Pesticide Residues - Finding out more and more about less and less
Introduction
Pesticide residues have been a catch cry of environmental and consumer groups since the mid-1960’s when Rachel Carson drew the public’s attention to the deleterious ecological effects of organochlorine insecticides which were in widespread use at that time. The complex role of pesticides in modern society with the associated problem of pesticide residues has undoubtedly contributed to the disenchantment with chemistry which is prevalent in Western society, often fuelled by misleading propaganda from special interest groups on residue issues.
Pesticide residues are closely regulated on foods in most countries using Maximum Residue Limits (MRL’s) which are based on recognised safe and reasonable use patterns. Increasing attention is also being given to environmental issues particularly to sensitive resources such as estuaries and water. The proposed moving of pesticide regulation in N.Z. from under the wing of the Ministry of Agriculture and Fisheries to the Ministry for the Environment (under an authority to be established under the Resource Management Act, 1991). recognises this need to take a very broad view.
Knowledge on residues has depended on advances in analytical chemistry, particularly gas-chromatography (GC), combined GC-mass spectrometry (GC-MS) and high performance liquid chromatography (HPLC). This article outlines some issues regarding pesticide residues in New Zealand with particular reference to the role of the analytical chemist.
Organochlorine Insecticides (OC’s)
DDT was widely used in New Zealand pastoral agriculture and dieldrin or lindane to a lesser extent. In the 1960’s we followed other countries and banned widespread uses of OC’s. In our pragmatic fashion, this was largely based on concerns about meat products, in which residues were coming under regulation in export markets.
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The OCs are resistant to microbial degradation and have a propensity to concentrate in lipid rich tissues. These properties lead directly to their most undesirable characteristics - the environmental persistence, bioconcentration, and biomagnification through the food chain of their residues. They share these characteristics with two other notorious classes of contaminants, the polychlorinated biphenyl’s (PCBs, electrical insulating fluids) and polychlorinated dibenzodioxins (PCDD’s, by-products of manufacture of some organochlorines and of incineration). Pentachlorophenol (PCP), another timber treatment chemical which was poorly regulated in the past, is a major source for PCDD’s in N.Z.
While residues of DDT, and its primary metabolite DDE, in the soil have declined to low levels in most parts of New Zealand (less than 0.1 mg/kg), levels of 1-5 mg/kg are not uncommon on farms in Canterbury where DDT use was high and where dry land conditions have lead to very slow degradation rates. High soil residue levels are an intractable problem which can result including excessive residues in meat or dairy products. Reducing them will depend on enhancing bioactivity in the soils including the use of irrigation.
OC residues are detectable in most of our estuarine areas and are bioconcentrated in shellfish and other biota in these ecosystems. The levels are not particularly high by international standards except in localised, highly contaminated sites adjacent to industrial areas such as the Mangere Inlet in Auckland (ref. 1). The gas-chromatogram of an extract of a marine sediment (Fig. 1) illustrates the power of modern analytical techniques. The high resolution capillary column and optimised electron capture detector (ECD) enable detection limits of 1 µg/kg dry weight (1 ppb). The chromatogram reveals the presence of DDT, DDD, DDE and dieldrin each in the 2-20 µg/kg range. Concentrations of OCs in organisms dwelling in this sediment are 10-20 times higher (ref. 2) - the start of biomagnification up the food chain which can lead to damaging levels in marine mammals.
Humans also carry body burdens of OC residues accumulated mainly through diet. The significance of these residues is a subject of intense interest. At typical levels (less than 1 mg/kg on a fat basis) they are generally thought to be relatively inert and non-carcinogenic. However current biomedical research is pursuing the theory that some OC’s and other xenbiotics mimic estrogens, leading to increased susceptibility to carcinomas in fatty tissues such as the breast.
Organophosphorus and carbamate insecticides
The banning of OCs encouraged the development of a wide range of OP and related carbamate insecticides which act by inhibition of insect nervous system cholinesterase. Early products such as parathion also had very high mammalian toxicities (LD50 rat <5 mg/kg body weight) which made them very hazardous to use. However more recent OPs are much less toxic, e.g. pirimiphos methyl (LD50 rat 2000 mg/kg), and are widely used in agriculture.
