The trees that are burning across Krasnoyarsk this year were, in a lot of cases, already dead. Not from fire, and not from drought, but from a moth.
Dendrolimus sibiricus, the Siberian silk moth, has spent the last several years stripping needles from fir, spruce, larch and pine across huge stretches of central Siberia, and a tree that loses its needles for enough consecutive seasons simply dies where it stands. It does not fall over.
It does not decompose quickly in a climate this cold. It stays upright, drying out, for years, until something eventually sets it alight, and this year that something has been an unusually hot, dry stretch of Siberian summer.
How a caterpillar kills a forest without ever touching fire
The Siberian silk moth’s caterpillars are defoliators, meaning they eat needles rather than wood, bark or roots, and a single bad outbreak year can strip a stand bare across an area running into the hundreds of thousands of hectares. Research into the moth’s life cycle, published this year in Frontiers in Insect Science, examined why some populations follow a one-year larval cycle while others take two — and found the split isn’t simply a matter of temperature or food availability. Laboratory tests that varied larval density, food supply and host plant didn’t reproduce the switch; the researchers found a distinct molecular signature instead, pointing to a more complex mix of cues and environmental history behind which path a population takes. Whatever sets the timing, a conifer can usually survive losing its needles once. It is much less likely to survive losing them in consecutive years, which is exactly the pattern an outbreak produces once it takes hold.
Remote sensing work tracking a more recent outbreak in central Siberia found the scale involved is not small. A study analysing satellite data from 2016 to 2020 documented Siberian silk moth damage across multiple sites in the Yenisei and Irbei districts, with the outbreak that ran from 2015 to 2018 alone disturbing an estimated 428,600 hectares of forest in the Yenisei region, and a second, smaller outbreak from 2018 to 2020 affecting a further 41,500 hectares in Irbei. Those are just two districts in one stretch of years. The moth’s outbreaks recur roughly every ten to eleven years and typically run for two to three seasons once they start, which means the deadwood now feeding Krasnoyarsk’s fires was very likely put in place by an outbreak that peaked years before anyone was thinking about this particular fire season.
This is not the first time, and it will not be the last
Russian forestry records on this moth go back further than most pest histories, and the historical numbers are large enough to be genuinely difficult to picture. A contingency assessment compiled for the UK’s plant health authorities, drawing on that longer Russian record, states that across the ninety years from 1855 to 1945, Siberian silk moth outbreaks killed at least 4 million hectares of Russian forest. A subsequent twenty-five-year stretch, from 1932 to 1957, saw the moth damage 7 million hectares of forest in Western Siberia and Chita Oblast alone, with the assessment noting that the insect killed the forest outright across roughly half that area rather than merely thinning it. These are not one-off disasters. They are what happens, repeatedly, when an outbreak-prone species with a ten-year cycle meets a forest type it is well adapted to defoliate, across a landscape too vast for any human intervention to meaningfully interrupt.
What makes an outbreak year different from a quiet one is not usually visible to anyone who is not specifically monitoring moth populations. The insect itself does most of its damage during a narrow larval feeding window each year, and by the time a stand’s needle loss is severe enough to be obvious from the ground, the underlying population boom that caused it may already be subsiding. The dead trees, on the other hand, do not subside. They remain part of the landscape for years afterward, and it is that lag, between an outbreak most people never noticed and the fire season that eventually finds the deadwood it left behind, that connects an entomological event to a wildfire emergency that looks, on the surface, like a purely meteorological one.
A predictable cycle that still catches everyone off guard
Part of what makes this connection so easy to lose track of is that the moth and the fire show up in completely different seasons, in front of completely different audiences. An outbreak plays out over successive spring and early summer feeding windows, tracked mostly by foresters and the handful of remote sensing researchers who specialise in this insect.
A wildfire emergency plays out in front of regional officials, emergency crews and, once smoke starts settling over towns, the general public. By the time the fire is the story everyone is watching, the outbreak that set the stage for it is old news, if it was ever news at all outside forestry circles.
That gap in visibility is one reason the deadwood explanation, when officials do offer it, tends to land as a surprising detail rather than something people already expected to hear.
Why this connection is easy to miss and hard to fix
Fire agencies fighting the current Krasnoyarsk fires have been explicit that the moth-killed deadwood is a major factor in how fast this year’s fires have spread, alongside the high temperatures driving the season generally. That is a useful diagnosis for understanding why containment has been difficult, but it does not translate into an obvious fix.
Clearing dead timber across hundreds of thousands of hectares of remote taiga, most of it far from any road, is not a realistic undertaking at the scale the outbreaks operate on, and the moth itself is native to the region, not an invasive species that can be targeted for eradication. Forest managers can watch for outbreak signals and plan around them, but they cannot meaningfully outpace an insect whose largest recorded outbreaks predate satellite monitoring by a century.
What the historical record does offer is a way of reading this year’s fires that goes beyond the weather report. A summer this hot would have produced a serious fire season regardless. It is the several hundred thousand hectares of standing dead timber, left behind by an outbreak most of the region never registered as news at the time, that is turning a bad fire season into one moving faster than heat and dryness alone would predict. The moth will not be back in force for years. The forest it already killed is still standing, and still burning, right now.