The fir leafminer, a tiny caterpillar that attacks the king of the forests

Did you know that a tiny caterpillar could change the future of our trees? The needle miner is proliferating today under the effect of global warming, and turns the needles of the king of the forests red, which then end up falling. Scientists, by studying its life cycle, hope to understand its spread before it affects more French forests… and your Christmas trees

Discreet for years, the needle miner (Epinotia follows), a very small green caterpillar a few millimeters long, is now causing a lot of trouble for forest managers, particularly in the fir forests of south-eastern France and the Southern Pre-Alps. Observers from the Forest Health Department noted massive defoliation there.

As a “miner,” this insect tunnels inside needles to feed, leading to defoliation and weakening of trees. Although this insect is indigenous, that is to say naturally present in the region for a long time, its populations are now expanding under the influence of climate change.

This small caterpillar represents a new threat to silver fir trees (Abies alba), an emblematic tree of these regions.

A native species that swarms

The first outbreaks ofEpinotia follows were reported in the Aragonese Pyrenees, in Spain, between 1995 and 1997. These episodes, characterized by a rapid and massive multiplication of insect populations, followed episodes of intense drought. They caused significant dieback in the affected massifs during the 1990s.

Since this period, the needle miner has remained largely discreet, before reappearing in 2017 with new outbreaks in the fir forests of south-eastern France.

Although the leafminer has been spotted in other French regions, such as Île-de-France or the Massif Central, it is the fir forests of the south-east of France, and in particular the Southern Prealps, which have since shown 2017 worrying signs of repeated and intense attacks. Initially concentrated in the South, these outbreaks now extend towards the North-East in the Upper Var valley, affecting new stands of fir trees.

The expansion capacity of this insect and its proliferation potential represent a risk for other forest regions. Their precise impact on tree health remains poorly documented. What is certain is that the weakening caused by defoliation could increase the vulnerability of fir trees to other environmental stresses, such as drought.

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Furthermore, the defoliation caused by the pest also disrupts the microclimate of the fir forests. In this way, it causes a degradation of this ecosystem, a consequence which deserves attention, particularly in the context of climate change.

Read more: Preserving the future of our forests: what research can bring

Study the leaf miner to better understand and prevent its attacks

In order to better understand the life cycle of the fir needle miner, two INRAE ​​research units in Avignon – the entomology and Mediterranean forest experimental unit (UEFM) and the Mediterranean forest ecology unit (URFM) – have set up caterpillar breeding under controlled laboratory conditions. This breeding proves essential, because the biology and impact of the fir needle miner remain poorly documented in the scientific literature.

Currently, the information we have about its life cycle only reveals overall information about its development.

The fir needle miner, Epinotia followsis a small butterfly of the Tortricidae family which measures up to 13 mm as an adult. Each year, the adults emerge from the ground at the end of April and their flights continue until the beginning of June. During this period, females lay between 4 and 12 eggs on the upper branches of fir trees.

It is in the larval stage (caterpillar) that this insect becomes a real defoliator. The young caterpillars, reaching 6 to 7 mm, penetrate the needles through their underside to feed there by digging galleries. After their second molt, they also attack older needles.

At the end of summer, the caterpillars drop to the ground on a silk thread to bury themselves in the top few centimeters of soil and transform into chrysalises. After spending the winter in the ground, they give birth to new butterflies the following spring, mainly in May.

Breeding this insect in the laboratory should allow research teams to fill some of these gaps by collecting reliable and detailed data on the development, life cycle and behavior of the insect.Epinotia follows.

Maintained in a stable environment (temperature of 19°C and humidity of 70%), the pupae were collected last spring in infested areas and placed in emergence cages (kind of small aviaries). The research teams were thus able to observe the complete development of Epinotia followsby studying all the stages of its life cycle, its average lifespan in captivity and its feeding behavior.

This laboratory monitoring made it possible to collect valuable data. A total of 465 butterflies emerged, with peak emergence observed on the fourth day. The average lifespan of adults in captivity has been measured at 12 days, with a balanced ratio of male and female births.

These data provide a solid basis for future research into the life history and population dynamics of this species.

Read more: What coded messages do butterfly wings carry?

The leaf miner not only makes holes, it also “nibbles”

The breeding set up made it possible to clarify the different types of damage caused by the feeding of the fir leaf miner. The majority of the caterpillars showed miner behavior, digging a hole in the needle to feed by forming a gallery. But some also caused “nibble” type damage, attacking the needles from the outside.

Observations carried out in the field made it possible to assess the extent of the damage caused by the needle miner after an epidemic phase. Initial results reveal that more than half of the needles of the attacked trees have been affected, with rates that can exceed 60% in some cases. These figures highlight the significant impact of these outbreaks on the health of fir forests.

The observations made during breeding made it possible to better describe this damage and to adjust the methods for monitoring attacks in silver fir stands.

Two types of monitoring are thus planned to better understand the dynamics of attacks.E. following.

• The first will consist of monitoring the geographic expansion of leaf miner populations towards the North-East, beyond its initial home in the Préalpes d'Azur park.

• The second will be dedicated to populations hitherto spared from proliferations, notably those of Mont Ventoux, in order to identify the biological and climatic factors favoring such proliferations.

Although the parasitoids ofE. following (that is to say, the organisms capable of developing to the detriment of the caterpillar and thus regulating its populations) are not yet known, the breeding carried out this year made it possible to highlight promising avenues. Hymenoptera of the family Eulophidae could play a role as a parasitoid in the natural regulation of leafminer populations.

Thus, this study shows that it is possible to develop strategies to preserve the resilience of French fir forests in the face of growing threats, both from pests such asE. following than those linked to drought and other climatic factors.