Three New Zealand species of stick-insects, or phasmids (from the Greek ‘phasma’ meaning phantom or apparition), have become naturalised in the UK over the last 100 years, and almost all are in south west England. This means that they live and reproduce here in the UK in the wild. The first UK record was of a Prickly Stick-insect (Acanthoxyla geisovii) found in a Paignton garden in 1909, with a further locality in 1943 from Tresco Abbey Gardens in the Isles of Scilly. The next species was the Smooth Stick-insect (Clitarchus hookeri) found in 1949, also in Tresco Abbey Gardens. The third species was the Unarmed Stick-insect (Acanthoxyla inermis) recorded at Truro, Cornwall, in 1979. Subsequently it was found that it had been established in Treseder’s Truro nursery since the 1920s, only a 100 metres from that 1979 record.
|Prickly Stick-insect (Acanthoxyla geisovii) |
© Malcolm Lee
Our naturalised stick insects all have similar annual life cycles. They breed parthenogenetically - meaning that the young hatch from eggs produced without fertilization by a male - laying several hundred eggs in summer and early autumn. These are simply dropped onto the ground below where the insect is feeding. They hatch out the following spring as miniature adults, some 12mm long, and climb up the first stem they meet. Nymphs grow quickly by shedding their skins five or six times to become mature in mid-summer. Adults typically live only three or four months, with few surviving into the winter.
The stick insects came to be here in the UK, thousands of kilometres from their native lands in new Zealand when plants, including Tree Ferns, from New Zealand were shipped to nurseries in south west England. In the Eucalyptus forests where the plants came from, stick insects live high in the canopy. Their eggs rain down on the forest floor, and many will be caught in the crown and the rough ‘bark’ of the Tree Ferns, to be transferred with the other plants to the UK. As stick insects can reproduce parthenogenetically a single egg arriving in the UK could lead to a viable colony.
|Unarmed Stick-insect (Acanthoxyla inermis) |
© Malcolm Lee
As the insects are surrounded by food (such as bramble and privet), and have no need to find a mate, they have little natural inclination to move far. Only in the autumn, when the leaves fall off the plant, will they wander to seek food or warmth.
As the weather turns colder, stick insects venture out onto south-facing walls to bask in the sun. This enables them to maintain their body temperature – and also makes them easy to spot! Most of the stick insects will die off in the first autumn frosts, but some individuals will survive the winter and thus small colonies have developed.
Unlike some 'alien invaders' these naturalised stick insects appear to have no negative impact upon native wildlife or plants, although further research is needed. In the meantime they are an exotic addition to the gardens and green spaces of south west England.
We need to find out more about where stick insects are living in the UK. If you find a stick insect in the wild, please record your sighting by clicking here