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Synthetic Pyrethroids Insecticides (SPs)
SPs are structural analogues of the natural pyrethrums and have a mode of action on insect nerve junctions similar to the OCs. Permethrin is one of the most commonly used. They have good biodegradability due to the ester linkage and are used at very low rates (5-50 g/ha); consequently environmental residues are uncommon but may be expected where industrial effluents accumulate. Their principle disadvantage is the very broad insecticidal activity which tends to eliminate many beneficials. Residue analysis (generally GC-ECD) is enlivened by the range of R,S or cis/trans isomers that may be present.
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Fungicides are widely used on crops in New Zealand as the climate encourages growth of plant pathogens. A diverse range of chemical classes have been developed with the emphasis in recent years being on biodegradability. As with modern insecticides, residues are rarely found in the wider environment. An exception is copper which accumulates in soils under crops treated with cupric oxide or the hydroxide (Bordeaux mix) - a conundrum for ‘organic’ proponents and a serious problem in Kenya where decades of heavy use have led to phytotoxic levels in the soils of coffee plantations.
Herbicides
This is the most important sector of the NZ pesticide market and contains an ever increasing diversity of chemicals. While many herbicides are of limited environmental significance due to rapid degradation or strong adsorption e.g. glyphosate, some herbicides designed for longer term control have more mobile and persistent residues with the potential to contaminate the wider environment. A number of herbicides have been implicated in contamination of ground water resources in North America and Europe. Recently low level residues of atrazine, terbutylazine and simazine have been detected in some wells in South Canterbury.
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The sulfonyl-urea class of herbicides has come into prominence recently due to their very high activity against many weed species and their very low mammalian toxicity. They act by specific inhibition of an enzyme in the pathway for biosynthesis of valine and isoleucine in plants. Residue analysis can be carried out by HPLC but has been difficult due to the very low rates of application. Recently we have developed conditions under which diazomethane can be used to produce the thermally stable dimethyl derivatives of sulfonyl-ureas (Fig. 3) so residue analysis can be carried out by GC (ref. 3). Figure 4 shows the GC-MS selected ion chromatogram for detection of several sulfonyl-ureas in water at 0.1 µg/Litre. The method has also been applied to field and laboratory experiments which showed metsulfuron residues in soil degrade with a half-life of about 20 days.
Residues in the Diet
Although modern insecticides and fungicides are readily degraded in the environment by soil micro-organisms, residues on treated crops such as fruit or vegetables often do not dissipate quickly. Residues still present at harvest are regulated through MRLs which in NZ are set in the Food Regulations.
A combined Department of Health/MAF survey of fruit and vegetables was carried out in 1991/92 (ref. 4). Multi-residue analytical techniques were used which can detect a wide range of residues. Sub-samples of crops were extracted with ethyl acetate and cleaned up by gel permeation chromatography to remove lipids and pigments. High resolution GC with effluent splitting to ECD and NPD allowed detection of over 80 pesticides. Figure 5 shows chromatograms for standards and an tomato sample containing residues of pyrimiphos methyl and permethrin. The large peak on both channels is the internal standard carbophenothion. A similar method has been developed for multi-residue analysis of wine and juices which uses SPE to concentrate residues (ref. 5).
Over 52% of the 740 samples tested in the survey had no significant residues and 43% had residues below set MRLs. 5% of samples had residues which either exceeded a set MRL or which had no set MRL for that pesticide on that particular crop. Most of the MRL violations were from moderate residues due to unregistered uses of a pesticide approved for other crops rather than uses of banned products or levels exceeding set MRLs. The results were in broad agreement with residue monitoring in other countries and confirmed Department of Health ranking’s which put microbial contamination of food as a much more serious health problem than residues.
Most pesticides lack systemic action and therefore residues are mainly on the exterior surfaces where they are amenable to removal in operations such as trimming, washing or peeling that most crops undergo before consumption. Further losses can arise during cooking. A review of the extensive literature on the effects of storage and processing on residues in food has recently been carried out the IUPAC Commission on Agrochemicals (ref. 6).
