Allium leaf miner hits RHS top 10 pests

A pest that bores into the stems of leeks, onions and garlic leaving plants open to infections makes its first appearance on the Royal Horticultural Society's annual list of the top 10 garden pests for 2014.


Allium leaf miner, which was first detected in Britain in 2002, is placed seventh on the list of the most frustrating pests. The small white headless maggots of allium leaf mining flies bore into the foliage and stems of their host plants rendering them inedible. When fully fed the maggots turn into brown pupae which can often be found in the stem.

First found in the West Midlands more than a decade ago, the flies are becoming established in new areas throughout Britain and are a serious threat to plants in the onion family (Alliaceae) such as leeks, onion, chives, shallot and garlic. This pest is likely to continue to spread to new areas over the coming years, and so may remain a top pest. Gardeners can protect plants from allium leaf miner by growing crops under horticultural fleece.

Rosemary beetle (5th), which feeds on the foliage of aromatic plants including rosemary, lavender, sage and thyme, and tortrix moth (=8th) both made a comeback to the top five after an absence of several years. Rosemary beetle previously appeared in the top half of the table in 2005, when it occupied fourth position, while tortrix moth has not been placed in the top 10 in more than a decade.

Initially found mainly in London in the late 1990s, rosemary beetles have become widespread in England and are now established in parts of Scotland and Wales.

Tortrix moth caterpillars feed on fruits and flowers and are found both indoors and outdoors. Often unseen while feeding, the caterpillars usually bind the leaves of plants together with silky threads and feed within this shelter. Damaged areas of the plant dry out and turn brown.

Slugs and snails again topped the list of the most troublesome pests, as they have done for eight of the past ten years.  

Damage from slugs and snails can occur all year round, affecting seedlings and many ornamental plants and vegetables. Plants often affected include potato tubers, hosta leaves and narcissus flowers.

Mice and voles, about which enquiries hit a 24-year high in 2013, slid down the table in 2014 to joint 8th place. These small rodents pose a number of challenges to gardeners: the tunnelling activities of voles can disturb the roots of plants and disfigure lawns, and they gnaw the bark from the roots and stems of tree and shrubs.

Mice can be a problem in storage areas, and in the garden they take fruit and chew off seedlings. Peaks and troughs in mice and vole populations are thought to be due to changing food supplies and the prevalence of predation and diseases.

Two pests outside the top ten, but which frustrated an increasing number of gardeners, were box tree caterpillar and glasshouse thrips.

Box tree caterpillar is a relative newcomer to the UK with gardeners first reporting the larvae in 2011. Originating in East Asia, box tree caterpillars can grow up to 3cm (1.25in), can completely defoliate box (Buxus) plants.

Glasshouse thrips, have been become an increasing problem outdoors and appear to be thriving in sheltered urban gardens, attacking a range of garden plants, particularly Viburnum, causing a silvery discoloration of the upper leaf surface.

RHS Senior Entomologist Dr Andrew Salisbury said: "The RHS saw a 42% increase in the number of enquiries it received about pests in 2014 compared with the previous year. One of the possible reasons for the increase could be the great gardening year we have enjoyed, which meant that more people were spending more quality time in their gardens and noticing the effects of pests.

"It is interesting that pests affecting food crops have been very prominent in the 2014 list. Perhaps this reflects a trend for gardeners to set aside some space to increase their food self-sufficiency and grow crops. However, regardless of the reason, the RHS will continue to monitor the spread of pests, work to identify suitable control methods and provide advice for gardeners on how to deal with the problem of pests."


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