Viburnum beetle is RHS's worst garden pest

The Royal Horticultural Society (RHS) has announced its top 10 pest-related enquiries received by its advisory service.

Viburnum beetle is RHS's worst garden pest - image: FlickR/Gnosticgardener
Viburnum beetle is RHS's worst garden pest - image: FlickR/Gnosticgardener

In most years slugs and snails have topped this list but in 2010 RHS members wanted to know more about problems associated with the viburnum beetle.

Viburnum beetle eats the foliage of various viburnums commonly grown in gardens, especially the evergreen shrub Viburnum tinus and the deciduous Viburnum opulus, also known as guelder rose or snowball bush.  Adult beetles cause some damage in late summer but it is the grub stage in April-May that can cause severe defoliation.

Principal RHS entomologist, Andrew Halstead said:

‘’Viburnum beetles are in our top ten list most years but they do seem to have become more troublesome over the last decade.  The damage to evergreen viburnums is more apparent because it can be seen all year round.’’

Enquiries into two sap-sucking insects, cushion scale and horse chestnut scale, have increased. Cushion scale infests the underside of leaves on evergreen shrubs, such as camellia, rhododendron, holly and Trachelospermum. Horse chestnut scale is seen on the trunks of horse chestnut, lime, bay trees, sycamore and maples.

RHS members were also concerned about the glasshouse red spider mite, which sucks sap from a wide range of greenhouse and garden plants.  Halstead said: ’’The relatively hot dry summer last year meant that red spider mite become more of a problem on outdoor plants. However, a benefit of the dry weather was that it also restricted the activities of slugs and snails which prefer wet, cool weather.’’

The number one plant in terms of pest enquiries in 2010 was grass. This was followed by viburnums, roses, apples, fuchsias, lilies, plums, maple, pears and bay. Lawns are usually the number one problem because they have a wide range of pest problems, although most of the damage is caused by chafer grubs.

TOP TEN requests for pest information:

1.  Viburnum beetle (Pyrrhalta viburni). This pest has been climbing up the top 10 ranks in recent years and makes the number one spot for the first time. The principal host plants are Viburnum opulus, V. lantana and V. tinus but some other Viburnum species may also be attacked.  Most of the damage is done by the larvae, which can reduce the foliage to lacework during May-June.  The adult beetles also eat the leaves in late summer but less extensive damage occurs at that time.

2.  Slugs and snails (various species).  In most years, slugs and snails are the number one problem in gardens and on allotments. The reduced number of enquiries in 2010 may be due to the long dry spell during the summer. Most damage occurs during spring to autumn, affecting seedlings, many ornamental plants and vegetables, especially potato tubers and narcissus flowers.

=3. Cushion scale (Chloropulvinaria floccifera). This sap-sucking insect occurs on evergreen shrubs, especially camellia, holly, rhododendron, Trachelospermum and Euonymus japonicus.  Although long established in Britain, it has become more widespread and troublesome over the last 20 years.  It excretes honeydew, causing infested plants to develop a thick black coating of sooty mould on their foliage over the winter months.

=3.  Chafer grubs in lawns (various species). Chafer grubs are the larvae of several species of chafer beetles.  Most of the damage in lawns is caused by grubs of garden chafer (Phyllopertha horticola) and Welsh chafer (Hoplia philanthus), which eat the roots.  During autumn to spring other animals, such as foxes, badgers and crows, rip up the loosened turf to feed on the grubs.
5.  Harlequin ladybird (Harmonia axyridis), This foreign ladybird was unknown in Britain until 2004 but has since spread rapidly throughout Britain.  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.  Adult harlequin ladybirds are highly variable in appearance, with many different colour forms and variable numbers of spots and other markings. It over-winters as adult beetles and likes to do so inside buildings; this habit helps to bring it to public attention.  

6. Vine weevil (Otiorhynchus sulcatus), The adult beetles eat notches in the leaf margins of a wide range of herbaceous plants and shrubs.  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 in most years is a top five enquiry. These days, the majority of vine weevil enquiries concern foliar damage by the adult beetles rather than the grub stage.  This may reflect the better control options currently available to amateur gardeners for dealing with vine weevil grubs.  Another contributing factor is likely to be the establishment and spread of several other Otiorhynchus spp. in Britain in recent years.  These cause identical patterns of leaf damage that cannot be separated from that caused by adult vine weevil.
7.  Lily beetle (Lilioceris lilii), Although established in England since 1939, this pest of lilies (Lilium spp.) and fritillaries (Fritillaria spp.) did not spread out of the south east counties until the 1980s.  It now occurs in all English counties with a more scattered occurrence in Wales, Scotland and Northern Ireland.  Both the larvae and adults eat the leaves and can cause severe defoliation.

8. Horse chestnut scale (Pulvinaria regalis),  This sap-sucking insect occurs on various trees and shrubs, including horse chestnut, sycamore and maples, lime, magnolia and bay trees. It becomes noticeable in early summer, when the mature scales deposit white egg masses on the trunks and larger branches of infested trees.  A common and spreading pest, especially on trees growing in streets and other urban situations

9.  Glasshouse red spider mite (Tetranychus urticae),.  A wide range of glasshouse plants and house plants is attacked by this small sap-sucking pest.  In late summer, damaging infestations can also build up on garden plants, especially in hot dry summers.  Vulnerable plants include cucumber, runner beans, strawberry, raspberry, fuchsia, rose, Brugmansia, Crocosmia, orchids, oleander and dahlia.

10. Ants (mostly Formica and Lasius spp.), Ants are often 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 should be tolerated.

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