Sandy’s Garden

Azolla Weevil

The list of imported plants that are now presenting problems in the United Kingdom is a long one, although not too many of the names are familiar to the average gardener. However, most readers will recognise some of these names: Giant hogweed, Himalayan knotweed, Indian (Himalayan) balsam, Japanese knotweed and Rhododendron ponticum. Virtually all of them were introduced into these islands by well-meaning horticulturists as ‘exotics’ … non-native plants that promised to make their gardens or their estates the envy of their neighbours, by reason of the size, or the flower colours, the foliage or the fast growth of these incomers. However, the introduction of these exotics took no account of the balance of nature, a natural balance in which predators, either animal, vegetable, insect or biological, keep native plant species in check. These problem plants had no natural enemies when they were brought here; and they have shown a real penchant for taking over their environment.

And here’s a name to add to the list, a name that may be new to many readers but will be familiar to anglers and water managers … the fairy fern or floating water fern (Azolla filiculoides). As its name suggests, it is an aquatic plant with delicate fern-like foliage. It comes originally from America and was introduced into the United Kingdom around 1840 as … had you guessed? … an ornamental garden aquatic. But it found the British Isles very much to its liking, soon escaped from ornamental ponds into the wild and, with the ability to double in size in less than a week during hot weather, forms mats on the surface of the water, blocking out light, killing water plants, hindering flood defences, interfering with aquatic sports like boating and angling, and reducing oxygen levels in the water, killing fish.

But Azolla is still being imported and sold by garden and aquatic centres around the country, despite being recognised as one of the most invasive plants in the United Kingdom. It is a very difficult plant to control, for it produces literally millions of tiny spores annually

which are released in Autumn and grow into new plants the following year. Since even tiny fragments of fronds left behind by any attempt to destroy the plants by mechanical means will soon form new plants; and since the only effective legal herbicides all contain glyphosate, which kills everything, the problem looks almost insuperable.

But help may be at hand, in the shape of a tiny weevil which is one of the Azolla plant’s few natural enemies. The Environment Agency recently issued a press statement saying that a colony of Azolla weevils has been bred in captivity and released into the River Till in Lincolnshire, where there is a blanket of fairy fern 2½ miles long, 50 feet wide and one foot thick. Environment Agency invasive species expert Trevor Renals said: “The weevils are real specialists and only eat water fern. They don’t harm other plants and often die out naturally once they have eaten their way through the Azolla. Thanks to this weevil, we are able to eradicate the weed without the need for dredging and chemicals.”

So that’s good news! But wait: as far back as May 2002, a research team at CABI Europe - UK, led by Corin Pratt, announced the start of a programme to deal with infestations of fairy fern by releasing Azolla weevils; and on 21 October 2004, seven years ago, the British Waterways Board invited journalists to join experts from CABI Bioscience to examine the results of a trial to control Azolla on a stretch of the Leeds & Liverpool Canal near Aintree, where thousands of captive-bred Azolla weevils had been munching through tonnes of the invasive Azolla filiculoides. Help may be at hand … but don’t hold your breath!

Sandy Simpson, Polmont Horticultural Society