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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
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.
Alder leaf beetle 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 widespread in north-west England. In 2014 it was re-discovered in southern Hampshire and is now widespread in that county and 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.
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Chemical labels explained
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RHS statement on pesticides in horticulture
Trees and shrubs: native to the UK
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