Threat Level: Medium

Pheidole megacephala is a common species in the Pacific with two waist segments, 12-segmented antennae, 3-segmented antennal clubs, propodeal spines, long thin pilosity, and a bimorphic worker caste. The minor workers are small, yellow to brown, small propodeal spines, and no antennal scrobes. Sculpture on the minor worker is restricted to the middle and posterior portions of the mesosoma. The major workers are larger and have oversized, mostly unsculptured heads without antennal scrobes.

Pheidole oceanica and P. fervens are the two ants in the Pacific most often confused with P. megacephala. The major workers are the easier subcaste to differentiate. Majors of Pheidole oceanica and P. fervens have strongly sculptured posterolateral lobes and antennal scrobes, while majors of P. megacephala have unsculptured posterolateral lobes and lack antennal scrobes. The minor workers of P. megacephala are best separated from the other two PIAkey Pheidole species by the swollen appearance of the postpetiole and the shorter antennal scapes.

Pheidole megacephala is known to cause significant damage to native biological diversity, including vertebrates, and also significant damage to agricultural systems. The species is considered to be one of the worst 100 invasive species in the world according to the IUCN/SSC Invasive Species Specialist Group (ISSG). Pheidole megacephala recruits strongly to baits and food resources and forms long and busy foraging trails. Minor workers are much more abundant than major workers, both outside of and within the nest.

Pheidole is the most diverse ant genus in the world, and many native and widespread species occur in the Pacific that are not included in PIAkey. If the specimen under investigation does not match any of the PIAkey Pheidole, explore images from Antweb and taxonomic literature pertaining to Pacific ants.

Table 1. Minor worker
Table 2. Major worker

Table 1. Minor worker

Waist with two segments (petiole & postpetiole)
Propodeal spines present
Antennal club with three segments
Antennal scrobe absent
Slope of mesosoma steep
Postpetiole swollen compared to petiole (for an example of an unswollen postpetiole, see P. fervens)
Antennal scapes relatively short

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Table 2. Major worker

Waist with two segments (petiole & postpetiole)
Propodeal spines present
Antennal club with three segments
Slope of mesosoma steep
Head size disproportionately large
Antennal scrobe absent
Posterolateral lobes with sculpture absent

Chart 1. Minor workers

 

	P. megacephala	P. fervens	P. oceanica
Head
Profile
Postpetiole relative to petiole	swollen	not swollen	not swollen
Antennal scapes relative to head width	< 1.5x	~ 1.5x	~ 1.5x
	P. megacephala	P. fervens	P. oceanica

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Chart 2. Major workers

 

	P. megacephala	P. fervens	P. oceanica
Head
Profile
Sculpture on posterolateral lobes	absent	present	present
Antennal scrobes	absent	present	present
	P. megacephala	P. fervens	P. oceanica

• Antweb: specimen images, data & maps
  
  
• Global Invasive Species Database: information about ecology, distribution, impacts, management, references, links and contacts
  
  
• Bolton, B. (1995) A new general catalogue of the ants of the world. Harvard University Press, Cambridge, Massachusetts, 504 pp.
  
  
• Wilson, E.O. & Taylor, R.W. (1967) The ants of Polynesia (Hymenoptera: Formicidae). Pacific Insects Monograph, 14, 1-109.