The studies described in this thesis sought, to establish the importance of Rattus exulans and Rattus rattus as pests of two crops, coconuts and cocoa, in Fiji. Movement patterns of R. exulans were investigated. Males moved greater distances in both crops with distances increasing with population density. The eye-lens technique proved to be useful for ageing rats up to about 100 days. Population levels fluctuated considerably at all sites but there was no marked seasonal trend. No simple relationship existed between rat densities and damage to the two crops. Damage only increased when marked population peaks coincided with a possible shortage of other sources of food. Both species attacked developing coconuts but R. rattus caused most damage because it foraged more extensively in the crowns of taller palms. R. rattus also caused most damage to cocoa despite both species being in constant contact with the crop. Attack of cocoa appeared to be a learned pattern of behaviour. Coconut damage and production was surveyed at 16 sites over three years. Damage varied considerably between palms, sites and years. Rats favoured coconuts aged three to seven months, stages of development during which sugar concentrations were highest. Because coconuts on particular palms were favoured a possible basis for this palm selection was sought. Despite concentration of attack, favoured palms did not produce fewer mature coconuts. A trial simulating rat damage established that the coconut palm was capable of compensating for the premature removal of nuts. Compensation was conservatively estimated to replace 50 percent of the nuts lost. Control of rat damage to coconuts in Fiji was not economical because of the low level of loss after compensation and the inefficiency of trunk banding, the most convenient method of reducing damage. Damage to cocoa was serious at some localities. No compensation occurred but control, using warfarin based baits, proved highly economical.
|Date of Award||1974|