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Slugs and Snails

(but not quite puppy dogs tails) – RHS ‘Top Ten’ Pests 2007
by RHS
February 3, 2008

Slugs and snails were the UK’s most troublesome garden pest of 2007, according to members of the Royal Horticultural Society (RHS). These slimy predators moved one place up on last year, just beating the recently established harlequin ladybird into second place. This announcement comes from the Entomology team at RHS Wisley, which has released their annual list of ‘Top Ten’ problematic insects and other garden pests, following twelve months of enquiries from RHS members.

The RHS offers a unique advice service to all its members. Gardeners can write, email, phone, or visit one of the four RHS gardens (Wisley, Hyde Hall, Rosemoor, Harlow Carr), to discover the answer to any horticultural question that is plaguing them. Last year the RHS Advisory Service, of which the Entomology department is part, answered over 50,000 member queries - 3,000 of which were solely pest related. Once they have identified the pest, the Entomology department can then offer the gardener a solution to their problem and advise them on the necessary action.

The wet and cloudy summer of 2007 played a big part in bringing slugs and snails into people’s gardens, where they wreaked havoc among the foliage and flowers of established plants and killed seedlings along the way. Second place in the RHS ‘Top Ten’ went to the highly publicised harlequin ladybird, which became established in south east England in 2004. The harlequin ladybird has a voracious appetite and both the adults and larvae eat aphids and other insects, including beneficial predators, such as native species of ladybirds, hoverfly larvae and lacewings.

Andrew Halstead, Principal Entomologist for the RHS said, “The harlequin ladybird rapidly established itself in England and there is no possibility of eradicating it or preventing its further spread. Gardeners need to take a pragmatic view and accept the fact that the harlequin ladybird is here to stay. Its food of choice is greenfly and other aphids, so it is going to help gardeners to control these troublesome pests.”

New entries into the ‘Top Ten’ can be found at numbers six and seven respectively. The rosemary beetle and berberis sawfly are relatively new pests in the UK, both of which have become widespread in England during the last ten years. However last year’s number one pest was another newcomer, the horse chestnut leaf-mining moth, which was first detected in Wimbledon in 2002. In 2007 this pest dropped to number 15, despite becoming more widespread. Andrew Halstead said, “Horse chestnut leaf miner causes severe browning of the foliage by late summer on common horse chestnut, Aesculus hippocastanaria. It was no less damaging in 2007 but probably produced fewer enquiries as more gardeners became familiar with the effects of this insect.”

The advisory service is just one of the benefits to becoming an RHS member; and as a charity, the RHS is committed to helping people of all ages and abilities to get more from their gardening. The RHS also provides a wealth of information and advice free to all on its website and at its shows and gardens: visit www.rhs.org.uk for more information.

Description of 2007 pests

1. Slugs and snails (various species). In a wet year, such as 2007, it is not surprising that slugs and snails were the runaway number one problem. Most damage occurs during spring to autumn, affecting many ornamental plants and vegetables, especially potato tubers and narcissus flowers.

2. Harlequin ladybird (Harmonia axyridis). This foreign ladybird was unknown in Britain until 2004 but has since spread rapidly throughout England and into Wales. It is not a plant pest but causes concern because of its reputation for eating native ladybirds and other aphid predators. Its prey of choice, however, is greenfly and other aphids, so it is of some benefit to gardeners. It can feed on a wide range of other insects when aphids are in short supply but it remains to be seen whether harlequin ladybirds will reduce the numbers of native beneficial insects. Harlequin ladybird likes to overwinter inside buildings and this habit helps to bring it to public attention. For a distribution map of this ladybird, see www.harlequin-survey.org.

3. Vine weevil (Otiorhynchus sulcatus). The adult beetles eat notches in the leaf margins of a wide range of plants. The larvae feed on plant roots, especially those being grown in pots or other containers. It is one of the few pests capable of killing plants and it is a top three enquiry in most years.

=4. Cushion scale (Chloropulvinaria floccifera). This sap-feeding insect is mainly found on camellia but also attacks the foliage of other evergreen shrubs, including holly, Euonymus japonicus, Trachelospermum, Rhododendron and Choisya. The foliage becomes heavily coated over the winter months with sooty mould that grows on the pest’s sugary excrement.

=4. Ants (mostly Formica and Lasius spp.). Ants tend to be most abundant in sunny gardens with well drained soils. They cause little direct damage to plants but the soil excavated from their nests can be a nuisance in lawns, on patios and in flower beds where low-growing plants may become partly buried. Although there are insecticides for ant control, it is difficult to eliminate nests from gardens and so the presence of ants has to be tolerated.

6. Rosemary beetle (Chrysolina americana). Since its establishment in the London area in the mid 1990s, rosemary beetle has become widespread in England. Both the adults and larvae eat the foliage of rosemary, lavender, sage, thyme and Perovskia. Feeding takes place from late summer to the following spring, giving heavily infested plants a shabby appearance by late winter.

7. Berberis sawfly (Arge berberidis). This is another recently established pest that has spread rapidly in England since its arrival in about 2000. The caterpillar-like larvae feed on the foliage of some deciduous Berberis spp. and mahonias. Berberis thunbergii is particularly susceptible and can be completely defoliated. This is the first time this pest has featured in the RHS top 20 pest enquiries.

8. Rabbits (Oryctolagus cuniculus). Rabbits eat the foliage of many low-growing ornamental plants and vegetables. They also damage the bark of trees and shrubs, sometimes fatally, especially in hard winter weather when other plants are frosted or snow-covered.

9. Lily beetle (Lilioceris lilii). Enquiries about this pest were down on previous years, when it has regularly been in the top five. Lily beetle is a voracious pest of lilies (Lilium species) and fritillaries (Fritillaria species), with both the larvae and adult beetles devouring the foliage during spring and summer. Lily beetle is continuing to spread, although it remains a local problem in Scotland, Wales and Northern Ireland.

10. Cypress aphid (Cinara cupressivora). Die-back of conifer hedges of Leyland cypress, Cupressus, Chamaecyparis and Thuya spp. has various causes, including infestation by cypress aphid. The aphids suck sap from the stems and cause a gradual yellowing and drying up of the shoot tips. This can result in large sections of a hedge becoming mostly brown by late summer.

About the RHS

The RHS believes that gardening improves the quality of life and that everyone should have access to great garden experiences. As a charity we help to bring gardening into people’s lives and support gardeners of all levels and abilities; whether they are expert horticulturists or children who are planting seeds for the very first time.

RHS membership is for anyone with an interest in gardening. Support the RHS and secure a healthy future for gardening. For more information call: 0845 130 4646, or visit www.rhs.org.uk

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