著者

寒川 一成 ケビン D.ギャラガー ピータ E.ケンモア

出版者

日本熱帯農業学会

雑誌

熱帯農業 (ISSN:00215260)

巻号頁・発行日

vol.38, no.4, pp.361-368, 1994-12-01

被引用文献数

1

---

The brown planthopper (BPH), Nilaparvata lugens has long been an outbreak-prone insect pest of rice since the introduction of paddy farming into Japan. BPH is also known as a monsoonic migrants originated in tropical Asia. The BPH was a rice-monophagous minor herbivore in the tropical paddy ecosystems of subsistent phase. However, it raised suddenly as a key-pest of high-yielding rice varieties (HYV) spread into tropical Asia during the "Green Revolution" in 1970s. The BPH outbreaks happened under the pest management with pesticides at International Rice Research Institute (IRRI) is an epitome of the BPH menace which is even now prevailing in tropical Asia. Pesticide use at IRRI farms started with γ-BHC against the rice stem borers in 1962. Since its first occurrence in 1964,BPH infestations magnified rapidly in spite of intensive control with diazinon and later with carbofuran. Eventually, IRRI farms were suffered by devastating outbreak of BPH in 1971-1976,until declining after sequential releases of BPH-resistant HYVs. Resurgence induced by pesticides, development of pesticide resistance, and biotype shifts in the BPH populations were attributed to the outbreak. Restraint of preventive use of pesticides was most effective to stop the BPH problems at IRRI. At present, the BPH density at IRRI farms is at far below the economic threshold level. The pesticide input is often mistakenly associated with increased productivity of the tropical rice, which is largely attributed to HYVs, fertilizers, and irrigation. In general, potential yield losses due to the insect pests are estimated about 10-20% by the on-farm assessments. Except for massive infestations, less than 10% can be caused by insect damages in a normal year. However, the on-farm economic assessment revealed unprofitability of pest control with high input of pesticides. Expected returns are generally low on high levels of pesticide use, and no control is often the economically acceptable option, and biological control in association with varietal resistance has consistently proven more profitable than pesticide use in long-term experiments. This is the reason why pesticides that were employed as a modern high-yielding technology needed official subsidy. In addition, the resurgence of BPH is the most expensive hidden cost of pesticide use. Population ecology of BPH and its natural enemies in the tropical paddy ecosystems demonstrated that pesticides readily led to explosive upsurge of BPH populations by destroying its natural enemy complex. High fecundity, short generation cycle, active dispersal, and tolerance to crowding are the biological properties of the BPH for its easy resurgence under the disruptive impact of pesticides to natural enemies. The BPH was initially uncurbed from natural enemies by pesticides used for controlling the other targeted insect pests such as stem borers. Escalated pesticide use against the upsurge of BPH infestations further promoted its resurgence. Eventually, such vicious cycle give rise to uncontrollable outbreaks of BPH, and disaster the rice production systems. Indonesia is a prime example of the country where the BPH crisis led to the establishment of ecologically sound pest management program in rice. Pesticides were initially adopted as an essential component of "Green Revolution" package for high-yielding technology, and 85% of their cost was subsidised. As increase of pesticide use, resurgence and outbreaks of the BPH prevailed throughout the rice granaries in the country in 1975-1979. Stagnation of rice production made the country the world-biggest rice importer in this period. Forcible planting of the BPH-resistant variety IR 36 suppressed the BPH outbreak, and led to the rice self-sufficiency in 1985. However, the BPH menace revived when IR 36 was replaced with new varieties having improved eating-qualities and high profitability. The BPH biotype adapted to the new varieties became epidemic over major rice areas in 1985-1986,in spite of as much as 10 times more pesticide supply. Collapse of the reliance to pesticides led to the switching of crop protection policy from pesticide-dependent to ecosystem-orientated IPM by the Presidential Instruction in 1986. Consequently, the 57 pesticides were immediately banned from use in paddy, and government subsidy for pesticides was completely abolished in 1989. At the same time, FAO Rice-IPM Program has launched. The principles of FAO Rice-IMP in the developing countries in tropical Asia is "Integration of biological control into crop production systems", where "Maximum conservation of natural enemies, minimum reliance on pesticides" is emphasized in implementation of the IPM. The concept arose largely in response to the crisis-driven outbreaks of BPH induced by prophylactic use of pesticides that had motivated by government policy and commercial promotion, as well as unprofitability of overdependence to pesticide technology for controlling the endemic insect pests in the tropical rice farming. Recognizing the central role of natural enemies of rice pests through on-farm paddy-ecosystem analysis by farmers themselves is the most strongest motive for farmers to change their pest control practices. The ecosystem analysis ensure that IPM is not distorted into a purely negative message "Don't spray pesticide this week". Instead, IPM is reinforced by positive reassurance "This field is in good condition this week". Pesticide use has effectively been reduced without spoiling productivity of rice by farmers trained IPM. Official support of FAO Rice-IPM have been promulgated in the Philippines, Indonesia, Malaysia, India, and Sri Lanka.