular habitats. Eggs are laid on the flower buds of wild thyme or marjoram. The larvae burrow into the flower heads and when they are about 4mm long drop to the ground and wait to be found by foraging red ants, attracting them with sweet secretions from a “honey” gland. The ants place them in their brood chamber and the larvae feed on ant grubs. They turn into butterflies, crawl above ground, and fly in midsummer.

Daneway Banks nature reserve in Gloucestershire is one of the sites where the large blue has flourished and by last year there were an estimated 4,000 of these rare butterflies there.

Greaves and his wife, Anna Pugh, both volunteers at Daneway Banks, with the reserve’s warden Alan Sumnall, showed the Guardian around on a sunlit day this week.

As the skylarks sang and the green woodpeckers bobbed past, Greaves and Pugh, both members of the charity Butterfly Conservation, were crumbling pieces of trifle sponge on the herb-rich slopes to try to attract the red ants and count them as part of ongoing efforts to check that conditions are as near perfect for large blues as possible.

Warden Alan Sumnall at Daneway Banks reserve in Gloucestershire where Phillip Cullen was found to have unlawfully collected the large blue. Photograph: Adrian Sherratt/The Guardian

“The cheaper and more sugary the sponge the better,” said Pugh. “They don’t seem to like the expensive version so much.” Within an hour, red ants were beating a track between trifle treats and their burrows and the couple declared themselves satisfied.

It is far too early in the season for the large blues to appear but other species such as the brimstones, the commas and orange tips are at large.

The Gloucestershire Wildlife Trust, which has managed the site for half a century and bought it last year together with the Royal Entomological Society, is not resting on its laurels. It is busy fencing off parts of the reserve to create different types of habitat for other rare species of butterfly.

Welsh ponies and sheep graze on the areas aimed at the large blue because they need closely shorn grass; another section is allowed to get more “tussocky” to appeal to the Duke of Burgundy; wild liquorice is encouraged in a third sector for the liquorice piercer micro-moth.

Despite the bucolic scene, butterfly-watching is not as genteel as might be imagined. Getting the perfect picture can be competitive. Greaves and Pugh tell how one enthusiast was pushed into a thorn bush by a rival trying to capture the perfect image; another photographer was seen taking a picture of a large bluethen hurling a stick close to the insect so it fluttered away before a second fan could take a snap.

Cullen, of course, went much further. Quite why remains sketchy. In Victorian times the large blue was highly prized by collectors because of its wonderful colour and its rarity, as its unusual life circle means it cannot be bred in captivity.

Pugh said there was a market still for butterfly specimens. “People buy up old labels and cases and add in newly killed specimens.” The magistrates who sat at Cullen’s trial were told large blues can fetch £300 each.

Mark Greaves, a volunteer at Daneway Banks, uses trifle sponges as insect bait to attract red ants, critical to the large blue’s life cycle. Photograph: Adrian Sherratt/The Guardian

But it isn’t easy money. Daneway Banks and Collard Hill in Somerset, another large blue hotspot where Cullen was found, are considerable journeys for Cullen and catching butterflies can be painstaking work. “I think some people are also just fascinated with collecting rare things,” said Pugh.

One twist in the tale is that Cullen was also a member of Butterfly Conservation.

The charity’s chief executive, Julie Williams, said: “Butterfly Conservation is utterly opposed to the illegal killing, capturing and possession of butterflies and moths. As a result of Mr Cullen’s actions, the Butterfly Conservation board of trustees moved swiftly to revoke his membership of the charity.

“More than three-quarters of the UK’s butterflies have declined in the last 40 years so for Mr Cullen to illegally catch and kill one of our most threatened species that is slowly recovering following extinction is unforgivable.”