Alder leaf beetle

The metallic blue alder leaf beetle (Agelastica alni) feeds 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

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?

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 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 and it is now widespread in north-west England. In 2014 it was re-discovered in Hampshire and is now widespread in adjacent areas. In some areas this beetle has become very abundant and can cause significant defoliation.


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.

Pesticide control

  • On trees that are too tall to be sprayed thoroughly there are no chemical control options
  • Pesticides are likely to be more effective against larvae than adults
  • Only consider treating heavy infestations which are likely to cause significant defoliation
  • Organic contact insecticides containing natural pyrethrins (e.g. Bug Clear Gun for Fruit & Veg, Ecofective Bug Killer). Several application of these short persistence products may be necessary to give good control
  • More persistent contact insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer) and cypermethrin (e.g. Py Bug Killer)
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
  • Follow label instructions when using pesticides 
  • 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


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

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