Bark Beetle

Gallery systems

Female bark beetles bore through the outer bark of a suitable tree and excavate a mating chamber in the inner bark (phloem). Males enter, mating occurs, and the female extends the gallery, laying eggs in niches along the sides. Larvae hatch and bore their own lateral galleries outward, feeding on the nutritious phloem. The completed gallery system — a central shaft with lateral arms radiating outward — is species-specific and can be used to identify the species even without seeing the beetle.

The primary pest

Ips typographus, the European spruce bark beetle, is the most economically damaging forest insect in Europe. Under normal conditions it kills only stressed and storm-damaged spruce. But warming temperatures and increasing summer drought stress trees, reducing their ability to produce defensive resin. Once this threshold is crossed, beetle populations can erupt — a single stressed tree produces pheromone signals that attract hundreds of beetles, overwhelm the tree’s defences, and kill it. Neighbouring trees are then attacked in turn.

Climate and outbreaks

Climate change has dramatically increased bark beetle outbreaks across Central and Northern Europe. The record outbreak of the late 2010s killed over 180 million cubic metres of spruce in Germany, Czechia, Austria, and Scandinavia — the worst forest destruction in European history. Warmer winters allow more beetles to survive; longer, hotter summers stress trees and allow additional beetle generations per year.

Fungal partners

Many bark beetles carry wood-staining fungi in specialised structures (mycangia) that they inoculate into the tree as they bore. The fungi kill trees more quickly by blocking water transport, and later serve as food for developing larvae.