Pheidole megacephala (Fabricius 1793)
Big-headed ant (Andersen 2002) (Generic and species common name). Also known as brown house-ant, coastal brown-ant, lion ant, African Big-headed ant
Synonyms (WWW5 )
Atta testacea Smith, Formica edax Forskal, Formica megacephala Fabricius, Myrmica laevigata Smith, Myrmica suspiciosa Smith, Myrmica trinodis Losana, Oecophthora perniciosa Gerstaecker, Oecophthora pusilla Heer, Pheidole janus Smith, Pheidole laevigata Mayr
Pheidole is among the largest of ant genera. Wilson (2003) estimates the number of its known species as close to 900, with a large number still to be described. Australia has some 53 species and subspecies. Three of these are established in New Zealand, P. megacephala , P. rugosula and P. vigilans. The former, a notorious tramp species, is almost certainly of African origin, but probably arrived here via Australia or the Pacific region, rather than directly.
Distribution (see map)
The first record of establishment in New Zealand, dated 10 February 1942, appears to be from a chocolate factory in Auckland. It had been intercepted a few years earlier, and has since been intercepted at ports on a regular basis. It currently appears to be restricted to coastal suburbs of Auckland, although it is likely that coastal areas north of Auckland would also be suitable. Berry et al. (1997) provide detailed distribution data.
The workers of Pheidole are dimorphic (major and minor workers). The antennae are 12-segmented, usually with a 3-segmented club. In side view the propodeum is depressed below the level of the pronotum and the forward section of the mesonotum, and these two regions are connected by the steeply sloping rear section of the mesonotum. The nest entrance typically has a small mound of soil extracted from the nest.
Diagnostic features of the major worker
Length 3.5 mm; head massive with the rear (occipital) margin deeply impressed in the middle; rugae (wrinkles in the cuticle) extending about half way from the clypeus to the rear margin of the head; propodeal spines (paired) longer than diameter of propodeal spiracle; colour light brown.
Diagnostic features of the minor worker
Length 2 mm; head with rear (occipital) margin shallowly impressed in the middle; head smooth; propodeal spines (paired) longer than diameter of propodeal spiracle; colour light brown.
Berry et al. (1997) provide a key to Pheidole species in New Zealand.
In spite of its status as a major pest in many countries, little is known about many aspects of the biology of P. megacephala. It shares with some other tramp species certain features that have contributed to their reproductive success. For example, it is multi-queened and is able to spread by budding off groups of workers along with inseminated queens (Passera 1994). It is also capable of forming interconnected super-colonies that cover tens of hectares.
It is known to tend homopterans injurious to garden and horticultural crops (Berry et al. 1997). Many Pheidole species (including megacephala ) are seed-eaters, and in Australia are the most widespread of seed-harvesting ants.
Internationally this ant displaces most native invertebrate faunas directly through aggression, and as such is a serious threat to biodiversity (WWW6). Evidence also exists of reductions in vertebrate populations where this ant is extremely abundant. Effects on plants and horticultural crops can be direct through the likes of seed harvesting, or indirect through the likes of harbouring scale insects that reduce plant productivity. It is known to facilitate the invasion of introduced plant species via moving seeds. This ant is known to chew on irrigation, telephone cabling and electrical wires, and to nest under paving stones displacing large amounts of soil and causing uneven surfaces.
Most of New Zealand is probably too cold for this species to realise its full pest potential, but the far north could support populations if it is transported there. Large populations have also been reported on the Kermadecs (C. Green pers. comm.).
Compiled by Warwick Don & Richard Harris