The bird-cherry ermine is rampant in Bergen
This year, ghost trees have again appeared in the forests in and around Bergen. Ten to twelve years usually pass between each time the bird-cherry ermine attacks the wild cherries. The last time they were here in such large numbers was in 2007.
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At the end of May, the bird-cherry is in full bloom with green and spring-fresh foliage. But gradually a change takes place, the trees lose their green color and are covered in a ghostly web-like silk. The bird-cherry ermine has struck again. The larvae have been on the tree for a while eating, but the first clear signs of the bird-cherry's transformation occur at the beginning of June. Beyond June, the spindly becomes more and more dominant, and eventually there is hardly a green leaf left. Some trees are only partially attacked, while others are completely transformed and become completely covered in a huge silk web. The trees that are most attacked eventually take on a ghostly appearance where the branches and trunk are completely wrapped in a gray silk web.
Why does the caterpillar make the silk?
It should primarily serve as a home and help the larva to move in the foliage between the branches. The spinning contributes to a cooler climate like in a small greenhouse, and it has been proven that the temperature on the inside is a couple of degrees higher than on the outside. The elastic web protects the larvae from birds and other predators. The protection is probably effective in that the birds apparently do not allow themselves to be tempted by the multitude of larvae, and the larvae thus do not need camouflage color to avoid being eaten. Parasitic wasps are reported to be the one of the predators that is perhaps the most effective at beating back the moth, and keeping it at bay for many years at a time.
But everything has an end, so also the feast of the caterpillars. When they have no more bird-cherry-leaf to graze on, they enter the pupal stage. The pupal stage begins a little towards the end of July and lasts approx. 4 – 6 weeks, but the length depends on the weather and is extended at low temperatures.
It is hard to imagine that the tree could have survived. But then, immediately after the larvae have begun to pupate, the foliage begins to bounce for the second time, and it is not many days before the tree has regained its green color. These trees may look somewhat reduced, and this lasts for the rest of the season. the bird-cherry recovers well through attacks from the bird-cherry ermine, at least one or two years in a row. It is likely that several years in a row will weaken the tree so that the risk of dying increases.
The bird-cherry ermine hatches and takes to the wings in late summer, in August or the end of July. After a few weeks of swarming, it lays eggs in clusters on twigs near the buds of the bird-cherry, and the oval clusters of eggs are covered by a shell. The eggs normally hatch in autumn, and the small egg larvae overwinter on the tree in the shelter of the bark. Hatching can also take place in the spring for those eggs that overwinter. When spring rolls around, the larvae bore into the buds and are ready to start a new cycle.
The bird-cherry ermine (Yponomeuta evonymella) is a butterfly that belongs to the group of spider moths.
It got its name because the larvae weave themselves into a dense silk-like web. There are five species in the spider moth genus in Norway.
