Final Observations on Pheidole Megacephala and Iridomyrmex Humilis in Bermuda

Pheidole megacephala (Myrmicinae) and lridomyremex humilis (Dolichoderinae) are two well known invasive pest species of ants which, within approximately the last century and a half, have achieved almost worldwide distribution within the tropical to semitropical habitats available to them. Both are extremely aggressive and are capable of displacing native ant faunas on an impressive scale. Thus P. megacephala, originating probably in middle Africa, radiated extremely actively in both the Old and New Worlds as a "tramp" species, and by 1852, judging by Heer’s vivid account (Heer, 1856) it had thoroughly occupied, among many other western European locations, the island of Madeira, exterminating a very large fraction (if not all) of the indigenous ant fauna. iridomyrmex humilis, the "Argentine Ant" was first reported from Buenos Aires in 1866, and described by G. Mayr in 1868 (Skaife, 1951, p. 7) (it may well have originally been indigenous to Brazil). By 1882 it had found its way (likewise as a "tramp" species exploiting human transport) to Madeira, which it occupied, in competition with the resident P. megacephala. It was a successful "occupation", and by 1898, according to Stoll (1898) P. megacephala had completely disappeared from the Island, leaving L humilis as the sole ant reported. Nothing is known of further details of the explosive interaction between these species in this limited island environment. P. megacephala reached Bermuda at least as early as 1902 (Dahl, 1902), where it behaved essentially as in Madeira, displacing the greater part of the indigenous ant fauna and "blanketing" the Islands. By the time that one of the authors made a survey in 1927, its situation as a ubiquitous field and house ant closely resembled Heer’s account of it in Madeira seventy-five years earlier.