Behavior of Pesticides in Rice-Based Agroecosystems: A Review

Abstract

Emphasis on agricultural production using modem techniques has focused atten-tion on the problem of yield losses due to pests and the need for adequate protection of the crop. Pesticide application is currently the most widely practiced method of pest control in rice and rice-based cropping systems. Because of their toxic nature there is a general concern with the potential hazards of pesticides to humans and the environment. The fate of pesticides applied in agricultural ecosystems is governed by the transfer and degradation processes and their interactions. Transfer is a physical process in which the pesticide molecules remain intact; it includes sorption-desorption, runoff, percolation, volatilization and absorption by crop plants or animals. Degradation is a chemical process in which pesticide molecules are split; it includes photodecomposition, microbiological decomposition, chemical decomposition, and plant detoxication. Transfer and degradation determine pes-ticide persistence or retention, its efficacy for pest control, as well as its potential for contamination of the soil and water resources. Growing evidence indicates that pesticides are present in food grown on the land in surface and subsurface bodies of water and in the atmosphere. Research 112 IMPACT OF PESTICIDES ON FARMER HEALTH AND THE RICE ENVIRONMENT has established the toxic effects of some pesticides on fish and other aquatic animals, on birds and other wildlife, and on human health. There is therefore a growing interest in understanding the processes relevant to the transport, trans-formation, and retention behaviors of pesticides used in agricultural production systems. This paper reviews the information on the fate of pesticides in rice-based agroecosystems with emphasis on wetland soils. Early studies of the fate of pesticides in soils have dealt mostly with upland soils. The bibliographic review on microbial transformation and decomposition of pesticides by Hill (1978) includes 334 references, among which only six refer to flooded soils. Similarly, the review of microbial degradation of insecticides by Matsumura and Benezet (1978) lists 133 references but only ten on sub-merged soils. During the 197Os, interest in pesticide fate in wetland rice soils developed, especially with the studies conducted by Sethunathan and his coworkers, who authored between 1969 and 1984 more than fifty papers on this topic. Work on microbial degradation of insecticides in flooded rice soils was reviewed by Sethunathan (1973a) and Sethunathan and Siddaramappa (1978). About 100 pesticide studies dealing with ricefields, rice soils, or organisms isolated from ricefields were published during the 1970s. The number falls to about fifty in the 1980s. Since 1990 very few pesticide studies have dealt with ricefields. 5.2. Pesticide Use in Wetland Rice Fields