The Ants of Africa
Chapter 4 - Economic Importance of Ants - Introduction
Ants as Vectors of Plant Disease

Ants and Pests of Cocoa

The entry to the literature here has to be Entwistle (1972). It, however, dealt with the pests of cocoa on a global basis and, although a whole chapter is assigned to ants, predates almost all the work on mosaics.

Interestingly, additional early information was collated by Bequaert who wrote on "Ants in their diverse relations to the natural world". His long list of references shows that, prior to 1922, there was only a modicum of information from West Africa (Part IV in Wheeler, 1922). For instance, there was work by Winkler (1905) in Cameroun, when it was found that Crematogaster kohli (as africana variety winkleri) gnawed the skins of cocoa pods, and Camponotus acvapimensis and Oecophylla longinoda were "accused of the same evil"; Camponotus brutus reportedly gnaws the base of "fruit-stalks" and licks up the sap, an effect of this was to cause the fruits to dry up or drop off (presumably the fruits were at the Cherelle stage). Winkler also reported the involvement of ants in the pollination of cocoa. Lamborn (1914a) wrote on trophobiosis between ants and coccids, membracids, jassids and psyllids in southern Nigeria; including a report of Camponotus acvapimensis (as akwapimensis variety poultoni) tending a membracid. In Zare, Bequaert himself had observed Pheidole punctulata with treehoppers and Myrmicaria eumenoides with membracids. The habit of tent or cowshed building by ants over coccids, etc., Bequaert noted as having first been described by P. Huber in 1810 for Lasius niger in Europe. An early English-language reference with "tent-building ants" in its title was given by Bequaert as Couper, W. (1863) Proc. Ent. Soc. Philadelphia, 1, 373-4. Incidentally, for anyone wishing to study the subject, Bequaert's list of references would serve as a very good source list of the early literature.

The collecting efforts of Prof. F. Silvestri, which yielded so many new species and new geographical records, came according to his report title from an "Expedition to Africa in search of natural enemies of fruit flies" (Silvestri, 1914, cited by Wheeler, 1922).


Ants and mirids, or capsids

Collingwood (1972, abstract only) reported that an analysis of samples of 100 individual trees from each of 64 cocoa farm plots in Ghana showed that both Oecophylla longinoda and Tetramorium aculeatum (as Macromischoides aculeatus) were significantly negatively associated with fresh capsid damage. Other dominant ants were either weakly negatively associated or, as in the case of the majority of Crematogaster species, positively associated with damage, i.e. protective of capsids. A further analysis showed that the net effect of 16.5% reduction in capsid damage was due to an ant-negative association with Distantiella theobroma but not with Sahlbergella singularis. From a large number of samples (presumably a different set) using both pkd and direct trunk counting on farm plots scattered throughout the cocoa growing area of Ghana, and using the relativities he had derived from the first exercise, it was calculated that the mean capsid reducing effect of the presence of ants, mainly Oecophylla longinoda and Tetramorium aculeatum was 15%. Therefore, on average over 80% of Ghana cocoa was not protected from capsid injury by ants.


Ants and Homopterans

Strickland (1951a, b) showed how most Homopterans have a close relation, perhaps dependency, with ants of one species or other. The dependency however has a high degree of mutualism, as the sap exudates of the Homopterans is a very useful source of energy for many ant species. For most of the ant species regarded as dominants that energy source is vital for the development and maintenance of their large colonies. Indeed, perhaps the only dominant which does not rely on Homopterans for energy is Tetramorium aculeatum.

Campbell (1994) provided a very useful list. Although his work was in Ghana, and dealt with only Crematogaster clariventris, Pheidole megacephala types (used as a convenient grouping for several morphologically and behaviourally similar taxa), and Tetramorium aculeatum, the list is applicable to Nigeria and elsewhere in West Africa.

Aphids - Toxoptera aurantii (Boyer de Fonscolombe) - associated with Pheidole megacephala and some Crematogaster spp. but Campbell did not do any detailed studies.
Mealy bugs - Pseudococcidae - frequency from Campbell (1994):
Planococcoides njalensis (Laing) - 47% - positively associated with Pheidole megacephala and negatively with Tetramorium aculeatum (although it will attend then if other ants absent). Apparently Crematogaster africana will attend this species on Canthium but not on cocoa.
Planococcus citri (Risso) - 43% - positively associated with Pheidole megacephala and negatively with Tetramorium aculeatum.
Phenacoccus hargreavesi (Laing) - around 5%.
Pseudococcus calceolariae (Maskell) - under 0.15%.
Pseudococcus concavocerarii James - around 0.8%.
Pseudococcus longispinus (Targioni-Tozzetti) - about 0.2%.
Maconellicoccus ugandae (Laing) - about 0.75%.
Soft scales - Coccidae - Waxiella sp. near zonata.
Hard scales - Stictococcidae - Stictococcus sjostedti Cockerell - the most abundant Homopteran, mostly attended by and highly positively associated with Crematogaster clariventris, a few tended by Pheidole megacephala, negatively associated with Tetramorium aculeatum (although it will attend them if other ants absent).
Jumping plant lice - Psyllidae - Mesohomotoma tessmanni (Aulmann) - as abundant as the stictococcids but apparently not attended by ants. It, however, was most abundant on trees occupied by more than one of the dominant ants, perhaps because there was less well-organised ant activity in such circumstances.

Campbell (1994) supported the suggestion by Bigger (1993) that the absence of honeydew exuding Homoptera is a key factor which allows Tetramorium aculeatum to occupy trees as sole dominant; an associated factor was felt to be the fact that Homoptera were absent from poorly growing trees, which could account for the association between Tetramorium aculeatum and shade. The observations at CRIN in Nigeria (Taylor & Griffin, 1981), during the 1975 study of the black pod disease epidemic, also showed that densely planted cocoa had poor productivity, with few pods, few Homoptera, and, in that instance (Block E5/1), the canopy dominant was Tetramorium aculeatum.

Contents Economic Importance - Ants as Vectors of Plant Disease
1998, 2003 - Brian Taylor CBiol FSB FRES
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