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. 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.