Host specificity of the Alcon blue Large blue butterflies can usually only survive and develop in the nests of a single species of Myrmica ant, the primary host. Although a few individuals may be able to develop in the nests of other secondary hosts, the vast majority of the individuals of any population will develop in nests of the primary host (Thomas et al. 1989).

The alcon blue is distributed patchily throughout much of western Europe, although it is not common in any country and is cited as "endangered" or "vulnerable" in most countries. The map to the left shows the large-scale distribution of the Alcon blue in western Europe (after Wynhoff 1998). The Alcon blue seems to be different from the other large blue butterflies in that it is known to use different primary host ants in different parts of Europe (Elmes et al. 1994) Populations in Spain use Myrmica scabrinodis as the primary host. Populations in the Netherlands use Myrmica ruginodis as the primary host, and Swedish populations use Myrmica rubra as the primary host. Although all these populations are called Alcon blues, it is possible that they actually represent different species which are very similar - so called cryptic species.

Denmark is a particularly interesting country to study the Alcon blue, because it represents an area between known Myrmica rubra and Myrmica ruginodis use. A previous genetic study by Rebekka Gadeberg and Koos Boomsma in 1997 suggested that there may be a transition in populations of the Alcon blue in Denmark from those using Myrmica rubra in the north to those using Myrmica ruginodis in the south, with little interbreeding of butterflies raised by the two host use type. This suggests that there may actually be two cryptic species of the Alcon blue in Denmark which use the two different primary hosts. Results of fieldwork carried out by researchers from the Universities of Copenhagen and Aarhus in 1997 and 1998 have confirmed that both Myrmica rubra and Myrmica ruginodis are used as hosts in Denmark. However, this is not according to a simple north-south gradient. There are also some populations where both ants are used as hosts simultaneously. Myrmica scabrinodis is never used as a host ant in Denmark, although all three potential host species are common in the areas where the Alcon blue butterfly is found.

The picture to the right shows one method of assessing the distribution of potential host ant nests. A grid is laid out on the field site and baits placed at its vertices. The density of foraging ants at these baits gives an estimate of the probability that an Alcon blue will be found by any particular ant species. The lower part of the figure shows contours of foraging ant density for one such grid, with positions of ant nests of each species overlaid. Foragers of each ant species are found mostly near to the nests of that species. In this particular case, all the Myrmica rubra nests were found to contain Alcon blue caterpillars, while none were found in the Myrmica ruginodis or Myrmica scabrinodis nests. Eggs were laid on gentian plants over the whole grid, so it seems that those caterpillars that were not adopted to Myrmica rubra nests did not survive. We are currently investigating the host specificity of the Alcon blue further in terms of the genetics of the butterfly populations and the communicatory signals used by the caterpillars to allow them to be adopted into and survive inside the host ant nests.