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ALIEN SPECIES

Invasive Garden Plants

Nowadays the chances of finding species alien to these shores are greater due to an increase in the movement of plant materials. A recent study has found that the incidence of alien pests and diseases in the UK has increased from about 150 per year in the 1970's to about 370 in 2003. More and more fruit and vegetables are imported from all over the world and they can easily give the interlopers a lift, eg. the spiders used for pest control on vines have been found in bunches of grapes. Plants are also raised overseas where the climate is better and labour is cheaper, eg. bedding plants in Israel and indoor plants in Florida.
Some imported material is supposed to have a "plant passport" or be declared, but most plants and trees are not checked for eggs or hidden animals. The increase in winter temperatures here could mean that these visitors will become established in the wild. Even non-organic materials can bring unwanted species, the Asian Hornet is spreading from Southern France after it is believed to have arrived there in a consignment of pots from China - they are very aggressive preying on Honey Bees and have caused some human deaths as well.

Taking any organism from one place to another is unpredictable as the new environment is unlikely have any of the natural controls acting on it where it came from.

Here are some examples which have been recorded recently:-

• Harlequin Ladybird (Harmonia axyridis) - Found in the South-east of England and is a danger to native Ladybirds. They also eat lacewing larvae, nibble at soft fruit and there have even been reports of biting people. If found, they should be reported to the Harlequin Survey. Given their use on the European mainland as a biological control and the alarming rate of increase in numbers there, it was inevitable that they should arrive in the British Isles. The native species will not be able to compete and could disappear as they have in many parts of North America.

• Berberis Sawfly (Arge berberidis) - from central and southern Europe found in Essex; it strips the leaves from Berberis. The adult fly is black; the larvae have a black head on a greyish-white body with yellow blotches and small black dots.

• Cypress Gall Mite (Trisetacus chamaecypari) - from North America found in Cheshire, It attacks the shoots on cypress trees, which become yellowish-white then dry up and turm brown as they die.

• Elaeagnus Sucker (Cacopsylla fulguralis) - found in Essex, Surrey, Sussex and Yorkshire; sucks the sap from shoot tips and foliage. They also exude honeydew which covers the leaves, reducing transpiration and it can become colonised with Sooty Mould.

• Horse Chestnut Leaf Mining Moth (Cameraria ohridella) - probably from Asia and found in Wimbledon. It could have arrived via mainland Europe; is was first recorded in Macedonia in 1985. The tiny caterpillars strip the flesh out of leaves causing brown blotches which become holes later. This can cause early leaf fall if the damage is severe. Similar damage can be caused by a fungal attack.

• Oak processionary moth (Thaumetopoea processionea) - The caterpillar of this moth is the culprit. It is natibe to continental Europe but was first reported in England in 2005. The caterpillars form processions nose to tail and several crearures wide as they move around giving rise to the common name. As well as stripping trees of their leaves they have hairs on their backs which break off and are irritant ausing skin rashes, conjunctivitis and if inhaled, respiratory distress or even anaphylaxis. Importing any Quercus species to the UK is severely restricted as a result.

• Box tree caterpillar (Cydalima perspectalis) - Native to East Asia it became established in Europe in 2007 and first sighted in England the same year it became established in parts of London in 2014. The greenish-yellow caterpillars with black heads defoliate Buxus plants producing a webb to cover the feeding area. This causes dieback of the plants similar to the Box Blight diseases.

• Red Lily Beetle (Lilioceris lilii - Native to mainland Europe and Asia it has spread to the British Isles and due to Global warming, is progressing northwards, reaching Northern Ireland at the turn of the century.

• Wisteria Scale Insect (Eulecanium excrescens) - from Asia and the United States, and found in south-west London. A large sap-sucking pest up to 10mm long, which can kill the plant. It also attacks fruit trees.

Many of the trees, shrubs and weeds around were introduced hundreds of years ago, little thought was given to the consequences. Some of the more wayward of the ornamental plants in the garden have liked it so well that they have moved beyond the flowerbed and are now regarded as weeds. Others have been here for so long that they are considered as native, eg. Beech, Sycamore and most of the conifers.

There are some introductions which have naturalised and are becoming a nuisance as they overwhelm native plants.

• Buddleja davidii, Rhododendron ponticum, Cherry Laurel (Prunus laurocerasus), Himalayan Cotoneaster (Cotoneaster simonsii)

• Bindweed, Ground-elder, Himalayan Balsam, Japanese Knotweed, Japanese Barberry (Berberis thunbergii) (the names give these ones away), Mind-your-own-business, Pineappleweed, Pirri-pirri burr(Acaena ovalifolia), Rosebay Willow-herb, New Zealand Willowherb.

• Many of the pond weeds sold for ornamental ponds have escaped and are creating havoc in waterways and lakes.

Some of the larger animals were brought for food, fur or just ornamental purposes and are now roaming free.

• Grey Squirrel - from North America, but is more successful and is replacing the native Red Squirrel. They carry a virus which does not affect them greatly, but is deadly to the Reds. Now an even more aggressive 'Black' Squirrel, which is a mutation of the Grey, is replacing the latter in parts of England. The more aggressive behaviour is thought to be due to higher levels of testosterone in the males and this makes them better at securing a mate, which in turn makes their genotype more successful - Darwinism in practice.

• Rabbits - brought by the Normans for food and their fur. A pest to farmers and gardeners. They were taken to Australia and were creating deserts before they were controlled. (There is a danger of a repetition in Africa with the introduction of goats, which seems like a good charitable thing to do, but if they escape and breed in the wild - as they are apt to do - they can strip the area bare, even climbing into trees to eat the foliage and twigs.)

• Mink - brought here to be farmed for their fur are now naturalised after escaping and when they were released by unthinking animal rights activists. Now they are threatening the existence of water voles. A larger fur animal, the Coypu was establishing itself, but it has been eliminated.

• Muntjac Deer - brought to Woburn Abbey from South East Asia as a curiosity, but some escaped and there are now over 100,000 roaming free around the South of England. Although they are relatively small they can do a lot of damage by removing young shoots and tree bark, also many plants which are considered poisonous. Although some were known to have been kept enclosed in Northern Ireland, recently some have been spotted in County Down so there are worries that a colony may develop to cause damage to the countryside.
  They, along with Chinese Water Deer, are the only deer in the UK do not have a closed season for hunting under the Deer Act 1991.

• Some of the Wild Boar which were being farmed for their meat, have escaped and feral herds can be found in larger wooded areas of southern England (though some say it might have been a deliberate release).

It can be seen from these examples that introducing an alien species, whether deliberately or accidentally can have far-reaching consequences. No species can be introduced to-day without extensive research. Some are being tested for use as biological controls of pests and diseases, many of which are due to careless releases in the past.

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