Sustainability & Environment (Agri-Food Systems) · Sustainable Agriculture & Agroecology · also Pesticide Residues and Environment, Environmental Fate of Agrochemicals, Pesticide Risk Reduction
What happens to pesticides after application and how to reduce harm: fate processes, residues, effects on non-target organisms, regulatory risk assessment and reduction strategies. Field IPM practice is in the crop protection subjects; this subject is the environmental science of pesticides.
In the exams: ICAR AIEEA-PG 3.4 Agricultural Chemicals; ICAR AICE subject 71 Unit 4 (pesticides); UGC NET Unit 6
Pesticides are substances used to prevent, destroy or control pests, including insects, weeds, plant pathogens, nematodes and rodents. They have contributed greatly to crop yields and to control of vector-borne diseases, and global agricultural use, according to FAO statistics, is of the order of a few million tonnes of active ingredients per year. But only a fraction of what is applied reaches the target organism. The rest moves into soil, water, air and food, where it can harm pollinators, aquatic life, soil organisms, birds and people. Rachel Carson's Silent Spring (1962), which documented the effects of DDT and other organochlorines on wildlife, began the modern environmental movement and led to the first bans and to the regulatory systems that now govern pesticide approval.
This subject is the environmental science of pesticides. It covers how pesticides are classified, the fate processes that decide where they go and how long they last, residues and maximum residue limits, effects on non-target organisms, persistent organic pollutants and the Stockholm Convention, risk assessment and registration, indicators and targets for reducing risk, integrated pest management as a risk-reduction strategy, biopesticides and botanicals, and spray drift and application technology. Field pest management practice is covered in the crop protection subjects; organic rules on permitted inputs are in organic-farming-certification, and ecological pest regulation in agroecology-principles.
Students preparing for the ICAR agricultural chemicals and environmental science papers and UGC NET will find the classifications, fate parameters, conventions and calculations they need. Practitioners in regulation, advisory work, residue laboratories and food companies will find the logic behind risk assessments and reduction programmes. Numbers given for toxicity classes, limits and triggers are taken from the systems named; they differ between countries and are revised, so the current rules of each jurisdiction should be checked.
Pesticides are classified first by target organism: insecticides, acaricides (mites), nematicides, fungicides, bactericides, herbicides, rodenticides, molluscicides, and plant growth regulators, among others. Within each group they are classified by chemistry and by mode of action. Mode-of-action classification matters for resistance management: international committees publish codes for insecticides (IRAC), fungicides (FRAC) and herbicides (HRAC) so that users can rotate products with different modes of action.
Major insecticide groups include: organochlorines such as DDT, lindane and endosulfan, persistent compounds that act on nerve ion channels and are now largely banned; organophosphates such as chlorpyrifos, malathion and monocrotophos (IRAC group 1B) and carbamates such as carbaryl and carbofuran (group 1A), both acetylcholinesterase inhibitors; synthetic pyrethroids such as cypermethrin and deltamethrin (group 3A), which keep sodium channels open; neonicotinoids such as imidacloprid, thiamethoxam and clothianidin (group 4A), systemic agonists of nicotinic acetylcholine receptors; and diamides such as chlorantraniliprole (group 28), which act on ryanodine receptors in muscle.
Important herbicides include glyphosate, which inhibits the enzyme EPSP synthase in the shikimate pathway; atrazine and other triazines, which inhibit photosystem II; 2,4-D, a synthetic auxin; and paraquat, which diverts electrons from photosystem I to form damaging radicals. Fungicides include multi-site contact products such as mancozeb and copper, triazole demethylation inhibitors, and strobilurins (quinone outside inhibitors), which are prone to resistance.
Acute toxicity is measured as the LD50, the dose that kills half of a test population, usually rats, expressed in milligrams per kilogram of body weight; a lower LD50 means higher toxicity. The World Health Organization's Recommended Classification of Pesticides by Hazard, in its recent editions aligned with the Globally Harmonized System, uses the acute oral LD50 in rats: Class Ia (extremely hazardous) below 5 mg/kg; Class Ib (highly hazardous) 5 to 50 mg/kg; Class II (moderately hazardous) 50 to 2,000 mg/kg; Class III (slightly hazardous) above 2,000 mg/kg; and Class U (unlikely to present acute hazard) 5,000 mg/kg or more. Dermal toxicity can raise the class.
In India, the Insecticides Rules of 1971 under the Insecticides Act of 1968 require a coloured triangle on the label according to acute oral LD50: red for extremely toxic (1 to 50 mg/kg), yellow for highly toxic (51 to 500), blue for moderately toxic (501 to 5,000) and green for slightly toxic (above 5,000). Acute toxicity is only one dimension of hazard; chronic effects such as carcinogenicity, reproductive toxicity and endocrine disruption, and environmental hazards such as persistence and toxicity to bees and fish, are assessed separately. The FAO and WHO define highly hazardous pesticides (HHPs) using criteria that include WHO Class Ia or Ib, classification as carcinogenic, mutagenic or toxic to reproduction, listing under international conventions, and evidence of severe or irreversible harm under conditions of use.
