CARIBBEAN FOOD CROPS SOCIETY 37 Thirty Seventh Annual Meeting 2001
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^fOOD CROps CARIBBEAN FOOD CROPS SOCIETY 37 Thirty Seventh Annual Meeting 2001 Trinidad and Tobago Vol. xxxvn Proceedings of the Caribbean Food Crops Society. 37:104-109. 2001 PESTICIDE RESIDUES ON LOCAL FOOD CROPS: REALITIES AND RECOMMENDATIONS Ivan Chang Yen, Dept. of Chemistry, The University of the West Indies, St. Augustine; Carlyle Kalloo, Environmental Management Authority, Port of Spain, Trinidad; Isaac Bekele, Dept. of Food Production, The University of the West Indies, Si. Augustine; Wade Lee Fook, Ministry of Agriculture, Centeno, Trinidad; Ronald Barrow, CARINET, St. Augustine, Trinidad ABSTRACT: A recent market basket survey of organophosphate insecticides in Trinidad clearly indicated improper use of this class of insecticides on produce offered for sale to local consumers. In several cases, crops appeared to have been harvested without regard for recommended pre-harvest intervals, as well as being treated shortly before harvest. On many samples, recommended maximum residue levels were exceeded, particularly on celery, on which mixtures of insecticides were frequently and consistently detected. Supervised field trials of five of the most commonly detected organophosphates showed that recommended pre-harvest intervals of each pesticide were often not in accordance with actual field degradation rates. In some cases, residue levels of pesticides declined faster than expected, raising the possibility of re-infestation of crops by pests within the recommended pre-harvest intervals. Our results indicate a pressing need for supervised efficacy and pesticide residue trials, of all pesticides used on food crops under local growing and storage conditions, and offered for sale locally or exported. The recent acceptance of HACCP for food production systems also mandates regular monitoring of edible crops for contaminants, including pesticide residues, to ensure the safety of consumers. INTRODUCTION The effective application of pesticides is a necessity in modern crop protection. For many local fanners, it often represents the only means of guaranteeing their harvests and their livelihood. Unfortunately, pesticide overuse often leads to the development of pest resistance, which in turn results in pesticides becoming ineffective at recommended rates of application, or being ineffective altogether. Consequently, increased rates of application may become necessary for pest control, or by the use of other pesticides or control strategies. However, the development of genetically modified pest-resistant crops, which may reduce the use of pesticides, is not without problems, particularly in Europe (1). The application of combinations of pesticides is a well-established practice in pest control, and is commonly done by local farmers (2). In addition, the "more is better" principle is often adopted in application rates, leading to problems of pesticide residues on harvested crops. Such residue problems are exacerbated by praedial larceny of freshly sprayed crops (3), as well as the non-observance of recommended pre-harvest intervals (PHI), following the final pesticide application. However, pre-harvest intervals are often developed under conditions which may differ significantly from local growing conditions and may thus be considered inappropriate by local farmers. Nevertheless, pesticide residues on crops are cause for concern for consumers, and maximum permissible levels in edible crops have been set for each pesticide (4). Not surprisingly, pesticide residues have occasionally been found on crops exported to the US from the Caribbean (5). Once maximum permissible levels are exceeded, such produce is rejected, and enhanced monitoring of subsequent shipments is implemented. In the face of strong competition from other regions, such action may result in permanent loss of markets for local and regional farmers. In this study, a survey of pesticide application practices by local farmers was undertaken to determine pesticide usage patterns, followed by a market-basket study on the levels of organophosphate pesticides on produce offered for sale to consumers. Supervised field trials on selected organophosphate pesticides were carried out, to determine their persistence patterns and PHI under field conditions, as well as to help explain the results of the market basket study. Summary findings are presented and recommendations made, for the benefit of both the farmer and the consumer. 