Epinotia and fir trees: native species, new damage – investigation into a recent imbalance

On a landscape scale, the damage is striking: defoliated fir stands, slopes where brown replaces the usual green of fir trees and, in places, loss of growth and increased mortality. These symptoms were first reported in 2017 in the Lachens massif by the Department of Forest Health (DSF) and its network of observer correspondents. The investigation quickly identified the culprit: a small caterpillar, E. subsequana, which eats the needles from the inside.

At the individual tree level, the tops of the attacked trees show clear signs: browned needles and premature needle drop. The initial diagnosis led the DSF to call on the URFM (INRAE) to develop a scientific response: what is the extent and spatial distribution of the damage? What are the seasonal and interannual dynamics of E. subsequana, and how does its biology interact with its host? Finally, how can the exponential increase in damage be explained: role of climate change, imbalance in predator and parasitoid communities, interactions with drought and other stressors?

For forest users, dead needles on the ground are evidence of damage. Their accumulation is likely to increase the fuel load (continuity and dryness of leaf horizons) and therefore raises the question of a potentially aggravating interaction with the risk of fire.

To answer these questions, a research and management network is being set up: INRAE, DSF, ONF and CRPF are conducting the first campaigns, and as the epidemic progresses northwards, new partners such as the Mercantour National Park and Natura 2000 site managers are joining the monitoring and data collection efforts. In the field, technical teams and researchers coordinate standardised protocols to feed into a shared database.

Need for studies and perspectives

Current knowledge remains fragmentary: there is an urgent need to quantify the damage precisely, characterise the biology and phenology of E. subsequana in these new conditions, and identify the triggering factors (climatic variations, interactions with drought, physiological weakening of trees, role of predators and parasitoids, etc.). Modelling work is already underway as part of Victor Fririon's post-doctoral research, and a thesis project will aim to understand the interaction between the physiological state of the tree and its vulnerability to the insect.

In summary, E. subsequana is a native species whose recent impacts highlight the complexity of forest responses to global change: better characterising this emergence requires joint efforts in monitoring, experimentation and modelling — and enhanced collaboration between managers and researchers to protect fir trees in the long term.

To learn more about our initial work on the biology of E. subsequana and our laboratory breeding programmes: https://theconversation.com/la-mineuse-du-sapin-une-toute-petite-chenille-qui-sen-prend-au-roi-des-forets-246349

We would like to thank all those who participated in this 2025 campaign and the upcoming follow-ups!

Translated with DeepL.com (free version)