Terrific tansy beetles

By Olivia Boult (NLOW Natural Heritage & Communities Officer)

Iridescent, jewel-like and shiny, tansy beetles (Chrysolina graminis) are a leaf beetle about 1cm long found beside riverbanks or in wetlands. They were once widespread across the UK, however are now currently endangered, not just in the UK but worldwide. They are only found in three locations across the UK: along the River Ouse in York and at Woodwalton Fen NNR and WWT Welney in Cambridgeshire. They were thought to be extinct in the East Anglia Fens, until rediscovered in 2014.

As part of the New Life on the Old West (NLOW) project, we were invited to help Natural England staff survey their tansy beetle population at Woodwalton Fen. I went along with a few NLOW volunteers and other Natural England volunteers to help. Coincidentally being in York the day before, I also visited the River Ouse population. The ecology of the populations differs, which I’ll mention below.

Monitoring involves walking through suitable habitat and recording the number of beetles spotted, their location and what plant they are found on. At Woodwalton Fen, they are still researching the beetles’ preferred food plants but they have been found feeding on: water mint (Mentha aquatica), gypsywort (Lycopus europaeus), hemp-nettle (Galeopsis bifida) and marsh woundwort (Stachys palustris). Whilst walking the transects, we were looking out for signs of nibbling on these plants. Our success in finding the beetles reflected the fact they are scarce but we found a few (nearly all on reeds above clumps of water mint). The transects and recording effort are used as a proxy for how many beetles are present at the site.

In York, the beetles contrastingly feed almost exclusively on tansy (Tanacetum vulgare), which is how the beetles got their name. Tansy is a distinctive yellow plant and when looking for clumps of it along the river, it didn’t take long to spot it and hundreds of tansy beetles! Because the beetles are found along stretches of the River Ouse, surveying them is much easier and counting them is possible without causing damage to their habitat. It’s therefore simpler to get an accurate estimate of their abundance than in the Fens.

In the York population, the beetles spend their entire lifecycle on and around tansy. Adults feed, mate and lay eggs around the tops of tansy plants from April until June. Eggs are laid on the underneath of the tansy leaves or surrounding plants and hatch between May and June into larvae. The larvae feed on the tansy leaves and then burrow underground at the base of the tansy plant until the pupae hatch in mid-July and can be seen again on the tansy plants until September. The beetles then burrow underground and spend the winter there until emerging again as adults in April.

In the East Anglian Fens population, how adults overwinter is less certain. The winter water table is often at or above ground level, and flooding (which is a known threat to the beetles) can be a common occurrence, so it’s perhaps less likely that the beetles burrow deep into the soil during the winter like the York population do. Further research is needed!

The severely restricted range and declining distribution of tansy beetles is a result of threats such as loss of their food plants, summer flooding, mowing of riverbanks, reduction in suitable wetland habitat, and invasive species, such as Himalayan balsam outcompeting native tansy plants. Even though the beetles have wings, they don’t seem to fly and are only capable of walking up to 200m – as such, ensuring there are regular pockets of their foodplants is so important. The beetle is now a UK Biodiversity Action Plan (UKBAP) priority species, meaning that public bodies have a duty to protect it, together with its habitat.

Attempts at reintroducing the tansy beetles to other areas, such as at Wicken Fen, haven’t been successful so far. Nevertheless, there is hope that the beetles are resilient enough to recover if given appropriate habitat conditions. The beetles were thought to have gone extinct at Woodwalton Fen in 1973 when they were last recorded at the site, but their rediscovery in 2014, and at WWT Welney in 2018, gives hope. More research into their ecology will be invaluable.

Many thanks to Natural England for letting us help with one of their surveys!