Alder leaf beetle

The metallic blue alder leaf beetle (Agelastica alni) feeds on the leaves of alder trees. It has become common in part parts of England and Wales since 2004.

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Adult blue alder leaf beetle

Quick facts

Common name Alder leaf beetle
Scientific name Agelastica alni
Plants affected Alder (Alnus) and some other deciduous trees
Main symptoms Holes in leaves, presence of blue beetles
Most active April-July

What is alder leaf beetle?

There are about 250 species of  leaf beetle (family Chrysomelidae) found in Britain, they range in size from 1 mm to 18 mm. The family contains many metallic and attractive species and all feed on plants (herbivores). 

Alder leaf beetle is an 7-8 mm long dark metallic blue leaf beetle that feeds on alder (Alnus) and is occasionally found on other deciduous trees such as beech (Fagus sylvatica), hazel (Corylus) and hornbeam (Carpinus betulus). It overwinters as adults which emerge in the spring, sometimes in large numbers.

The black caterpillar like larvae also feed on the leaves of alder and other trees and reach 11 mm in length.  Larvae can be found on the leaves in spring and summer. The beetle has one generation a year. Adults emerge from soil and leaf litter where they have been overwintering in early spring, they are winged and capable of flight. New generation adults can be found from mid summer, although may enter a summer diapause (aestivation).  

Alder leaf beetle was considered extinct in Britain with almost no records of it between 1946 and 2003. In 2004 larvae and adults were found in Manchester. It is not known how the beetles reached Manchester, but it is possible they arrived with plant imports. The beetle is now widespread in north-west England and has spread into north Wales. In 2014 it was discovered in Hampshire and it is now widespread in the south east. In some areas this beetle has become very abundant and can cause significant defoliation.

Control

It can be impossible to control alder leaf beetle particularly on taller trees. Fortunately, although the damage they cause can be unsightly, it is something that the trees will survive and the beetle can be tolerated. 

Check susceptible trees frequently from spring onwards so action can be taken before a damaging population has developed. When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce the damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.

Non-Pesticide control

  • Where possible tolerate populations of plant feeding beetles
  • Remove beetles by hand where practical
  • Encourage wildlife in the garden, many animals including birds, frogs and predatory insects such as ground beetles and social wasps will eat the larvae and sometimes adult beetles.

Pesticide control

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.
  • On trees that are too tall to be sprayed thoroughly there are no pesticidal control options
  • Pesticides are likely to be more effective against larvae than adults
  • Only consider treating populations on very young trees where significant defoliation may occur
  • Organic contact insecticides containing natural pyrethrins (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer). Several applications of these short persistence products may be necessary to give good control
  • More persistent contact-action insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
  • A broad spectrum systemic insecticide containing the active ingredient Flupyradifurone (Provanto Smart Bug Killer) is available for use on ornamentals and selected edibles
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
Plants in flower should not be sprayed due to the danger to pollinating insects
Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

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Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

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