Berberis and Mahonia have no significant defoliators other than the larvae of the berberis sawfly (Arge berberidis). The sawfly was confirmed as present in the UK in 2002 from Essex, and is now widespread in England.
The caterpillar-like larvae are up to 18mm long, creamy white with black spots and yellow blotches.
The adult sawflies are 7-9mm long, bluish black with dark grey wings and upswept antennae. It has several generations a year and can be found throughout the summer months.
Hosts and life cycle
Berberis sawfly larvae are found most frequently on Berberis thunbergii and its cultivars; they also feed on B. vulgaris, but that is a relatively scarce wild plant in Britain. Other cultivated Berberis species and hybrids on which larvae have been found include Berberis 'Georgei', B. jamesiana, B. mitifolia, B. x ottawensis, B. praecipua and B. virescens. The larvae also feed on the foliage of some Mahonia species and hybrids.
Adult berberis sawfly can be found flying around Berberis and Mahonia in the spring and summer and have been observed visiting the flowers of Eryngium caerulescens and fennel (Foeniculum vulgare), where they feed on nectar and/or pollen.
The female sawflies deposit their eggs in batches of up to seven, which are inserted under the epidermis of the lower leaf surface of its host plants.
Berberis sawfly is likely to be mainly a garden insect in the UK, as that is where its food plants are most likely to be found. There are two or three generations during the summer and autumn and so larvae can be found at almost any time between May and the end of October. When the larvae have completed their feeding, they pupate in soil inside silk cocoons.
Establishment and spread in the UK
Berberis sawfly is a European species that was first confirmed as being present in Britain in April 2002. An adult female was sent to the RHS's Members' Advisory Service from a garden in Church Langley, Essex, where Berberis thunbergii plants had been defoliated in the previous year.
Other records of this pest in the London area came to the attention of the RHS during 2002, and it was clear that in some gardens severe leaf loss on Berberis plants had been occurring since at least 2000.
As Berberis has no significant defoliators other than the sawfly, it is reasonable to assume that this pest first entered Britain, possibly with imported nursery stock, at some time during the late 1990s.
Records of the sawfly's distribution have been kept in subsequent years, primarily through enquiries via this web survey and those sent to the RHS Advisory Service. Initially, the sawfly made slow but steady progress from the original area of infestation in the counties around London, but by 2004 there were records near Lacock, Wiltshire, and Dover, Kent. These may have resulted from the movement of infested plants, rather than flight dispersal by adult sawflies.
By the end of 2010, berberis sawfly had become widespread in England and has recently reached Wales. It is likely that berberis sawfly will continue to spread and before long may reach Scotland.
- The larvae of berberis sawfly feed in relatively exposed positions on the foliage and are susceptible to most insecticides.
- If damage is to be avoided, host plants should be inspected carefully throughout the spring and summer in order to detect the young larvae.
- Removal of larvae by hand can be effective for light infestations.
- More extensive infestations can be dealt with by spraying with insecticides containing pyrethrum, deltamethrin, thiacloprid or
lambda- cyhalothrin. Insecticides must not be used during the flowering period as they are harmful to pollinating insects.