Every nine years, on average, the tiny larch budmoth reproduces explosively. Its caterpillars strip larches until they lose the fight against their host tree and their natural enemies.
The larch budmoth (Zeiraphera griseana) finds its habitat on larches in the Engadin and other Alpine valleys in Europe. Its caterpillars feed on needle clusters, causing them to wither and turn yellow-brown.
Around every nine years, the number of caterpillars increases by a factor of up to 30,000. Infested trees flush new needles in summer, but grow less. This can be seen in the much narrower tree rings.
Soon, however, the tide turns. In the following few years, trees flush their needles later, and produce shorter and less nutritious needles. Many caterpillars starve to death. In addition, more and more caterpillars are the victims of parasitic wasps. Within a few years, starvation and natural enemies thus regulate the number of larch budmoths, and tree growth returns to normal.
Using wood samples from living larches and beams from historic buildings, WSL tree-ring researchers have reconstructed these larch budmoth cycles in Valais spanning 1,200 years.
In 2018, after almost 40 years without a major outbreak, there was again a caterpillar infestation in the Engadine - the race of nature between larch and larch budmoth continues.