Over the course of two weeks, all of the around 4,000 meadows in Åland that contain ribwort plantain (Plantago lanceolata) and/or spiked speedwell (Veronica spicata) are surveyed. These are the only host plants of the Glanville fritillary butterfly in Finland. Simultaneously, powdery mildew on ribwort plantain is also surveyed.
The survey is conducted by about 35–40 students hired as research assistants, who work in pairs in the field. Each pair always consists of at least one experienced field worker who has previously participated in the survey and one new assistant or trainee. Each pair must have access to a car to reaching the meadows and moving between them often involves driving 50–100 kilometers per day. During a typical day, a pair can survey on average 15 meadows, depending on their size, the number of Glanville fritillary nests and the amount of mildew present. Navigation to the meadows is done using topographic maps, a map application in the database, and a GPS device.
The time spent on each meadow searching for larval nests and powdery mildew depends on the size of the meadow. If no Glanville fritillary nests are found within a set time limit, the meadow is surveyed again. The aim is to minimize the number of meadows that are incorrectly classified as unoccupied.
Each year, several variables describing patch quality are recorded in the database for every meadow. The proportion of grazing is estimated as a percentage of the total meadow area, and the species of the grazing animal is noted. Grazing includes all activities that keep the meadow open, including mowing and lawn cutting. Overgrowth is the main reason for the loss of meadows suitable for the Glanville fritillary, which is often caused by a reduction in grazing.
Sometimes a meadow cannot be fully surveyed. This can happen e.g. due to the presence of livestock or the landowner denying entry. In such cases, the percentage of the area that could not be surveyed is recorded in the database.
The abundance of the Glanville fritillary’s host plants, ribwort plantain and spiked speedwell, is assessed for each species on a scale of 1 to 3. This variable describes the quality of the meadow as habitat for the butterfly. The larvae live in family groups of 50–150 individuals, that consume several rosettes of the host plants during spring and summer. If a host plant is not found on a meadow in a given year, its abundance is recorded as zero. If both host plants receive a value of zero, the meadow is not viable for the Glanville fritillary that year.
The proportion of dried host plants on the meadows are also estimated as a percentage for each host plant separately. The drying of the host plants during the larval growing season can have a dramatic effect on larval survival. The height of the surrounding vegetation also affects the success of both the host plants and the butterfly. Host plants that prefer sunny and dry conditions can become shaded by taller vegetation, and in that case the ectothermic larvae, which require high temperatures for growth, receive less solar heat energy. Thus, grass encroachment and overgrowth are important factors affecting meadow quality, and for this reason the occurrence of host plants in low vegetation is assessed each autumn as a percentage, separately for each species.
The number of overwintering nests of Glanville fritillary larvae varies greatly between regions and according to yearly weather conditions. The web-like winter nests are found at the bases of the host plants. Each nest found on a meadow is marked as a point on the database map using a GPS device. In addition, it is marked in the field with a wooden stick bearing the nest’s serial number on that meadow. The developmental stage of the larvae and the host plant that the nest was found on are also recorded.
The following spring, the nests are revisited using the GPS points. The timing of the spring survey depends on the progress of spring: the team travels to Åland as soon as the snow has melted and the thermal growing season begins. At this stage, the larvae emerge from their winter nests and feed on the small leaves of the host plants in the immediate vicinity of the nest. Because only the meadows that are occupied by the butterfly are visited in the springtime, the number of workers is much smaller than in autumn – about 4–15 people depending on the year – and the survey lasts only one week. During the spring survey, overwinter mortality of the nests is assessed. The number of living larvae in each nest and their developmental stage are recorded in the database.
During the autumn survey, the workers look for powdery mildew on ribwort plantain in addition to searching for butterfly nests. If mildew is found on a meadow, a mildew survey is conducted after the butterfly survey has been completed. The search for the mildew is more detailed than the nest survey which would bias the time spent searching for nests if both were conducted at the same time. Thus, the work is divided into butterfly and mildew rounds.
The surveyors report the absolute and relative numbers of infected plants using a four-step scale. Infected plants must also be examined closely to assess the presence of the resting spores (chasmothecia) that are necessary for the mildew to overwinter. This too is recorded on a four-step scale.
Regardless of whether mildew is present, the total cover of ribwort plantain is estimated in square meters. From some meadows, ribwort plantain seeds are collected and can be used in experiments, for example to study host plant resistance to disease and differences between populations or between years.
In the years when genetic samples of the mildew were collected during the autumn survey, more detailed information on the epidemic was also gathered. Each sample was marked on a map, and the number of healthy and infected leaves on the sampled plant was counted. In addition, the surveyors counted all ribwort plantain individuals, as well as the numbers of infected, flowering, and dried plants on several one-square-meter quadrats.