Recently, browning of alder leaves has been observed, caused by the larvae of the alder leaf beetle and the green alder leaf beetle. This does not harm the trees, and forest owners do not need to take any protective measures when they notice damage to alder leaves.

Currently, the alder leaf beetle (Agelastica alni) is frequently observed in Latvia, where it can be found in large numbers on alder leaves during the summer. These are small, dark blue beetles with bright, metallic-sheen elytra. The alder leaf beetle primarily feeds on alder leaves, but it also damages the leaves of birches, aspens, bird cherries, cherry trees, apple trees, plum trees, roses, and willows, and often gnaws at the bark of shoots.

The green alder leaf beetle (Melasoma aenea), also known as the shiny alder leaf beetle, is 6.5-8.5 millimetres long; it is oval and green, though blue specimens are occasionally found; the beetle has a metallic sheen and fine punctures. This pest is found throughout Latvia and damages grey alders, black alders, and birches; it is only harmful to young alder stands. Both the alder leaf beetle and the green alder leaf beetle are herbivorous and are not dangerous to humans.
https://www.vmd.gov.lv/lv/jaunums/alksnu-lapas-boja-lapgrauzi