Stick Insects

Stick Insects in Britain

Stick insects are among the most unusual insects found in Britain.

Resembling twigs or stems, they rely on extraordinary camouflage to avoid detection. Unlike many native British insects, however, stick insects are not indigenous to Britain.

All established populations originate from accidental or deliberate introductions, primarily from New Zealand.

The most widespread species is the Prickly Stick Insect (Acanthoxyla geisovii), accompanied locally by the Unarmed Stick Insect (Acanthoxyla inermis) and the Smooth Stick Insect (Acanthoxyla prasina).

These species have become naturalised in parts of southern England and Wales.

Origins and Establishment

Stick insects were first recorded breeding in the wild in Britain in the early 20th century.

The genus Acanthoxyla originates from New Zealand, and it is believed that:

Eggs were accidentally transported on imported plants, or

Captive individuals from private collections escaped or were released.

Because these species reproduce without males (see below), even a single individual can found a new population.

Today, established colonies are concentrated in:

• Devon

• Cornwall

• South Wales

• Parts of southwest England

Their distribution remains patchy but locally stable.

Identification

Stick insects are elongated, slow-moving and highly cryptic.

General Features:

Slender, twig-like body

Length typically 6–10 cm

Brown or green colouration

Long legs held at angles resembling branch offshoots

Slow, swaying movement mimicking vegetation in wind

The Prickly Stick Insect (Acanthoxyla geisovii) is the most frequently encountered and can be recognised by small spines along its body and legs.

Because they depend on camouflage rather than speed or defence, they are often overlooked even where present.

Diet and Feeding Ecology

Stick insects are strictly herbivorous.

Main Food Plants in Britain:

Bramble (blackberry)

Rose

Hawthorn

Ivy

Other hedgerow shrubs

They feed mainly at night, chewing leaf edges and leaving characteristic scalloped damage. During daylight, they remain motionless along stems or branches.

Their feeding rarely causes serious plant harm due to relatively low population densities.

Reproduction and Life Cycle

One of the most remarkable aspects of Britain’s stick insects is their mode of reproduction.

Parthenogenesis:

All established British populations consist almost entirely — or entirely — of females.

They reproduce by parthenogenesis, meaning:

• Eggs develop without fertilisation

• Offspring are genetic clones of the mother

• A single individual can establish a colony

Egg Stage:

Eggs resemble small seeds

Dropped individually to the ground

Hatch after overwintering

Development:

Young nymphs resemble miniature adults

Grow through several moults

Reach adulthood by late summer

Adults typically die in autumn after laying eggs.

Population Numbers in Britain

Precise population estimates are not available. Unlike birds or mammals, stick insects are not subject to nationwide systematic counting.

Current understanding suggests:

Populations are localised but stable in southwest Britain

Numbers fluctuate depending on climate and habitat

Colonies may contain dozens to hundreds of individuals in favourable areas

Because they are nocturnal and highly camouflaged, true numbers are difficult to assess.

Conservation Status

Global Status: the New Zealand Acanthoxyla species established in Britain are not globally threatened.

Status in Britain (Not native)

• Naturalised but limited in distribution

• Not considered endangered

• Not protected under specific conservation legislation

They are generally regarded as benign introductions with minimal ecological impact.

Unlike some invasive species, British stick insects:

• Do not spread aggressively across the country

• Do not significantly displace native insects

• Remain largely confined to mild coastal regions

Their reliance on warmer climates may limit further expansion northwards.

Ecological Role

Though introduced, stick insects now occupy a small niche within hedgerow ecosystems:

They contribute to leaf herbivory

Provide prey for birds and small mammals

Add structural diversity to invertebrate communities

Because they reproduce clonally, genetic diversity is low. This could limit adaptability to environmental change, potentially restricting long-term expansion.

Climate Influence

Britain lies near the climatic limit for many stick insect species. Warmer winters and milder summers in southern England have likely aided their survival.

Climate change could:

• Enable gradual northward spread

• Increase survival of overwintering eggs

• Expand suitable habitat range

However, severe cold spells can still reduce local populations significantly.

Outlook

Stick insects in Britain represent an unusual example of successful establishment through parthenogenetic reproduction. While never native, they have persisted quietly for decades in sheltered southern habitats.

Their future in Britain will likely depend on:

Continued mild climatic conditions

Availability of hedgerow habitat

Absence of significant predation or disease pressures

Though modest in number and distribution, they remain one of Britain’s most intriguing invertebrates — living proof that even a single individual can found a population under the right conditions.

Photo: copyright Alan Gilchrist

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