Immature Nutfall of Coconuts in the Solomon Islands. II.—Changes in Ant Populations, and their Relation to Vegetation
Authors: Brown E. S. 
Published: July 2009

In the British Solomon Islands Protectorate, the incidence of immature nutfall of cococuts that is caused by the Coreid bug, Amblypelta. (chiefly A. cocophaga China), depends indirectly upon certain species of ants, some of which protect the palms against Amblypelta, whilst others do not. There is evidence that populations of the ants can sometimes change quite rapidly, and that local fluctuations in nutfall are largely dependent on such changes.

Observations on ant populations were maintained over a period of 2½ years (1954—1956) to find out if such changes are of common occurrence and to attempt to explain their cause.

An account is given of ideas, current at the beginning of the author's investigation, on the effect of vegetation and other factors on changes in ant populations, and in particular of the cover-crop or creeper theory, the advocates of which suggest that the presence of such vegetation, prevalent in plantations during and after the war, provided conditions favourable to beneficial species of ants.

The actual progress of recorded changes in ant populations is described and discussed. Rapidly changing populations usually involve the ants, Pheidole megacephala (F.), Oecophylla smaragdina (F.), and sometimes also Anoplolepis longipes (Jerd.); more stable populations usually involve Iridomyrmex myrmecodiae Emery, and occasionally Oecophylla and Anoplolepis. An account is given of the ‘battles’ which often occur during replacements of one species by another.

An account is given of the vegetation found in coconut plantations in the Solomon Islands, with special reference to the plant communities associated with each of the four species of ant of special importance in immature nutfall, and also with mixed ant populations. It is concluded that heavy creeper growth is not particularly associated with O. smaragdina (as was thought by supporters of the creeper theory), nor with the other beneficial species A. longipes, but rather, if anything, with P. megacephala, a non-protective or harmful species; there is thus no good reason to suppose that a growth of creepers will encourage beneficial species. Iridomyrmex myrmecodiae is the only species of the four which tends to be associated with a characteristic type of vegetation, including several species of epiphytes and often heavy creeper growth. But there are exceptions even to this, and there is evidence that the epiphytic vegetation may to some extent be the result rather than the cause of the presence of Iridomyrmex. There are, in fact, exceptions to every rule, and it is concluded that vegetation is not an important factor controlling ant distribution, except in so far as heavy growth of creepers and other plants may encourage a more mixed ant population in general, including several indigenous species which are not normally conspicuous in well-maintained plantations; if there is any effect on the four ‘major’ species, it is to encourage Pheidole and Iridomyrmex at the expense of Oecophylla and Anoplolepis.

An account is given of two experiments which involved control of creepers and other vegetation by repeated cutting down or ‘brushing’ in marginal zones between areas occupied by two different ant species, to see if this practice would alter the course of changes in population that were already taking place. One experiment involved a marginal zone between I. myrmecodiae and A. longipes, and the other between O. smaragdina and P. megacephala. It was found that in both cases there was no appreciable difference in the course of events between ‘brushed’ plots and plots adjacent to them in which the vegetation was not controlled. This bears out the previous conclusions that the presence or absence of creepers and other vegetation has little, if any, effect upon changes in ant populations.

At Rua Vatu, where an extensive replacement of P. megacephala by O. Smaragdina was taking place, accompanied by recovery from a condition of heavy nutfall, detailed studies revealed that this replacement was not a direct one, but that, at all events for the most part, a succession of normally unimportant ‘transition’ species first replaced Pheidole, and were in turn replaced by Oecophylla. The details of this succession are described and discussed. Evidence is produced to suggest that a similar succession may have been involved in recorded examples of recovery from nutfall in other places, notably in that which occurred in the Kukum/Lunga/Tenaru group of plantations after the second world war. These conclusions present a picture not so much of direct antagonism between Pheidole and Oecophylla, but rather of the initial disappearance of Pheidole owing to unknown intrinsic factors, followed later by the infiltration of Oecophylla in the wake of a variable succession of indigenous ‘transition’ species, which have been allowed to increase in numbers temporarily during the interim period when the area is not dominated by either of the two major species. In view of this, it is not so surprising that a factor such as vegetation should have little or no direct effect upon the change as suggested in terms of the creeper theory.

As regards the reverse change, the replacement of Oecophylla by Pheidole, there is little evidence to go on, but such as there is suggests that the replacement in this case is more in the nature of a direct one.

The fact that recovery from nutfall took place recently at Rua Vatu, at a time and place where insecticides had not been applied, contradicts the theory that the earlier and similar recovery in the ukum/Lunga/Tenaru group of plantations was the result of a differential effect upon ant species of the application of insecticides during the war for mosquito control.