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Top five exotic pests

Citrus longhorn beetle (Anoplophora chinensis)

Citrus longhorn beetles are large (21-37mm) and black with variable white markings on the wing cases. Their antennae are longer than their bodies and are black with white bands.

The species has a wide host range including apple, oak, poplar, maple, alder, pine and beech. Damage is caused by the larvae, which bore through the trunks and roots of host trees leaving them susceptible to disease and wind damage. It is difficult to detect and control; the only certain method of control is to destroy infested trees.

When buying trees, particularly acers, ensure that they come from areas free from this and similar pests.

(Source: Defra)

Rosemary beetle (Chrysolina americana)

The rosemary beetle is an attractive 8mm long metallic green beetle with purple stripes.  It is a native of southern Europe that has recently become established in Britain, probably imported with plants.

Both the adults and the larvae feed on the leaves and flowers of rosemary (Rosmarinus officinalis), lavender (Lavandula spp.) and thyme (Thymus spp.).

It was first seen living out of doors in the UK at RHS Garden Wisley, Surrey, in 1994. By the end of 2003 the beetle had become widespread in London and East Surrey. It is also established in Norwich and a few adult specimens have been found in other areas of the country including Leicestershire and south east Yorkshire.

The entomology laboratory at Wisley recently established a laboratory culture of the rosemary beetle and investigated its life cycle.

Rosemary beetle can be controlled either with insecticides or hand picking of the adults and larvae. There are no recommended insecticides that can be used on rosemary and other plants if they are to be used for culinary purposes.

(Source: RHS)

Red lily beetle (Lilioceris lilii)

The red lily beetle is an 8mm long bright red beetle with a black head and legs. Both adults and larvae damage lilies and fritillaries, primarily by defoliating them, but in heavy infestations the flowers, seed capsules and stems will also be eaten.

The lily beetle is not a native species; it has been accidentally imported into Britain on several occasions in the past. It was first noticed at the end of the 19th century, with a handful of short-lived infestations reported from England and Wales in subsequent years. By the end of 2004 this pest had been found in almost all counties in England from Yorkshire southwards. In 2002 the beetle was reported for the first time from Scotland and Northern Ireland.

The increase in the distribution of lily beetle over the past two decades has been coupled with a rise in its frequency as an enquiry to the RHS Members’ Advisory Service.

At present gardeners rely on chemicals or hand picking to control this pest.

The red lily beetle is the subject of a three-year PhD research degree project being undertaken by RHS Entomologist Andrew Salisbury, which started in 2004 and is funded by the RHS and the Horticultural Development Council. It is hoped this will lead not only to a better understanding of the beetle’s biology but also improved methods for its control.

(Source: RHS)

Colorado beetle (Leptinotarsa decemlineata)

Colorado beetles are a notifiable pest and have been intercepted on a wide range of plant produce including parsley from Italy, salad produce from France and Spain and on ware potatoes from a number of other countries.

Host plants are potatoes and other plants from the potato family (Solanaceae) such as tomato, aubergine and pepper. Adult beetles are usually imported as 'hitch-hikers' on plants and produce they do not feed on, such as leafy vegetables and grain which have been grown in fields that previously had potatoes in the rotation. They can also be introduced direct on imported potatoes themselves.

(Source: Defra)

Allium leaf miner (Phytomyza gymnostoma)

The allium leaf miner is a pest of Allium crops, particularly onions, spring onions, garlic and chives and potentially of ornamental Allium plants. It mines the leaves and bulbs, making the plants soft and susceptible to bacterial and fungal infections, such as white rot. Heavy infestations completely destroy plants. Adult flies are about 3mm long with a black and yellow underside and black legs with yellow ‘knees’.

The pest is causing widespread damage to crops in an increasing number of European countries. The first UK outbreak was discovered in December 2003 in Wolverhampton. A further 12 outbreaks have since been found locally.

