Tansy Beetles

Earlier in August I attended a tansy beetle workshop organised by Buglife.

Tansy beetles (Chrysolina graminis) are small, green beetles that feed predominantly on the tansy plant.  They are a conservation priority species, and are found only in Norfolk and along a stretch of the River Ouse in York.

Many enthusiastic staff and volunteers are involved in trying to save this enigmatic little beetle, and help by managing the riverside habitat, growing tansy plants, and even breeding the beetles in captivity so that they can be released into the wild.

In the morning we had a session learning about the ecology and lifecycle of the tansy beetle. We had the chance to look at specimens at each stage of their life including eggs and larvae.

The eggs are just a few millimetres in size and are a pale yellow/cream colour. They are laid in April/May in small clusters on the tansy leaf, and stand upright.

The eggs hatch in May/June and feed on the tansy leaf. The larvae are small, 'slug-like' creatures with a black head. They go through four 'instar' stages before burrowing underground in July and going through a pupal stage.

The young adults emerge from the ground in August and spend the next few months feeding on the tansy to prepare themselves for their winter hibernation.  From September, they burrow underground and remain in these retreats during the cold winter months.

The adults re-emerge in April, when they mate and lay eggs ... and the cycle starts again.

Tansy beetles live along river banks. Whilst the adults can tolerate flooding, eggs and larvae are vulnerable at this stage. The larvae are also liable to be predated.

Another threat against the beetle is its reliance on tansy as a food plant - any reduction in the amount of tansy has a direct impact on the beetles. Tansy (Tanacetum vulgare) is a perennial plant. As agriculture expands and field margins disappear, tansy is at risk.

Another concern is tansy's close resemblance to ragwort (Jacobaea vulgaris). The leaves are very similar, and though tansy has a more compact flower-head, when ragwort's petals are closed they are hard to tell apart. Ragwort is believed to be toxic if eaten by horses and livestock. It is often removed from fields and meadows as a weed - and tansy is being pulled up by mistake.

Threats to habitat are bad enough, but the situation is exacerbated by the beetles reluctance to fly. Although adults possess wings, they do not use them. Beetles therefore have to walk from plant to plant, and their range is only about 200 meters. Thus, habitat fragmentation can have a large impact by isolating populations of tansy beetles making them even more vulnerable to local stochastic events.

In the afternoon, we had a stroll around Rawcliffe Meadows to search for tansy beetles.  Although it was a little early in the season, some adults had emerged from underground and could be seen on the tansy. 

Other invertebrates were also found in the area. As well as numerous frisky soldier beetles, I saw my first comma butterfly of the season. (The bottom photo shows the butterfly resting on ragwort - compare the flower to the tansy above).

It was a very enjoyable day, and I even came away with my own tansy plant to rear and re-plant in future!