Butterflies: reintroduction

Two butterflies (large blues) that had gone extinct in the Netherlands have been reintroduced in Noord-Brabant. Another, the small pearl-bordered fritillary, which had disappeared from some regions, has been reintroduced. The reintroductions have had mixed results.

Trend in large blues

In 1990, about 20 years after they had gone extinct in the Netherlands, 86 large blues and 70 dusky large blues from Poland were released in the Moerputten reserve (south of 's Hertogenbosch). Both these species live on damp infertile to lightly fertilised grassland, and their host plant is the greater burnet. In their larval stage they are parasites of certain ant species, so therefore they are dependent not only on the great burnet but also on the presence of these host ants.

There is now only one Dutch population of the large blue. Numbers fluctuate greatly from year to year. There are three small populations of dusky large blue; after increasing sharply, this species has been declining for several consecutive years and is thus once again at risk of going extinct in the Netherlands. One of the reasons for the decline is that the vegetation of its habitat is becoming rank, and grazing animals are preventing the host ants from increasing. In addition, inappropriate mowing is making much of the habitat of the dusky large blue unsuitable.

Trend in the small pearl-bordered fritillary

The small pearl-bordered fritillary used to be common in the Netherlands. It frequents damp, nutrient-poor grassland and marshes with flower-rich reed areas, where its host plant (the marsh violet) and nectar plants grow. The species has disappeared from many areas because of drainage, land reclamation and the intensification of farming.

In 1993 the small pearl-bordered fritillary was reintroduced in the nutrient-poor grasslands of the Meije reserve (Zuid-Holland), after careful management had brought about an increase in the marsh violet and nectar plants. In total, 79 butterflies were released, in two phases. The population increased sharply, especially in the first few years. It increased again in 2001.

In 2001 20 specimens of the small pearl-bordered fritillary were released in the Ilperveld reserve (Noord-Holland).

Both large blues and also the small pearl-bordered fritillary are on the Red List of butterflies.