Prickly leaf beetle is found in the UK for the first time

At just a few millimetres across the prickly leaf beetle might be easy to overlook, but this little insect has just made history. For the first time ever, this beetle has been discovered living in a garden in Britain.

The first specimens from Surrey were sent to scientist of the Royal Horticultural Society who identified the species as the rockrose prickly leaf beetle (Dicladispa testacea), before sending the specimens on to the Museum. Almost simultaneously, Dan Hall from the Museum found a second population in Kew, while reports from social media suggest there may be another population in south-west London.  

Max Barclay is a curator of beetles at the Museum who used the collections to figure out where the interloper might be able to live in the UK.

'Of all the insects established in the UK in recent decades, the rockrose prickly leaf beetle is one of the strangest, and is not closely related to any other insects that occur in the UK,' says Max. 'The kind of rockrose on which it feeds is not a native plant but is deliberately planted as a garden ornamental, so it is not a species that can occur in natural habitats but will remain restricted to gardens.'

'It poses no threat to any native plants or animals because it feeds only on a single plant which is not native.'

A full scientific report on the beetle has been published in the journal The Coleopterist. 

What does the prickly leaf beetle look like?

The prickly leaf beetle is incredibly distinctive and unlike any other beetle found in the UK.

It is small, at only around five millimetres long, but its ruby-red body is covered all over in long black spines. While it is too small to be of any harm or concern to humans, the prickly covering would likely put off many potential predators.

It is an annual species, which means that its life cycle is about a year. Most of this is spent within the leaves of the plant, during which the larvae feed and grow bigger. They will then emerge as adults in the summer months and will live for about another six weeks.

During this time the adults will gather on the ends of the leaves, where they will mate, lay their eggs and then die. 

Dr Michael Geiser, a beetle curator at the Museum who was also involved in the population modeling of the newcomer, says, 'The adults survive for only a few weeks. The rest of the year they will be larvae, pupae and eggs. Most of their yearly cycle is spent as a larva inside the leaves and that is how they overwinter.'

'It is about now that they emerge as adults, and it would be very useful to get any additional records so we can understand more about their distribution in Britain.'

If you think you have found a prickly leaf beetle, you can email in any pictures you have to Max and Michael at bug@nhm.ac.uk 

Where did the prickly leaf beetle come from?

The prickly leaf beetle belongs to a group of leaf beetles which contains about 800 different species. Most of these are found in the tropics of Africa and Asia, but five species can be found living in Europe.

One of these is restricted to the Canary Islands, while the other four are found on mainland Europe. The rockrose prickly leaf beetle's distribution matches that of the rockroses it feeds on, which is typically around the northern edge of the Mediterranean Sea and up the western coast of France.

The reason that the beetles (and their host plants) are not native to the Britain is likely because it was simply too cold for them to survive here when the landmasses were still connected. When Britain became more favourable as the environment warmed, the channel had already formed and prevented any dispersal attempts.

'Because the English Channel was formed during a colder period, many warmth-loving insects never reached the British Isles,' says Max. 'For example, the insect fauna of the north of France or the Netherlands or Belgium is much richer than that of southern England.'

This means that the beetle likely came to Britain not by itself, but in the pots and plants of the rockroses imported for gardens.

'The reason it was introduced and overlooked so far is because it is a leaf miner,' explains Michael. 'Leaves with the small larva in it will not be very conspicuous, so unless the plant is really carefully checked at customs the beetle is not going to be detected very easily.'

'You can only detect the beetle once the larva has grown bigger meaning it is very easy for those plants infested with eggs or young larvae to slip though.' 

Will the prickly leaf beetle cause damage?

The beetles feed exclusively on rockroses in the genus Cistus, all of which are naturally found in the drier climate of the Mediterranean. The rockrose has a strong, robust root system, thick, pointy leaves and lovely flowers ranging in colour from pink and purple to white, with a yellow centre.   

While not native to the UK, the flowering bush is often planted in gardens and parks as an ornamental plant.  

The prickly leaf beetle is entirely dependent on the Cistus plants for its lifecycle. The beetle is what is known as a leaf miner, meaning that its larvae burrow into the leaves of the plants and live within the structure.

This can help people to see if their Cistus plants may be hosting the beetles.    

'The plant will get a brown patch in the centre of the leaf where the larvae are developing, in between the layers of the leaf,' explains Max. 'It won't be very noticeable when the larvae are little, but when they get bigger you start to see the brown patch which some gardeners don't like.'

The impact of the prickly beetle in general is very minimal. While occasionally they might cause a leaf or two to fall off, it is very unlikely to lead to any significant damage or harm the plant.

'They will cause brown spots on the leaves, and there will be spiny beetles mating on the ends of the leaves,' says Max. 'Some people might not like to see things like that going on in their rockroses. They do mate a lot.'

The reliance on a single group of plants which are themselves non-native and requiring very specific growing environments means that there is very little chance that the beetle will become and invasive pest.

So far the beetles have only been detected in Surrey, but any reports of the insects can be sent to the Museum scientists at bug@nhm.ac.uk