Colony take-over and brood survival in temporary social parasites of the ant genus Formica

Parasites reduce host fitness, and so instigate counter adaptations by their hosts. In temporary social parasitism, usurpers must not only enter the colony unharmed, but also have their eggs reared by the host workers. We introduced parasitic Formica lugubris and Formica aquilonia queens into queen right and orphaned fragments of three host species, Formica cinerea, Formica picea and Formica fusca, and show that workers of all three host species kill over 40 % of the introduced queens within 10 days, regardless of the presence/absence of a resident queen, and parasite species. More parasite queens died in F. cinerea than in F. picea and F. fusca. There were no major differences in survival between the parasite species (except that F. lugubris survived longer than F. aquilonia in F. fusca colonies compared to F. picea colonies), but parasite queens survived longer in orphaned than in queen right fragments of F. fusca. Experimental introduction of parasite (F. aquilonia) eggs into orphaned colonies of F. fusca showed that none of the parasite eggs were reared until pupation; whereas on average, 12 % of the con-specific hetero-colonial eggs introduced in the same manner were reared until pupation. In all colonies that received parasite brood, all offspring consisted of worker-laid males, whereas the corresponding value was 50 % for colonies that received con-specific hetero-colonial brood. Thus, when the risks of entering host colonies and brood failure are combined, the rate of successful colony take-over is very low. Moreover, the host workers can to some extent alleviate the costs of parasitism by producing a final batch of own offspring.