After application, a pesticide is distributed among the target, crop surfaces, soil, water and air. Its fate depends on its properties (water solubility, vapour pressure, sorption, stability), on the environment (soil texture and organic matter, pH, temperature, moisture, rainfall, slope) and on application practice. Fate processes fall into transfer processes, which move the compound without changing it (sorption, volatilisation, leaching, runoff, drift, uptake by plants), and degradation processes, which break it down (microbial degradation, chemical hydrolysis and photolysis).
Sorption is the binding of a pesticide to soil particles, mainly organic matter and clay. It is described by the soil-water distribution coefficient Kd, the ratio of the concentration sorbed to soil (mg/kg) to the concentration in solution (mg/L) at equilibrium. Because organic carbon dominates sorption of many non-ionic pesticides, Kd is normalised to the organic carbon content: Koc = Kd / foc, where foc is the fraction of organic carbon. For example, a pesticide with Kd = 2 L/kg in a soil with 1 percent organic carbon (foc = 0.01) has Koc = 200 L/kg. Conversely, a pesticide with Koc = 300 L/kg in a soil with 2 percent organic carbon has Kd = 300 x 0.02 = 6 L/kg. High Koc (thousands or more, as for glyphosate, paraquat and pyrethroids) means strong binding and low mobility; low Koc (tens) means high mobility. Ionisable pesticides depend strongly on pH, and Koc is less useful for them.
The octanol-water partition coefficient (Kow, usually given as log Kow) measures how strongly a compound partitions into fat relative to water. High log Kow (above roughly 4 to 5) indicates potential to bioaccumulate in organisms and to sorb to organic matter.
Microbial degradation is the main route of breakdown in soil for most pesticides; it is faster in warm, moist, biologically active soils and can accelerate after repeated use as microbes adapt (enhanced degradation). Hydrolysis is reaction with water, often pH-dependent; organophosphates and carbamates hydrolyse faster in alkaline conditions. Photolysis is breakdown by sunlight on plant and soil surfaces and in clear water.
Volatilisation, loss to air as vapour, depends on vapour pressure and Henry's law constant (the air-water partition) and is highest from moist surfaces in warm conditions; some soil fumigants and older herbicides are notably volatile. Volatilised pesticides can travel and redeposit far away.
Leaching is downward movement in soil water towards groundwater. It is greatest for mobile (low Koc), persistent (long half-life) compounds in sandy, low-organic-matter soils with shallow water tables and heavy rain or irrigation soon after application. Preferential flow through cracks and macropores can carry even sorbed pesticides rapidly downward. The groundwater ubiquity score (GUS) combines persistence and sorption: GUS = log10(soil half-life in days) x (4 - log10 Koc). Compounds with GUS above 2.8 are classed as likely leachers, below 1.8 as improbable leachers, with a transition zone between. For example, a compound with half-life 30 days and Koc 100 has GUS = 1.48 x 2 = 2.95, a likely leacher, while one with half-life 10 days and Koc 1,000 has GUS = 1 x 1 = 1.0.
Runoff carries pesticides dissolved in water or attached to eroded soil to surface waters, especially when heavy rain falls soon after application on sloping, bare or compacted land. Strongly sorbed compounds move mainly with sediment; weakly sorbed ones mainly in solution. Vegetated buffer strips, conservation tillage, cover crops and avoiding application before forecast rain reduce runoff losses; see conservation-regenerative-agriculture.
Persistence is how long a pesticide remains in the environment. Degradation in soil is often approximated by first-order kinetics, in which the rate of loss is proportional to the amount present: C = C0 x e^(-kt), where C0 is the initial concentration, k the rate constant and t time. The half-life, or DT50 (time for 50 percent dissipation), is DT50 = ln 2 / k = 0.693 / k. Under first-order kinetics, DT90, the time for 90 percent dissipation, equals DT50 x ln 10 / ln 2, about 3.32 x DT50.
Because each half-life halves the remaining amount, after n half-lives the fraction remaining is (1/2)^n. For example, a pesticide with a soil DT50 of 20 days has k = 0.693/20 = 0.035 per day; after 60 days (three half-lives) 12.5 percent remains, and DT90 is about 66 days. A residue of 2 mg/kg with DT50 of 10 days falls to 0.25 mg/kg after 30 days. To fall from 4 to 0.5 mg/kg, an eight-fold decrease, takes three half-lives; with DT50 of 7 days that is 21 days. Students often make the mistake of assuming linear decline, which would wrongly predict complete disappearance after two half-lives.
Half-lives vary with the compound and the environment, so a single value is never a fixed property. Organochlorines such as DDT and dieldrin have soil half-lives of years; many modern insecticides and herbicides have half-lives of days to a few months. The same compound may persist far longer in cold, dry, low-organic-matter subsoils or anaerobic sediments than in warm, moist, biologically active topsoil. Laboratory DT50 values measured at a standard temperature are corrected for field conditions, and field dissipation studies include losses by leaching and volatilisation as well as degradation. Many pesticides show biphasic dissipation, fast at first and then slower as the remaining residue becomes less available, so simple first-order models can underestimate long-term persistence.
