At the end of August, birch stands across Estonia revealed a surprising and somewhat unsettling sight. Young and middle-aged birches that had regrown their green foliage by mid-summer were once again left with sparse, blackened or even completely bare crowns. In some places the damage was severe, in others milder, and it affected both downy and silver birches.

Just months earlier, the same trees had been devastated by a mass outbreak of the northern winter moth (Operophtera fagata). By June and July, however, their crowns had filled with leaves again. But not for long. In the second half of summer, another pest — rarely seen in Estonia until now — appeared: the birch sawfly (Arge pullata). The result was birch forests stripped of foliage or left with thinning crowns, resembling late autumn rather than the end of summer.

Bare birch stands and the synergy of pests

While the spring outbreak of northern winter moths left vast areas of birch forest completely bare, the damage caused by the birch sawfly this August was not as widespread. Still, it gave nature observers pause, as it revealed the growing pressure of a new pest rarely seen in Estonia until now.

The cycles of winter moth population surges are well known in Estonia — their mass breeding events periodically inflict extensive damage on broadleaf stands. Typically, birches can recover after spring defoliation, regrow leaves during the summer, resume growth and prepare for possible damage again the following spring. This time, however, their recovery period was cut short. By late July and August, the larvae of the birch sawfly had already launched a new assault. As a result of the combined impact of both pests, the trees lost their foliage twice in the span of a single growing season.

Such repeated defoliation in such a short time seriously weakens the vitality of trees and diminishes their ability to recover. Mass outbreaks of leaf- and needle-feeding pests are among the gravest threats to forests, as they reduce leaf area, disrupt metabolic processes and undermine overall tree health. Trees already weakened in this way are then far more vulnerable to secondary pest infestations and fungal diseases. It is also common for outbreaks of this kind to recur over several consecutive years, which can cause irreversible decline in tree condition.

From rare species to invasive

The birch sawfly is a widely distributed species, found from Western Europe to the Far East, including China and Japan. Mass outbreaks have been recorded in the forest-steppe zone, especially in southern Russia and Ukraine. In Central Europe, the birch sawfly has so far been found only in scattered small populations, though some larger-scale damage and mass outbreaks have been documented.

Until recently, the birch sawfly was rare in Northern Europe and was even listed in Finland's Red Data Book. In the early 2000s, it was thought to have disappeared from Finland altogether, until 2005, when a massive outbreak in South Karelia killed thousands of young birches. In Estonia, the species became more widely known during a localized outbreak in Tabivere between 2011 and 2014.

Now, a decade later, the same pattern is repeating in southeastern Estonia — only on a broader scale. In Sweden, the birch sawfly has been classified as an invasive species, as its range has expanded rapidly in recent decades.

A similar trend can now be observed in Estonia. While just a few decades ago it was rare and poorly understood, today's mass outbreaks and multiple centers of damage suggest that environmental conditions are shifting. Studies confirm that climate change — warmer winters and longer growing seasons — creates favorable conditions for the rise of new and previously uncommon forest pests. In this light, the defoliation of Estonia's birch stands may not be a mere coincidence, but a sign of a wider ecological shift.

The birch sawfly belongs to the order Hymenoptera and the family Argidae. The adult insect, metallic blue-black and 7–14 millimeters long, emerges in late May. Females lay eggs singly into small slits cut at the tips of birch leaf teeth, with up to 25 eggs per leaf. These egg-laying sites swell and darken, making affected leaves easy to identify.

The preferred host plants are different birch species: silver birch in northern Europe and downy birch in western Europe. When food is scarce, larvae may also feed on alder. The newly hatched larva — known as a false caterpillar — begins feeding at the leaf margin, often in groups. Together they can strip a leaf entirely, leaving only the petiole behind. As a result, an entire tree may be defoliated.

The larvae are yellow-green with shiny black heads, three pairs of thoracic legs, 7–8 pairs of abdominal legs and longitudinal black dotted lines along their pale backs and sides. They go through four larval stages while feeding. Damage typically begins in the lower crown, often on the southern side of stands. Because egg-laying occurs over an extended period, birch stands often contain larvae of different ages and sizes at the same time. Young larvae are easily knocked down by wind or rain, but they usually climb back up the trunk into the crown.

A mature larva grows up to 28 millimeters long before dropping into the soil, where it spins an elongated oval, cream-colored cocoon with a double-layered wall, up to 15.5 millimeters in length. It overwinters there as a pupa. The full life cycle usually takes one year.

The impact of the birch sawfly's larvae is not limited to their voracious feeding. Biochemical studies have shown that they contain a toxin called lophyrotomin. This substance protects the larvae against natural predators, which is why birds usually avoid eating them. However, several parasitoid species do help to reduce their numbers in outbreak areas. The toxin is chemically similar to the compounds found in fly agaric mushrooms and can cause acute liver damage in livestock that consume larvae fallen from trees.

In Australia, related sawfly species have caused the deaths of hundreds of cattle and sheep. Denmark has reported at least one incident in which 50 sheep died after ingesting birch sawfly larvae along with grass. While no such cases have yet been reported in Estonia, this serves as a reminder that the insect's impact extends beyond forest health and could also pose risks to agriculture.

A sign of climate change and need for monitoring and research

The current situation in Estonia suggests that the birch sawfly is no longer just a rare, occasional visitor but has emerged as a significant forest pest that requires systematic national attention. Monitoring is needed to map its distribution and population dynamics, along with scientific research to clarify its interactions with other associated species. Only then will it be possible to assess how serious a threat it poses to Estonia's birch forests in the coming years.

This summer's bare birch stands are not just a local phenomenon but may signal how changing climate conditions, the spread of invasive species and mass outbreaks can quickly alter the face of forests. The question is no longer simply whether the damaged trees will recover next year, but whether Estonia is prepared for the rise of new forest pests and their long-term impact on forest ecosystems.

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