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Reader's question: What ghastly larvae have covered Kadriorg Park's trees?

A tree near the Kadriorg stadium has been completely covered with silken webbing this week. Juhan Javoiš, an entomology researcher at the University of Tartu, said the phenomenon has been caused by the ermine moth.

Ermine moths are common fruit tree pests in Europe. Javoiš said the Yponomeutidae moth family contains many species and the two most prominent types in Estonia are bird-cherry and apple tree ermine moths. These insects feed on specific tree species and, as a rule, they do not attack different species.

For apple growers, Javoiš says, the apple ermine moth can cause havoc.

"As you can see from the pictures, they may leave the tree entirely defoliated and it might produce less fruit for several years as such an attack puts it under considerable stress," said the researcher. Although the trees may become stressed by moths voracious feeding, they are not likely to die.

The ermine moth lays its eggs on tree branches in the autumn so that larvae can start feeding in early spring directly off the leaf buds, which is also where they are usually found. The eggs are so tiny and well-camouflaged that it is difficult to spot them on the tree, said Javoiš.

Depending on the year, different numbers of moths appear. "I haven't noticed exceptionally many of them this year," he said. In the first half of the summer, the feeding off trees reaches its peak and thereafter apple trees are seldom left completely defoliated. However, he says, the bird-cherry ermine moth might leave its trees completely bare.

Scientists are not quite sure why some years have many more ermine moths than others. "One might think that if they are prolific in one year, there might be even more of them in the following year, but that's not always the case", said the researcher.

Javoiš said that, interestingly, even though two trees can stand close to each other, one of them can be covered in moths while the other will be completely unharmed. "We don't know exactly how and by what means these insects decide that one tree is better suited to their needs than the other. And I wouldn't dare to say that any tree is absolutely protected from the moths," said the researcher.

Ermine moths are difficult to deal with because you might not even realize they are there until they start bustling about on the tree.

If the owners of trees inspect their property often, they might very well notice the moths. Before attempting to feed on a leaf, larvae will glue the leaves together and that is how one could tell that the pest is active on that tree. Probably not before.

A simple method to get rid of the pest, Javoiš said, is to cut off the nest from the tree whenever you spot one. "Hopefully, in that way, not too many moths grow large enough to destroy the leaves in large quantities. This is probably not feasible on an industrial scale, but for handling a couple of trees it is not too difficult," said the researcher.

While it is often possible to fight pests by triggering other species to eat them, with ermine moths this is difficult to achieve. "Birds don't like feeding on them and so they reproduce in abundance," said the researcher. Perhaps, the reason is that the ermine moth contains defensive chemicals that make them distasteful to birds.

"They are not palatable to humans either, and eaten in large quantities, who knows, they may cause digestive problems. While the production of human food from insects is currently hampered by European regulations, work is underway to comply with the norms and it is probably inevitable in the foreseeable future that we will feed on them," says Javoiš.

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Editor: Kristina Kersa

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