More Aliens in Elmbridge

The addition of alien species to the list of British wildlife seems to be never-ending, due of course mostly to human activity, but perhaps also influenced by climate change.  It might even be speeding up, judging by the list of recent introductions.  All groups of organisms are involved, from mammals, including wild boar and probable big cats, to (more evident in Elmbridge) birds such as the ring-necked parakeet (see article on p. 6), but also including invertebrates, plants and fungi.  The awful Water pennywort (Hydrocotyle ranunculoides) from America, currently choking our waterways, and the similarly invasive Himalayan balsam (Impatiens glandulifera) and New Zealand pigmyweed (Crassula helmsii) are three examples.  These are all now well-established here and need urgent control to protect native species.  Even in Elmbridge, examples of alien introductions (though not abductions as far as I know!) are many.  Dutch elm disease, which had such devastating effects on native elm trees, was caused by a, then undescribed, fungus (Ophiostoma novo-ulmi) probably introduced from the Himalayas, and wood chip mulch now supports dozens of apparently alien fungi never before seen in Britain.  The spectacular Starfish fungus Aseroë rubra, found in the Northern hemisphere only at Oxshott is another local example, introduced presumably from Australia and first found here about eleven years ago.  Invertebrates are also involved, and recently the Mediterranean ladybird Epilachna argus was discovered in Molesey, the first and only established British locality known for this species.  Unlike most ladybirds this is a vegetarian, feeding on cucurbits, though so far only found on White bryony (Bryonia dioica) and not yet a scourge of allotment courgettes and marrows.  Actually, it now seems to be fewer in numbers and may be disappearing again.

Two micro-moths are the latest additions to the list of British aliens, both of them now present in Elmbridge.  Like most micro-moths they are hard to identify as adults, though both are attractive species with white chevron-like markings on the wings. Both belong to the Gracillariidae, and can be more easily identified during the larval stages as both are leaf miners. Mining leaves is a useful strategy, affording protection from many potential predators and a constant food larder immediately available, that has been adopted by other insect groups as well.  Some beetles (Coleoptera), many sawflies (Hymenoptera) and lots of true flies (Diptera) also have leaf-mining larvae. Like galls, leaf mines offer useful identification characters, varying in position on the leaf, in shape (from blotches to linear, snake-like mines), as well as season. Most species are also quite specific in their choice of host.

The first of these moths, Phyllonorycter leucographella, mines leaves of Pyracantha and is duly known as the Firethorn leaf miner.  The larvae feed under the epidermis and cause large blotches centred over the midrib on the upperside of the leaf, appearing whitish and quite conspicuous. Later, the larva spins silk across the mine, which soon contracts and folds the leaf into a pod-like shape. It subsequently pupates in the mine.  Some bushes along the Thames in Molesey are quite densely infected.  This moth was accidentally introduced into the UK in the mid-1980s and has spread quite rapidly since then so that it is found now throughout most of the Southeast, apparently aided in its spread by the garden-centre trade in Pyracantha.  The moth can occasionally also be found now on hawthorn.

The second moth is the Horse chestnut leaf miner, Cameraria ohridella, which causes conspicuous, blotchy mines on the upperside of leaves of Aesculus.  These mines, often exceedingly abundant with many per leaf, can be quite disfiguring and can also cause considerable defoliation.  It is considered the most dangerous pest of Horse chestnut in Europe. Having no natural enemies in Europe, it is becoming a severe pest as trees defoliated annually can become severely weakened.  Cameraria ohridella was a completely unknown species, described as new to science only in 1986 when mines were first found at Lake Ohrid in Macedonia in 1985.  It has now spread though Austria, Hungary, Germany, Slovakia and the Czech Republic and much of the rest of Europe since then, and can move quickly, with up to four generations a year.  It is obviously an introduction from somewhere else, though its native home is unknown.  Curiously, an alien fungus, the Powdery mildew Uncinula flexuosa, has also been found recently on Horse chestnut in Europe. It was found in England in 2000, including Elmbridge, and is now quite common at least in the South.

BRIAN SPOONER