Relative distributions of ant species in cocoa plantations in Papua New Guinea.
Abstract
In Papua New Guinea the most important cacao pest, Pantorhytes szentivanyi Mshl., is controlled by the ant Anoplolepis longipes (Jerd.) [cf. preceding abstract], while in Ghana Oecophylla longinoda (Latr.) controls cacao Mirids [cf. RAE/A 60, 3052, 3054]. The author describes the ant fauna of cacao in Papua New Guinea cocoa and compares it with that found on cacao in Ghana [cf. 60, 3037]. Two methods of sampling canopy ants were used: pyrethrum knockdown and hand-catching. Both gave essentially the same results. Hand-catching in a 1-m2 quadrat was used to sample ants on the ground. Two hundred knockdowns, 135 hand-caught and 60 quadrat samples took a total of 121 species. These species and their frequencies of occurrence in each set of samples are listed. Species richness was greater on the ground than in the canopy. A greater range of microhabitats on the ground and the ease with which indigenous ants could invade the new cacao ground habitat are suggested as explanations. The mosaic distribution pattern of dominant ants found elsewhere was confirmed. The dominant species A. longipes and Technomyrmex albipes (F. Sm.) were abundant, O. smaragdina (F.) was moderately so, and Crematogaster sp. (R114) was scarce in the canopy. On the ground, the dominant ants Brachyponera croceicornis Emery, Pheidole megacephala (F.), Odontomachus simillimus F.Sm. and Rhytidoponera araneoides (Le Guillou) were moderately abundant in addition to the common canopy species. Specific communities of non-dominant ants appeared to be associated with each dominant in the canopy. The number of species in each community seemed to be related to the degree of specialisation of the dominant ant with which it was associated. T. albipes had the fewest species in its community, and it was the only dominant to nest in a potentially limiting resource, dead wood. T. albipes and Oecophylla smaragdina were not mutually exclusive, and it is suggested that competition between them was slight partly because of the difference in size of their foragers. Ant species diversity in the canopy was greater in Ghana than in Papua New Guinea, probably because cacao in the former is grown in thinned forest. Dominant ant species in Ghana were all indigenous, while the two commonest dominants in Papua New Guinea were tramp species. In both countries, the dominant ant species most positively associated with pest situations is the one that nests in dead wood.