Back from "extinction", what's going on with the Alder Leaf Beetle? More...

The Alder Leaf Beetle, Agelastica alni, is a dark metallic blue beetle measuring around 7mm in length. It feeds mainly on Alder but is occasionally found on other trees such as Beech, Hazel, Hornbeam and others.  The beetles and their black caterpillar-like larvae can cause a lot of damage to the foliage of affected trees. By the 1960's it was considered extinct in the UK, but was rediscovered in 2004 and since then has become increasingly common. The Royal Horticultural Society recommends ... leaving it alone and letting natural predators sort it out (well done RHS!), or if you're obsessed, picking them off by hand (good luck with that one)! 

So why the huge population swings in this species? We don't really know. It has been suggested (without much evidence) that its decline was because of fewer undisturbed Alder forests. But why has it come back? Not because there are more Alder trees. It's tempting to say "climate change" and leave it at that, but while the weather is probably a factor it is rarely the complete answer when species undergo population swings such as this. Usually rapid expansions in insect species are down to changes in feeding behaviour, such as the ability to use more host plants - which fits with this species. Other, less obvious changes could also be possible such as various anti-predator adaptations or behaviour. The most likely explanation is "all of the above". 

It is helpful to get your records for this beetle.  Good quality photos are required to distinguish it from other Leaf Beetles (Chrysomelidae) and as ever, as much additional information as possible, such as the plant the insect was found on.