Immature nutfall, caused by the Coreid, Amblypelta cocophaga China, was prevalent in the coconut plantations of the British Solomon Islands before the late war, and was increasing in extent and intensity. Good crops were borne by palms inhabited by colonies of Oecophylla smaragdina (F.), as this ant destroyed Amblypelta; and the same was true, to a lesser extent, of another ant, Anoplolepis longipes (Jerd.). The smaller species, Pheidole megacephala (F.) and Iridomyrmex myrmecodiae Emery, did not destroy Amblypelta and consequently palms bearing these species suffered from nutfall. These non-beneficial ants were extending their areas of occupation and becoming dominant in the plantations of the Protectorate.

Attempts to control Amblypelta by introducing parasites from Indonesia, Queensland and Fiji have all proved ineffective, and so also have past efforts to change the balance of ant populations in the plantations in favour of the beneficial species.

After the war, it was noticed that certain plantations in Guadalcanal, which had suffered severely from nutfall for several years, were recovering and bearing well. In 1948, it was found that this recovery was correlated with changes in the ant populations, Oecophylla having occupied much of the area, displacing Pheidole, which had previously been dominant. The investigators concluded that this was due to ecological changes in the. plantation undergrowth, resulting from neglect of cleaning and brushing during the war.

The present paper deals with a re-examination of the position in 1952–53. The author found a further improvement in the yield and an extension of the area of recovery on the plantation in question; Pheidole had completely disappeared, but in many parts Oecophylla had been driven back by Anoplolepis, which had now become the dominant ant. The only nutfall areas that remained here were occupied by Iridomyrmex.

A fuller examination of these and other plantations did not support the theory that nutfall recovery was due to ecological changes in the undergrowth, for these had occurred everywhere in the Protectorate during the war, yet on most other plantations yields had decreased, and Pheidole had greatly extended its pre-war area of occupation. The areas in which recovery had occurred were adjacent to wartime military installations that had been subject to regular weekly anti-malarial spraying from the air, and it is suggested that this had affected Amblypelta, both directly, and indirectly by having differentially affected the populations of Pheidole and Oecophylla. It seemed more likely that it had been caused by the long-sustained spraying programme carried out by the United States forces during and after the war.

Small-scale tests carried out with insecticidal sprays confirmed this theory: the harmful species, Pheidole and Iridomyrmex, were found to be very severely affected, while the beneficial ones, Oecophylla and Anoplolepis, suffered far less. In plots with mixed ant populations it was found possible to increase temporarily the number of palms occupied by beneficial species by spraying the palm-bases with insecticide. With the equipment available it was not possible to spray the boles and crowns: consequently, these trees were often reoccupied later by the harmful species Iridomyrmex from colonies in the crowns.

The sprays were also found to be fatal to Coreid bugs, both on contact and for a considerable residual period.