(Source: Central Science Laboratory)

Top five exotic diseases

Dutch elm disease (Ophiostoma novo-ulmi)

Dutch elm disease was first identified by scientists in the Netherlands. Its origins are unknown but are thought to have been in Asia, where elms show some natural degree of resistance to infection. In the late 1960s an aggressive strain of the disease, was imported into the UK from the USA via diseased timber.

It spread rapidly across southern England, and by 1977 was estimated to have killed 50% of the elms in the area. It eventually resulted in the lost of around 30 million elm tres.

The disease is spread by beetles that lay their eggs in the bark of dying elms. Affected branches wilt rapidly and do not recover. The disease has no cure and long-term solutions lie in breeding a selection for resistance.

Sudden oak death (Phytophthora ramorum)

A disease known as sudden oak death in the western USA and as ramorum dieback in northern Europe has been recognised since the early 1990s. It was first found in the UK in imported Viburnum in West Sussex, Dorset, Lincolnshire and Lancashire in 2002. Since then there have been more than 300 cases in the UK, most in containerised plants but some more recently in trees and shrubs in established gardens.

The disease is caused by the fungus Phytophthora ramorum, which gives rise to different symptoms. The most common hosts are viburnums and rhododendrons but occasional cases have been detected on Camellia, Pieris, Leucothoe, Syringa, Hamamelis, Taxus and Kalmia. There have also been cases in the UK in southern red oak (Quercus falcata), beech, horse chestnut, Turkey oak, holm oak and sweet chestnut.

In rhododendrons the disease appears as dark blotches on the leaves and on the bark of twigs, whereas in viburnums it can also cause wilting as a result of bark cankers at the base of the plant. In other UK hosts it mainly causes leaf blotches similar to those on Rhododendron. The country of origin is unknown.

Alder dieback (Phytophthora alni)

Alder dieback is present in England, Scotland and Wales and is most active in the south-west and south-east of England. In 2000 and 2001 an estimated 600,000 trees were affected: around 12-14 percent of the alder population. In addition to the UK, the disease is present in 10 other European countries.

Movement of infected plants has probably facilitated its rapid dissemination and infected stock may start new foci of infection when planted out. Research is continuing to further improve methods of managing the disease and diagnosing the pathogen. As with most phytophthoras, control is difficult and eradication from already infected riparian habitats would be impossible. As this particular disease is so widespread, quarantine controls would be ineffective.

Box blight (Cylindrocladium buxicola)

The disease was first reported in the UK in the mid-90s and was widespread in 1998. It affects box (Buxus), one of the oldest garden ornamental plants, which in the last two decades, has been rediscovered and become one of the most fashionable plants in the UK.

Box blights are fungal diseases and the origins of Cylindrocladium buxicola are unknown. The blight causes the leaves to turn brown and fall, often spreading to affect and kill off whole branches, which is very damaging to topiary box bushes.

At present, no fungicides are approved for either of the box blights. All affected leaves and branches, together with any fallen leaves, should be removed and destroyed as soon as the disease is noticed.

(Source: RHS)

Jarrah dieback (Phytophthora cinnamomi)

This disease is an introduced fungal species, which has been estimated to directly affect more than 2,000 of the 9,000 native plant species in the World Heritage status area of Southwest of Western Australia. The fungus attacks in particular Eucalyptus marginata (jarrah) and Corymbia calophylla (marri) trees.

It is listed as a 'key threatening process' to Australia's native species and ecological communities by the Australian Government and it threatens 181 species in Tasmania. The parks and wildlife service of New South Wales has identified at least 10 animal species that are under threat as a direct result of the fungi affect on the indigenous vegetation.

The root parasite thrives in warm, moist soil environments so has attacked low-level Bush, leaving taller plants under threat as their soils tend to dry out, which in turn reduces the shelter available for new seedlings.

Phytophthora cinnamomi was also introduced into the UK sometime in the past 2-300 years. It causes root rot on a wide range of plants, but fortunately is not particularly damaging to key trees in native ecosystems, unlike in Australia.

The word Phytophthora comes from the Greek ‘plant destroyer’.