Export Surveillance
While environmental and dietary concerns are of increasing importance to pesticide policy, the issue of residues in our export agricultural products continues to dominate thinking in the agricultural sector. Very large financial and technical inputs are made by the major commodity groups (meat, dairy, pip fruit and kiwifruit) into residue testing to ensure consignments meet the requirements of various overseas markets. Multi-residue techniques are supplemented by ELISA format immunoassays. Residues found in meat and dairy products are usually confined to low levels of DDE/DDT but a wide range of fungicide and insecticide residues may be present on horticultural crops.
Meeting quarantine and MRL standards while producing blemish free fruit can be a difficult exercise for growers, particularly as requirements vary between countries. Producer boards setting recommended spray programmes for growers need good insight into the residual properties of the pesticides. HortResearch has developed a predictive compute model which extrapolates field data into residue estimates for particular spray scenarios (pesticides, dates of application, rates, date of harvest). The program is based on the following simple two stage first order model with 3 parameters (pesticide dependant) for the quantity of pesticide remaining on a fruit at time t after spraying:
The initial quantity of pesticide deposited, Do, is determined by the rate of application and size of the fruit. The first exponential term accommodates initial rapid volatilisation of pesticide while the second term covers slower losses of more firmly retained residues. A residue concentration is then calculated which allows for fruit growth over time t. This model has proved able to provide remarkably accurate predictions of residue levels on apples or kiwifruit. Figure 6 shows calculated and predicted residues for chlorpyrifos on kiwifruit through a growing season when 6 spray applications were made.
Spray Drift
This is a controversial issue affecting horticulture in two ways. Firstly through crop damage due to drift from pastoral or forestry spraying of herbicides and secondly through the ire of neighbours subjected to drift from orchard spraying. Many crops are extremely sensitive to the ‘hormone’ herbicides such as 2,4-D or triclopyr. Analysis of affected foliage has revealed that residues as low as 2 µg/kg can indicate damaging exposure to drift. Reliable detection of residues at these levels requires GC-MS.
Pesticides from orchard spraying are detectable outside sprayed blocks. Deposited droplets can be extracted off filter papers and very fine particles or vapour phase trapped in air samplers on polystyrene resin beads (XAD-4). Experiments have been conducted in kiwifruit orchards (ref. 7). Typical levels of deposited drift were 30-300 µg/m2 at about 25 meters downwind from the shelter, which is less than 1% of deposits in the sprayed crop. Levels rapidly dropped with distance and were non-detectable beyond 50-100 m. Aerial drift levels were typically 0.5-5 µg/m3 with diazinon showing higher levels than the other les volatile pesticides. These drift levels are low by any objective health standards but may still cause concern to close neighbours who see drift as an unwarranted intrusion from the orchard.
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The severe regulatory pressures that have operated for some years has meant that most registered uses of current pesticides are very safe in regard to both human and environmental health. Despite many scares and individual incidents, the overall risk/benefit analysis is actually rather good, particularly if one takes as a base the multiple and manifest problems caused by the OCs. However pressures are continuing to mount world-wide for even stricter standards and, as a small country highly dependant on agricultural exports, we must try to keep ahead of these trends. Residues in export commodities and our environment will be of even greater significance to N.Z. post-GATT as countries turn to non-tariff barriers. Thus analytical techniques for low level residues will remain a focus for research, development and regulation of pesticides.
Local communities are also demanding more say in agricultural practices and insisting on extremely high environmental standards. The recent collections of unwanted pesticides from farms by some local councils have contributed to decreasing the risks. All pesticides have the potential for misuse and community groups have a role in highlighting problems and assisting with education. Chemists must play their part in this process or risk further alienation.
Acknowledgements
Denis Lauren, YinRong Lu, Don McNaughton, Colin Malcolm, Tania Trower and John Maber, Bill May (Lincoln Ventures) collaborated on various aspects of the research.
References