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Insect cycle leaves moth-eaten forests

Defoliated larch forest in the Italian Alps. Werner Baltensweiler

Every eight to ten years, Alpine larch forests are stripped of their foliage thanks to the activities of the larch budmoth.

This content was published on October 23, 2003 - 19:03

Using aerial surveys from the last 40 years, Swiss and United States researchers have shed new light on how and why this defoliation occurs.

They have shown that the population explosion and dispersal of the moths occur in waves.

“It’s a unique case,” said Werner Baltensweiler, formerly of the Institute of Entomology of the Federal Institute of Technology in Zurich.

“It’s one of the most regular periodic population dynamics, we know worldwide.”

Baltensweiler, who is now retired and lives in Hombrechtikon in canton Zurich, has studied the larch budmoth since 1954.

“The larch budmoth devastates the forest in cycles of eight to ten years and the forest looks as if it has been ravaged by fire,” he told swissinfo.

“There are so many larvae feeding on the large trees that the nibbled needles dessicate in the dry subalpine climate and turn red-brown.”

Cantons Graubünden, Ticino and Valais in Switzerland are particularly affected, as are parts of the Austrian and Italian Alps.

Life cycle

In the Engadine valley in canton Graubünden, the moth is active from August to October when it lays an average of 150 eggs, which hatch the following spring.

The young larvae feed on sprouting larch needles until late June or early July.

During the larval stages after feeding, they increase in size and moult four times, then descend on silken threads to the ground, where they pupate for three weeks.

The moths emerge from the soil at the end of July or beginning of August.

Every year, the population multiplies ten-fold. Theoretically, after five years, the population can have increased 100,000 times, and it is at this point that the larch trees are completely stripped.

The cycle breaks down the following year because once defoliated, the larch tree has no further mineral reserves in its branches and stem, and it only produces short, coarse, needles.

“The larvae have a tough time to survive in the year following defoliation because of the poor food quality,” said Baltensweiler.

Wave theory

Scientists say the browning of the forest occurs in "waves" as the larch budmoth population travels east to west in search of pure larch.

“These populations grow to high densities, first in the western Alps,” said Baltensweiler. “When the moths emerge from their pupa stage, they leave the defoliated forest and disperse.”

“We have quantified the moth flying all over Switzerland, going out east with the prevailing westerly winds and that is an important means of dispersal.”

The research is based on 28 maps showing larch defoliation in the European Alps from 1961 to 1998.

Baltensweiler mapped the defoliation in collaboration with the national forest services of France, Italy, Switzerland and Austria.

Threat to tourism

Research into the larch budmoth started after the Second World War.

“In 1947, the entire Engadine was all brown, all defoliated,” said Baltensweiler. “There was a great fear that tourists would not come back to the forest.”

Scientists were asked to control the insect with the pesticide DDT, which had just come on the market.

At the height of Swiss research into the larch budmoth, five scientists and about 30 other staff were engaged throughout the Alps. The Federal Institute of Technology in Zurich installed a field station in Zuoz, Engadine, which Baltensweiler managed.

Control operations were successful but due to the dispersal of the moths and the migration into green forests, populations immediately built up again and defoliation occurred the following year.

Walkers beware

“We just have to live with it,” said Baltensweiler. He added that because it was deciduous, the larch tree lost its needles in the autumn.

“Once the forest is defoliated, there are lots of larvae falling down from the tree, lots of frass (droppings) and lots of webbing which people walk into."

“From the forestry point of view, there is some reduction in growth of the tree if they have no needles, but it is very rare that the trees die - a few per cent only.”

However, Cembran pines, which grow beneath the taller larch trees, are killed by the activity in the canopy above them.

swissinfo, Vincent Landon

Larch budmoth

Every 8-10 years, the larch budmoth ravages Alpine larch forests.
Swiss and US researchers have shed new light on the pattern of defoliation.
Cantons Graubünden, Ticino and Valais are affected in Switzerland.

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