62 MATERIALS AND METHODS (a) Farmers ' survey Farmers (N = 69) in north, central and south Trinidad were interviewed, using a questionnaire. Information obtained on pesticide application practices, including the use of mixtures of pesticides, types of pesticides used on various crops, and the observance of recommended pre-harvest intervals were collated. Results of this survey are presented in Tables 1 and 2. (b) Market basket study A total of 200 samples representing 17 crops, was purchased in local markets and supermarkets, for analysis of organophosphate pesticide residues, from September 1996 to May 1997. This number was calculated to represent all produce containing pesticide levels above the Codex Alimentarius (4) Maximum Residue Levels (MRL), determined within 0.05 units at the 95% confidence level (6). Codex protocols on minimum sample size and items per sample were closely followed in sample collection. Summary results are shown in Table 3. (c) Supervised field trials Five organophosphate (OP) insecticides were applied individually to Pak Choi Brassisca sinensis L), using a randomized block design and 200 plants per treatment. Eight spray applications were made at recommended rates of application for each pesticide during the dry season. In comparison, five applications were made during the wet season, since applications were made only during periods of little or no rainfall, until close to crop maturation. The method of analysis used (7) was validated prior to sampling, and quality control measures implemented during the entire programme. Calculated values of pr-harvest intervals required for each pesticide to fall below its MRL are summarized in Table 4. RESULTS AND DISCUSSION (a) Farmers ' survey Table 1 shows the summary data on use patterns of pesticides, excluding herbicides, by local farmers. A wide range of different chemical classes are used in crop protection, with the most used pesticides being profenofos, used as two different formulations, and Cypermethrin. In addition, the same formulation of a pesticides could be used on a range of crops to control different pests. For example, for OP pesticides, up to 34 different formulations, containing 27 OP pesticides, were used on tomatoes, while 27 different formulations with 16 OP were used on cabbages (Table 2). Likewise, mixtures of pesticides were applied simultaneously, often using the "more is better" principle. Spreaders and stickers were also used to enhance pesticide persistence, mainly during the wet season, but appear to be increasingly used in the dry season as well. Pesticides were normally applied to cash crops at 4-5 day intervals in the dry season, and even more frequently during the wet season. Recommended pre-harvest intervals were often not strictly adhered to, this practice being encouraged by consumers preferring unblemished produce, as well as incidents of praedial larceny. 63 (b) Market basket study Of the 200 samples analyzed, 22% contained detectable OP pesticides, of which 10% exceeded Codex Alimentarius MRL (4). The worst affected crop was locally grown celery, with 15 of 18 samples containing detectable OP residues, 13 above the MRL. The most commonly detected OP was methamidophos, followed by triazophos, profenofos, diazinon, ethion, pirimiphos methyl and dimethoate. These results in Table 3 were consistent with our farmers' survey data, with respect to pesticide use patterns and practices, including the observance of pre-harvest intervals. For example, the 40.20mg/kg triazophos and 15.74mg/kg methamidophos levels on celery are well above the MRL and pose a significant threat to the health of the consumer. Similarly, the 8.52mg/kg profenofos level, also on celery, strongly implies that harvesting was carried out soon after spray application, since this pesticide decays rapidly once applied in the field (8). Our market basket data strongly suggest the need for increased monitoring of produce offered for sale to consumers. Continued education of farmers on the observance of safe pre-harvest intervals is also required. (c) Supervised field trials For each pesticide, the residue (Y) at time (t) fitted and exponential decay model of the following form, representing the pesticide decay rate: Y = a + b e"cl where Y is the fitted pesticide concentration; t is the time (days) after spray application; a, b, and c (c>0) are the estimated parameters; (a + b) denotes the initial pesticide level. The time taken for each pesticide to decay to below the MRL was calculated for both wet and dry seasons. Results are shown in Table 4. Of the five OP pesticides investigated, triazophos was highly persistent in both wet and dry seasons, while methamidiphos was more persistent under dry than wet conditions. In comparison, phenthoate, profenofos and pirimiphos methyl disappeared rapidly under wet or dry conditions. Our pre-harvest intervals, calculated for pesticide levels to fall below the Codex MRL, compared well with those recommended by the pesticide manufacturers for pirimiphos. However, for phenthoate and profenofos, our rates were about half those recommended by the manufacturers;