Alder leaf beetle

The metallic blue alder leaf beetle (Agelastica alni) feds on the leaves of alder trees. It has recently become re-established in some parts of England after an absence of more than 60 years.

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

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What is alder leaf beetle?

Alder leaf beetle is an 7-8 mm long dark metallic blue beetle that feeds on alder (Alnus) and is occasionally found on other deciduous trees such as beech (Fagus sylvaticus), hazel (Corylus) and hornbeam (Caprinus 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 from May to July. 

This species was considered extinct in the UK with almost no records of it between 1946 and 2003. In 2004 larvae and adults were found in Manchester and it is now spreading in north-west England. In 2014 it was re-discovered in southern Hampshire (Southampton) and is also spreading in that county. In some areas this beetle has now 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.

Chemical control

  • On trees that are too tall to be sprayed thoroughly there are no chemical control options
  • Pesticides are likely to be more effective on larvae than adults
  • Only consider treating heavy infestations which are likely to cause significant defoliation
  • Organic insecticides containing natural pyrethrins (e.g. Bug Clear Gun for Fruit or Defenders Bug Killer, ecofective Bug Killer (also contains fatty acids)). Several application of these short persistence products may be necessary to give good control
  • Synthetic pyrethroid pesticides such as lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), or deltamethrin (e.g. Bayer Sprayday Greenfly Killer) can be used
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) can also be used 
  • 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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