Agelastica alni (Linnaeus, 1758)
Alder Leaf Beetle

This widespread and locally common species occurs throughout Europe north to the UK and Fennoscandia, it is also present on most of the Mediterranean islands and extends east through Asia Minor, Russia, Ukraine and northern Kazakhstan into Siberia and is now established in the United States and Canada following introductions during the 19th century. It was formerly considered extinct in the UK; it was recorded a few times from widely scattered localities across England during the mid-20th century but following its rediscovery in 2004 in northwest England it has spread rapidly and it is now locally common and often abundant across the south of England and the midlands and it seems to be increasing in range and abundance. Adults are present almost year-round and are active over a long season from early spring until June or July and the next generation from July until the autumn although they may overlap, the usual host plants are common alder, Alnus glutinosa (L.) and grey alder, A. incana (L.), other broadleaf trees such as birch, hazel, willow and hornbeam only very occasionally host the species and in southern Europe they sometimes feed on the foliage of various fruit trees. They spend the winter among leaf-litter or moss close to host plants and emerge from late March or early April, maturation feeding begins immediately and they seek out freshly opening alder buds but if these are not available they will consume other foliage for a while, mating begins in April and females oviposit during May and June. Females attach groups of eggs to the underside of host foliage, usually between 50 and 70 eggs in each batch and each will lay between 200 and 250, when gravid they display physogastry, with the abdomen greatly swollen as also seen in Gastrophysa females. Larvae emerge 10-12 days later, they pass through 3 instars and are fully grown within 25-30 days; they feed communally but in the last instar disperse throughout the host plant, they are very distinctive and easily recognized; elongate and pale grey when small and shiny black as final instars, and during June they may be present with adults as well as batches of the bright-yellow eggs. Full grown larvae drop to the ground and construct elongate earthen cocoons just below the surface in which they pupate although they sometimes pupate among litter etc. and adults emerge after about three weeks over a long season from July to September. New-generation adults usually emerge and feed and so at this time foliage can become extensively damaged but early adults usually enter moss or litter and undergo a summer diapause in response to higher temperatures. Eggs and larvae may occur over a long season as development is strongly influenced by food supply and temperature, and in good years adults may appear in abundance. They are likely to occur wherever the hosts are abundant, usually in wetland marginal situations but also in permanently damp woodland etc., sampling is usually a simple matter of recognizing the feeding damage and then searching among foliage for larvae or adults but random beating or sweeping will sometimes produce them in numbers.