|Introduction||The Ants of Africa
CHAPTER 3 - Mosaics - Evidence from Ghana
1 - Strickland & Cornwell
|Ghana - 2||Ghana - 3||Ghana - 4||Ghana - 5||Ghana - 6|
The area within which most of the ant research within Ghana was conducted is shown in the General Map of Ghana
Strictly speaking, Strickland appears to have done limited work which could be described specifically as concerning the ant mosaic. What he provides, possibly uniquely for Ghana, is quantitative data, allowing some appreciation of the relative frequency of the commoner ants found on cocoa. Apart from general observations on the wider occurrence of species, however, the vast majority of his collections were made at CRIG, Tafo. He described the collections as being random and a single "collection" as referable to anything from a single specimen to a nest of hundreds of individuals, thus, his results do not give information on absolute abundance. Two further provisos have to be made when considering the findings - first, the state of taxonomy led him to lump some species and, second, that there is a mix of precision and vagueness (for instance he varies from writing simply "less abundant" to noting "only found three times").
The earliest paper (Strickland, 1948) was essentially a description of the coccids attacking cocoa and ant information was ancillary. The taxonomic problem was illustrated graphically, however, by his relating how nearly 50 species of ant had been taken attending colonies of the mealybug, Planococcoides njalensis, in the field but only nine of the ants had been identified (probably by Donisthorpe, see Chapter 2) - Crematogaster africana (as Crematogaster africanum), Crematogaster cuvierae, Crematogaster ?kneri, Crematogaster luctans, Crematogaster stadelmanni, Crematogaster striatula, Crematogaster wellmani (as Crematogaster boxi), Meranoplus inermis (as Meranoplus nanus) and Monomorium pharaonis (only in the laboratory). With another mealybug, Planococcus citri, he had found 20 ant species, of which only two were named, Crematogaster ?kneri and Crematogaster luctans, plus eight other Crematogaster species, nine Pheidole and one Camponotus. Arising from slightly earlier work on mealybugs and other pests of cocoa, Donisthorpe (1945a) described new species of Crematogaster, including Crematogaster boxi (now known as a junior synonym ofwellmani, see Bolton, 1995) and Crematogaster cuvierae. He also wrote a note on ants associated with coccids (Donisthorpe, 1945b).
From the second paper (Strickland, 1951a), by extrapolation and some guesswork, the frequency of individual species can be deduced, from a total of 4644 collections at CRIG (some species names are revised to present use), and is given in the Table .
A study of the attendance of Planococcoides njalensis by Crematogaster africana and Pheidole megacephala gave the interesting finding that the latter ant was capable of repelling its larger competitor and attacking and carrying off the mealybug. Crematogaster species F 257 also was reported as being able to repel Crematogaster africana.
In the third paper (Strickland, 1951b), he described felling 2880 cocoa trees, infected with swollen shoot virus, to examine ant and mealybug occurrences. In the first year, he studied trees from 10 one-acre (0.4 ha) plots chosen at random from a possible 350 numbered plots at CRIG, then being developed from an area of farmers' cocoa. In the second year, he added five more plots adjacent to the original Plot 1, so as to determine mealybug distribution in a compact six-acre block. Some 90% of the mealybugs were found at 4.5-6.0 m above ground level, and 80% of the mealybugs were obscured from view by ant-constructed carton tents. He selected three "dominant arboreal ant genera" - Crematogaster, Oecophylla and Macromischoides. Modern taxonomy has the last two "genera" to be two species only - Oecophylla longinoda and Tetramorium aculeatum. The Crematogaster was further subdivided into the subgeneric groups (Sphaerocrema), (Atopogyne) and (Crematogaster). Associations of the three dominant genera were examined for Camponotus, Polyrhachis, Atopomyrmex and Pheidole, again without species names being given. One or more ant species were found on 2230 trees, but only 169 trees had three or more species present; only thirty trees had more than four species; three trees had five species; one tree had six species of ant - Tetramorium aculeatum, two Polyrhachis, one (Atopogyne), one Camponotus and one Platythyrea. The (Atopogyne) species were negatively associated with Oecophylla and Tetramorium aculeatum. There was an indication that Oecophylla longinoda could exist on the same trees as (Sphaerocrema) and (Crematogaster). A total of 189,267 individual Planococcoides njalensis were collected; of these 185,945 were directly associated with Crematogasterine or Pheidoline ants of "known coccidophilic habit"; 121,651 of those were associated with (Sphaerocrema) species. He gave an ant-association table which showed mealybug attenders as - (Sphaerocrema), 760; (Atopogyne), 251; (Crematogaster), 269; Pheidole, 114; and Atopomyrmex, 20. Non-mealybug attenders were - Oecophylla longinoda, 952; Tetramorium aculeatum, 313; Polyrhachis, 207; and Camponotus, 50. A final note was that Oecophylla longinoda was "the dominant ant species inhabiting cacao in the Eastern province of the Gold Coast".
This was one of a series of papers by P.B. Cornwell, reporting
primarily studies on the vectors of virus diseases of cocoa,
essentially continuing Strickland's efforts at CRIG. In this report he
evaluated the effect of "cultural conditions" on the vectors, mealy
bugs, and their associated ants. The study area was within "the same
square mile of cocoa at Tafo as that surveyed by Strickland (1951b)"
and utilised the same technique of felling and examining cocoa trees.
Two habitats were defined - Habitat 1, well maintained cocoa,
irregularly spaced with closed canopy; and, Habitat 2, cocoa
growing in dense secondary bush, like semi-abandoned farms, with
aetiolated cocoa and high humidity. Eighteen plots each of two
adajacent sub-plots, one being of the Habitat 1 type and the
other Habitat 2, were defined and 29 cocoa trees were selected
at random in each subplot. Between 5.i.1954 and 11.ii.1955, four cocoa
trees were felled on each of nine working days in 29 fortnightly
periods; a total of 522 trees from each habitat. Every part of each
felled was examined immediately for virus vectors. On the afternoon
prior to felling, at 1400 h because that was when "ants are most
active", a 15-minute count was made of the number of ants of the genera
Crematogaster and Pheidole moving on the trunk of each
of the four trees. The results of the counts showed that -
In Habitat 1, 151 trees (28.9%) had a total of 3237 Crematogaster, whereas only 35 trees (6.7%) had 796 Pheidole.
In Habitat 2, 222 trees (42.5%) had a total of 5613 Crematogaster and 14 trees (2.7%) had a total of 120 Pheidole.
Seasonal trends were plotted but showed no great variation in ant populations, the highest levels being at the beginning of the wet season (March-May). Ultimately, the failure to separate the actual species of ant means that, from the point of view of ant bionomics, the paper has limited value. Even the evidence of there being more Crematogaster in Habitat 2 conveys nothing without species differentiation or information on the presence or absence of large trees, on which the large Crematogaster, such as africana, could nest. The focus on mealybugs presumably was why other ant genera were not recorded but in light of later work the data on Pheidole remains useful.
|Mosaics - Introduction||Mosaics - Ghana - 2||
©1998, 2003 - Brian Taylor CBiol FSB FRES
11, Grazingfield, Wilford, Nottingham, NG11 7FN, U.K.