(Warning: This blog contains “gorey” content. Some of the pictures below may not be for the most sensitive people)
Although our travels are well behind now, the work has continued at our home institute, the Swedish University of Agricultural Sciences in Uppsala. Most larvae are now in their final instar and will pupate within a week or have already pupated if they are not parasitized or virus diseased. So far we have seen quite a few cases of virus disease in our rearing, particularly from Gratangen (Norway, location C in the map in the previous blog), where the densities were quite high this year. The work with the larvae dead of virus continues in the laboratory by our colleagues, who will look for DNA and RNA viruses with genetic methods. The investigation will hopefully further our mission of revealing the role and abundance of viruses in the mountain birch – geometrid system.
The mountain birch forest and Geometrid moths – A dynamic system
On our trips we have taken samples from geometrid populations from 6 locations in northern Fennoscandia. These areas have in general been long studied for the cyclic dynamics and outbreaks the moth species exhibit by research groups in Sweden, Finland and Norway (see for example here and here) The populations we selected for our sampling are in different phases of their population cycles and outbreak cycles. The different locations also represent forests of different age structures and successional stages. The larval densities at Abisko and Kilpisjärvi reached very high levels in 2013 and large areas of mountain birch forests were defoliated. Repeated outbreaks and successive years of defoliation will inevitably affect the whole forest more or less. Dieback of shoots, branches and older tree stems are direct results of the defoliation and tree death due to secondary factors as fungi and wood boring beetles may also occur.
Phenology and especially matching the host tree phenology in the spring is an important factor defining the success of the winter and autumnal moths. This year leaf advancement in relation to larval developmental rate was fairly variable at Abisko and at Kilpisjärvi sites had early instar larvae and newly flushed leaves at the time of our sampling, marking a fairly good match. On the other hand, the birch leaves at Gratangen, Nuorgam and Neiden sites were well advanced while larvae were fairly early. The fluctuation in spring temperatures is a significant signal for the birch leaf bud burst in the spring. It seems that a period of cold weather has probably slowed down the larval development rate maybe more than the rate of leaf development in Gratangen, Nuorgam and Neiden. The larval densities in the previous year, and hence the stress on the birch, may also affect the budburst and the following phenology match. More on that hopefully later, as our observations inspired further investigations!
In addition to the role of viruses and phenology matching, a major factor driving the moth cycles are parasitoids. At least in the continental parts of our study area they seem to follow the regular fluctuations of moth densities in a delayed density dependent way. Overall about 15 species of parasitoids are known from our study Geometrids in the north. We have also had several different parasitoid1 species emerging from the collected larvae: one gregarious2 ektoparasitoid3 and some solitary4 endoparasitoids5plus one a bit particular gregarious endoparasitoid(2+5). Compared to the ecology of the host species, much less is known about the parasitoid complex. Just last year, for example, molecular and morphological analyses helped to identify a new species6
Above you can see examples of the parasitoids in our system. First from the top: Many Copidosoma chaconotum pupae filling up an Epirrita larva almost to a bursting point. Second from the top: A free flying parasitoid (a braconid) female searching the mountain birch leaves for hosts. Third from the top: gregarious ektoparasitoid larvae of Eulophus larvarum feeding on a still living Epirrita autumnata. Fourth from the top: A solitary endoparasitoid (Cotesia sp. or Protapanteles sp.) has come out of its host and formed its own pupa next to the host carcass. Lowest: An Ichneumonid parasitoid (Phobocampe sp.)has thoroughly consumed its host and left behind an empty skin to form its own black cocoon next to it. All photos by Helena Bylund.
The large fluctuations in geometrid densities are affecting other organisms at all trophic levels from soil dwelling microorganisms, plants, other herbivorous and predatory insects and invertebrates as well as birds and mammals. Defoliating outbreaks relocate nutrients and affect the forest structure, light conditions and composition of fungi, herbs, and grasses at the forest floor for years. The changes in plant species composition affect herbivores and predators of all kinds during the following years: more grass for grazing animals and seed feeders and so on. During years with high larval densities many birds are successfully reproducing with high survival of fledglings due to abundant food resources. However, the higher abundance does usually not persist the following year since winter survival is low.