Nature is full of fascinating animals with special names. Take the letter setter, a small beetle that causes a big havoc in the Drenthe forests.
The letter setter is a small, dark brown torret with a hairy body. With a length of only four to six millimeters, it is easy to overlook. But make no mistake: this mini beetle is a real tree killer. Especially fine spruce must believe it. Weak trees, affected by drought or storm damage, are put on the menu by the letter setter without mercy.
First the males drill their way inside and dig a mating room. Then they separate a chemical to lure females. Once inside, the ladies go hard to work: they dig long corridors under the bark and lay their eggs there. The larvae that come out eat their way through the wood and thereby make crosses. After six weeks the larvae are fully grown, they drill their way outside and fly to the next tree to start the tune again.
The name ‘Letterzetter’ almost sounds like a medieval profession. And actually that is not surprising. The corridors that dig the beetle and his larvae under the bark form a pattern that is reminiscent of old printing letters or handwritten texts.
If you have ever seen a affected tree, you know what we mean: winding, graceful lines that run through the entire inside of the bark. It almost seems like a printer has gone over it with a letter cupboard.
Some people think that the letter setter is an invasive exotic, just like the infamous tiger mosquito. But no, this beetle was born and raised here. “The letter setter is not an invasive exotic. He is native and just belongs here,” Bosecologist Ronald Sinke said before. The beetle did not come with imported fine spruce, but always lived here.
That does not mean that he does not cause any problems. “When trees get into stress, for example due to great drought, they are much more susceptible to a scourge like the letter setter,” says Hans Hasper of the Drenthe landscape. That was good noticeable in 2020, when years of drought weakened a lot of fine spruce, so that the letter setter spread at lightning speed. Forests in Drenthe were thinned by this.
This is also the case with the Asserbos. “The attack was so great that hundreds of fine spruce had to be felled. It didn’t make sense to leave it. They were dead,” says Hasper. The main reason that the letter setter could make his move so rigorously is because a lot of fine spruce used to be planted together.
This monoculture, as it is called, was planted in the past because it was useful for the logging. “The fine spruce were put together en masse,” explains Hasper. “Large boxes full of the same species remain vulnerable to pests and diseases. Just look at potato fields, for example, a lot has to be sprayed there to keep the plants healthy. That is also the case with fine spruce. If there are a lot together, the letter setter can easily spread.”
A large part of the Asserbos died in 2020. Those dead trees have been cut down and replaced by deciduous trees. “Beech, elm, maple, oak, everything mixed together. That keeps the forest healthier.” And with results, because in the Asserbos the letter setter is now much less a problem than five years ago. “There are it and there are still tufts, but if one is affected, the whole forest plot will not go on at once,” says Hasper.
Whether the letter setter is a real pest, that depends on how you look at it. It is a disaster for the wood industry and lovers of fine spruce. But from an ecological perspective, the beetle ensures rejuvenation of the forest. Weak trees die, creating space for new species. The next time you see a tree with mysterious patterns under the bark, you know that the letter setter has been working. And that he lives up to his name, like a tiny engraver in the wood.
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