The UK's rarest spider is to be safeguarded from extinction by conservationists releasing the species in a new home in Dorset – using plastic water bottles.

The ladybird spider – Eresus sandaliatus – is one of the most colourful arachnids in Britain, but by the 1990s only 56 were left. There are now more than a thousand, thanks to the efforts of conservationists, and the spider is ready to be released into new areas.

The first of these introductions begins on Thursday with the release of 30 ladybird spiders into the RSPB's Arne reserve in Dorset. They will be monitored in their new home and, if successful, more will be released in the next few years.

Toby Branston, the RSPB's senior warden at Arne, said: "The hope is that this will establish a self-sustaining population here so that if one of the other satellite populations gets damaged by fire, for example, then the species will not become extinct in the UK … It will be safeguarding the species in Britain." Wildfires in May hit heathland in parts of the Scottish Highlands, Ireland, north-west England and Berkshire with devastating results for wildlife, though the spider's current home elsewhere in Dorset was unaffected.

The spiders will be taken to their new home in plastic water bottles, each filled with heather and moss. Then the bottles will be buried to allow the spiders to crawl out in their own time.

"Burying plastic bottles in the heathland may seem a little strange to some of our visitors, but the experts have found that this is the best way to translocate the spiders," said Branston. "This is an ideal habitat for them so we will be keeping a close eye on the new colony and carrying out regular surveys to see if they take to their new home."

Scientists had believed that the ladybird spider was extinct in the UK, but in the 1980s one small colony was found in Dorset. The species was deeply affected as its heathland habitat was disappearing, being lost to farming, forestry and housing.

The Arne nature reserve – the spiders' new home – is an insect hotspot that covers a range of habitats. It already has 240 spider and hundreds of insect species, including a subspecies of the rare silver-studded blue butterfly and the Purbeck mason wasp (which even appears on a set of stamps of endangered British insects issued by Royal Mail in 2008).

It is the mature male ladybird spiders that have the bright red bodies covered in small black spots, which give the species its name. The females – with body lengths between 10 and 16mm, almost twice that of the colourful males – and juveniles are a velvety black.

Ladybird spiders live in a hole in the ground, a tube which they line with silk and decorate with the remains of their prey, often beetles. The females rarely leave their burrows and both sexes feed off insects that become entangled in the fine strands of web at the hole's entrance.