CC9 Moths

>> Background information

Development

The occurrence and abundance of moths have been monitored nationally since 1993. The monitoring bases on light traps that capture the moths. Especially since 1998 the monitoring has focused on forest habitats.

A rising trend can be seen in the annual number of species per observation site (A). The trend is most notable in southwestern Finland where on average 45 more species are observed now than in the beginning of the monitoring period. The rise of the species number levels off towards the north. On the whole, the number of moth species is many times greater in the south than in the north.

Most moth species produce only one generation during the short Finnish summer. Some species, however, can have two or more generations in one year, but this usually happens only in long and warm summers. The occurrence of multiple generations depends on the combination of the temperature sum and day length. The number of species with multiple generations has increased over the monitoring period especially in southern Finland (B). A rising trend can be seen also in central Finland, but not anymore in the north.

Many factors may drive the changes of moth communities. On the level of single observation sites, especially forestry alters the environmental conditions affecting moth populations. However, on the level of the whole country the impacts of forestry and forest succession are likely to have contrasting impacts and do not therefore explain changes over large areas.

The warming of the climate may be regarded as the primary cause behind the increasing moth species richness and the increased occurrence of second and third generation individuals. Further, the long-term soil eutrophication caused by nitrogen deposition has increased the food available for moths feeding on nitrophilous plants. The decrease of sulphur deposition (i.e. acid rain) has also contributed to the increasing population trends of moths feeding on lichens.

Increasing and decreasing species

Among the species that increased in abundance over the monitoring period are many southern species which distribution areas have also expanded towards the north. One of such species is the black arches (nun moth) which population has increased 80-fold since the early 1990s. Its larvae feed primarily on spruce and pine. The species has caused substantial forest damages in central Europe and it is possible that forests defoliated by black arches larvae may be seen also in Finland in the future. The warming of the climate has helped the black arches to grow in numbers and to colonize new areas in the north.

Another increasing species is the dotted carpet, but its increase is mainly due to other reasons than climate change. The dotted carpet has benefitted especially from the steep decrease of sulphur deposition. The species declined strongly in 1970s and 80s due to acid deposition caused by the burning of fossil fuel. Acid deposition eradicated the Usnea and Bryoria lichens, that the dotted carpet larvae feed on, from vast areas. As a result of the removal of sulphur from stack gases, the amount of acid deposition started to decrease in the late 80s. The dotted carpet was evaluated as vulnerable in the Red list assessment of 1991, but its status could be lowered to near-threatened in 2010.

An example of a decreasing species with a northern distributional range is the brindled ochre. The species occurs in southern and central Finland in moist and cool deciduous and mixed forests as well as in urban parks. The brindled ochre starts to fly in mid-September and flies until night time temperatures drop below zero for several consecutive nights. The species overwinters as an adult and therefore flies again in the spring right after snow melt. The brindled ochre has a boreomountainous range. Although it occurs in Finland primarily in the southern and central parts of the country, it is clearly a mountainous species in central Europe. As a species of a cool climate it is presumed that the brindled ochre suffers from a warming climate.