Alder leaf beetle
The metallic blue alder leaf beetle (Agelastica alni) feeds on the leaves of alder trees. It has become common in parts of England and Wales since 2004.
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), hornbeam (Carpinus betulus) and apple (Malus). 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 northern England and has spread into north Wales. In 2014 it was discovered in Hampshire and it is now widespread in the south east and spreading into the midlands. It may become widespread across much of England and Wales. 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. Although the feeding 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. Although damage on established trees may look severe the trees will usually survive with no long term effects on health or vigour. When choosing management options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section and avoiding pesticides. Within pesticides the shorter persistence products (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. Pesticide treatments are likely to kill natural enemies and are only likely to be successful if the entire plant can be reached.
- 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
The RHS recommends that you don’t use pesticides. Most pesticides (including organic types) reduce biodiversity, including natural enemies, impact soil health and have wider adverse environmental effects.
Where you cannot tolerate alder leaf beetle, manage them using the information above as your first course of action.
Pesticide treatments are likely to kill natural enemies and so reduce the likelihood of natural control and can lead to resurgence of the target animal.
The pesticides listed are legally available in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally.
Always follow the instructions on the products. For edible plants, make sure the food plant is listed on the label and follow instructions on maximum number of applications, spray interval and harvest interval.
Homemade products are not recommended as they are unregulated and usually untested.
Be aware that products such as Neem oil are not registered for use in the UK and we cannot advise on their use.
Plants in flower must not be sprayed due to the danger to bees and other pollinating insects.
- 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 Ultra 2, Neudorff Bug Free Bug and Larvae Killer) although broad spectrum have a very short persistence and so may require reapplication to keep beetle numbers in check
- Further information about the use of pesticides is available on the pesticides for gardeners leaflet
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
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