A critical review of the development of rat control in Malaysian agriculture since the 1960s
B. J. Wood and Chung Gait Fee
Crop Protection Volume 22, Issue 3 , April 2003, Pages 445-461
2003
บทคัดย่อ
Systematic rat control was developed in oil palm plantations in the 1960s and 1970s by comparing bait mixtures and application techniques mainly for anticoagulant poisons, in trials with related ecological studies. Rattus tiomanicus populations of 100–600/ha were estimated in plantings of a range of ages and localities, and numbers fluctuated slowly within these limits in a single plot without control, monitored over 20 years. Optimum control was with maize based wax-bound baits (ca. 12 g), applied one per palm (generally 114–138/ha) with "replacement rounds" of those taken, at 4-day intervals until acceptance declined below 20% (usually about 5 or 6 rounds), doing large areas at 6-month intervals to minimise intermediate build up. Potential losses are estimated at 5–10% of the palm oil product, worth, within the wide price limits of recent years, from $(US)48 to 288/ha. Baiting cost is around $15/ha. Events since 1982 include appearance of warfarin resistant populations (but "second generation" anticoagulants remain effective); the replacement of the formerly ubiquitous and virtually sole rat of mature oil palms, R. tiomanicus, by R. rattus diardii, in several localities; and some populations displaying non-acceptance of baits.
Biological control, by placing nesting boxes to enable barn owl numbers to build up, is practiced by several plantations, although evidence of exact effect is inconsistent. Investigation is needed to determine rat population sizes and losses in the long term if very large areas are left unbaited so that natural control agents can build up; also to confirm whether the owls actually add to their effect. This could include further development of technique to combine owls with limited baiting, which was the original expectation for their usefulness in practice. Optimum strategy where labour shortage affects fruit harvesting efficiency needs to be considered. Alternative poisons (including "biological rodenticides"), chemosterilants and pheromones merit further investigation.
Heavy losses can be caused in rice by R. argentiventer. It cuts down growing stalks (the biggest cause of losses) and eats developing grain. The replacement round technique is effective. Loss sustained depends on site-suitability for burrowing, shelter, and alternate food. These are related primarily to size of "bunds" (earthbanks separating individual paddies) and area of interspersed non-paddy land. A median gain of ca. 1.8 t/ha was found in trials in a range of localities, presently worth about $(US)250 (farmgate), against bait cost of about $2. Nevertheless, implementation still has been greatly limited by socio-economic factors. Rice growing environments differ widely in suitability for rat build up in growing seasons, and maintenance between them. This depends primarily on the availability of alternate food and shelter. Non-baiting control measures include reducing this site suitability, inter alia by minimising both interspersed land that is not rice cropped, and the size of the earthbanks ("bunds") that separate the paddies. Another approach is large-scale fencing with associated trapping (the trap/barrier or TBS system). Owls have been established in some rice areas, but there is no published study demonstrating their impact on rat damage, something evidently desirable. Combinations of these methods appear worth investigating together with a reduced baiting regime.
Other crops are grown on a large scale, although none approach the extent of oil palm and rice. Coconuts, on a world basis, sustain heavy losses to rats. In South East Asia, R. tiomanicus (and probably other species) damage the nuts, but this has not been quantified in any detail. Cocoa as a plantation crop expanded from about 1970, in Malaysia and other territories in SEA, and proved subject to rat damage. The crop proved unprofitable and most was removed from about 1990. Rat populations appeared not to have established in monocrop cocoa, but damage potentially was heavy alongside infested oil palms, or in plantings under coconut shade. Rats can cause severe losses in a range of fruit orchards, although squirrels are usually more serious. Among field crops other than rice, specific reports on rat damage are scarce. Sugar cane can be attacked, and in a pilot project in Malaysia, R. exulans caused heavy losses. Replacement round baiting has seemed effective where tried. Owls take rats in cocoa, but there are no reports of their potential in practical control being tested in that or any of these other crops.
The changes that may occur with regular baiting make the development of "sustainable" alternatives desirable. This requires ongoing study, particularly, of rat population ecology, and the economics of losses. The availability of effective baiting is a powerful tool for the latter.