Blog Posts Markus Melin Climate, Forestry

Traditionally, we have seen the cold climate of Finland as a preventive measure against two unpleasant phenomena: high population density of humans and presence of severe pest insects. Unfortunately, one can’t have the best of both worlds and so Finland, as cold and isolated as she still is, has seen a rapid increase in the abundance of insects typically associated with Central and Eastern Europe. Of the various examples, I deem the nun moth (Lymantria monacha) as worthy of some special attention.

A sudden increase in abundance

The nun moth is a nocturnal moth, whose larvae can feed on needles of various trees, but they are especially keen on defoliating Scots pine and Norway spruce (Finland’s two main tree species). While any healthy forest ecosystem is filled with insects feeding on vegetation, there is something special about the nun moth: it has the ability to form mass outbreaks. I am not talking about minor things here; an outbreak in Poland (1978–1983) caused damage in ca. two million hectares of forests (Bejer 1988). Yet, those events did not really concern us: while the first encounters of the nun moth in Finland date back to the 1950s, for the next 50 years, these were just rare individual occurrences. Then something happened (see Figure 1).

Figure 1. Number of nun moth observations made by lepidopterists using light traps and/or visual sightings, 1950-2017. Data adopted from (datacentre of Finnish biodiversity)


Note from the author: the trend in Figure 1 is not explained by another trend where observing of lepidopteran moths suddenly became cool. Nay. That hobby is still considered just as freaky as it always has, and ever shall be. Nonetheless, it was only after the rapid population growth that we began to associate the insect with the prefix ‘pest’.

Figure 2. The type of funnel trap used in the survey. The zoomed image shows trapped moths that have lost the typical zebra-like colouring of their wings, which is still intact on the moths in the two right-hand side images.

Next, researchers from Natural Resources Institute Finland (Luke) and University of Eastern Finland (UEF) hatched a plan for mapping the abundance of the species. This was to be done with pheromone traps. The method was piloted in 2018, and for the summer of 2019, we were fully equipped to complete the mission.

A network of pheromone traps was set out across Finland, within and beyond the known range of the species. The used traps were funnel traps equipped with a sexual pheromone mimicking that released by females. In another words, we were luring hundreds of lovesick males inside small plastic containers, which to them, still had that dazzling female scent (Figure 2).

Results went beyond the expected

With three researchers working in the field throughout the summer, and with numerous volunteers aiding us by looking after single funnels, the effort was good, and it was not in vain. We managed to find the northernmost nun moth individuals so far, and more importantly, confirmed that the species is present throughout Central and Southern Finland.

At many sites, the abundance indicated the presence of a locally reproducing viable population; one that is happily feeding and breeding (Figure 3). The northernmost catches were made up of only lonely males. These kinds of wind riders are typical for the species: after a successful copulation, male nun moths have a few more weeks to live. So, they venture for a walkabout, going where the winds take them – right into our traps in this case.

Figure 3. The results of the nun moth pheromone trappings. Image on the left shows the trap locations symbolized by catch size (moths per trap per season). The image on the right shows an interpolated surface based on the results of the left-hand side image. The figure is adopted from the open access research article Melin et al. (2020), where more details on the results and the used methodology are outlined.

As the Finnish climate has become warmer (Mikkonen et al. 2015), and the country is vast with coniferous forest, the outlook is positive for the nun moth and its apparent range expansion. As far as our knowledge goes, the species is a winner of the current climatic change, especially up in the Nordics. Indeed, the northwards expansion of the nun moth, as well as its adaptation to cooler Finnish climate, was discussed already in Fält-Nardmann (2018). Her excellent research on the topic was as topical as it gets, as supported here by our pheromone games.

Now, our minds are set on further studies such as trapping of the larvae, or the linking of nun moth abundance to variables of weather and landscape structure. This future research will give us hands-on information about the damage potential of this species – one that a mere 20 years ago was only a rare inhabitant.

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