Regulators use persistence thresholds. Under the Stockholm Convention screening criteria, a chemical is considered persistent if its half-life exceeds two months in water or six months in soil or sediment. EU rules classify substances as persistent, very persistent, bioaccumulative and toxic using similar half-life thresholds, and such PBT substances face hazard-based exclusion. Degradation products (metabolites) must also be considered: some, such as DDE from DDT, are themselves persistent and toxic, and metabolites of some herbicides are frequently found in groundwater.
Part of a pesticide can become bound to soil organic matter in forms that cannot be extracted by normal methods. Bound residues may be gradually released, mineralised or remain permanently incorporated; their significance is debated. In crops, the decline of residues after the last application is the basis for the pre-harvest interval (waiting period) on the label: the minimum time between the last application and harvest so that residues fall below the maximum residue limit when good agricultural practice is followed.
Dietary risk assessment begins with toxicology. The no observed adverse effect level (NOAEL) from the most sensitive relevant animal study is divided by uncertainty factors to give the acceptable daily intake (ADI), the amount that can be ingested daily over a lifetime without appreciable risk, in mg per kg body weight per day. The default uncertainty factor is 100: 10 for differences between animals and humans and 10 for variation among humans. For example, a NOAEL of 5 mg/kg bw/day gives an ADI of 0.05 mg/kg bw/day, which for a 60 kg adult is 3 mg per day. The acute reference dose (ARfD) is the corresponding value for a single day's intake, used for compounds with acute effects. In the United States, the Food Quality Protection Act of 1996 requires an additional tenfold safety factor for infants and children unless data show it is unnecessary.
A maximum residue limit (MRL) is the highest level of a pesticide residue legally tolerated in or on a food or feed, in mg/kg, when the pesticide is applied correctly according to good agricultural practice. MRLs are set from supervised residue trials at the highest permitted use and are then checked against the ADI and ARfD through dietary exposure estimates; an MRL is a trading and enforcement standard based on good practice, not itself a safety threshold. A residue above the MRL indicates misuse or contamination but does not necessarily mean a health risk; a residue below it is expected to be safe if the dietary assessment was done properly.
Internationally, the Codex Committee on Pesticide Residues (CCPR) recommends Codex MRLs on the basis of evaluations by the Joint FAO/WHO Meeting on Pesticide Residues (JMPR). In the European Union, MRLs are set under Regulation (EC) 396/2005, with a default limit of 0.01 mg/kg, typically the limit of quantification, where no specific MRL exists. In India, MRLs are set by the Food Safety and Standards Authority of India under its regulations on contaminants, toxins and residues, and pesticides must be registered for the crop concerned. Differences between countries' MRLs, and the low EU default, are a recurring trade issue for exporters of rice, spices, tea and fruit.
Dietary exposure is estimated by multiplying residue levels by consumption. A theoretical maximum daily intake (TMDI) uses MRLs, an overestimate since most food has residues well below the MRL; refined estimates use median residues from trials and processing factors (washing, peeling and cooking often reduce residues). For example, if a person eats 0.2 kg of a vegetable with residue 0.5 mg/kg and 0.3 kg of a cereal with residue 0.1 mg/kg, intake is 0.1 + 0.03 = 0.13 mg per day; for a 60 kg person with ADI 0.01 mg/kg bw/day (0.6 mg/day), this is about 22 percent of the ADI.
Monitoring programmes measure residues in food, water and soil. Surveys in many countries find most food samples with no detectable residues or residues below MRLs, a small percentage above, and multiple residues in some samples. In India, a national monitoring scheme analyses samples of vegetables, fruits, cereals, spices and water. Water standards differ in approach: the EU drinking water limit is 0.1 micrograms per litre for any individual pesticide and 0.5 micrograms per litre for total pesticides, a precautionary limit not derived from toxicity, while WHO guideline values and US limits are derived compound by compound from toxicology. Sediments and soils accumulate persistent, strongly sorbed residues, including legacy organochlorines decades after their use stopped.
Bees and other pollinators are exposed through contact with sprays, residues on flowers and leaves, contaminated pollen and nectar, guttation water and dust from treated seed at sowing. Acute toxicity to honey bees is expressed as the LD50 in micrograms per bee by contact or oral exposure; the US Environmental Protection Agency classes compounds with LD50 below 2 micrograms per bee as highly toxic, 2 to 10.99 as moderately toxic, 11 to 100 as slightly toxic and above 100 as practically non-toxic. Many organophosphates, carbamates, pyrethroids and neonicotinoids are highly toxic to bees.
Neonicotinoids received particular attention because they are systemic, appear in pollen and nectar of treated crops and wild flowers, persist in soil and water, and have sublethal effects on foraging, navigation and colony growth in some studies. The European Union restricted three neonicotinoids (imidacloprid, clothianidin and thiamethoxam) on bee-attractive crops in 2013 and extended the ban to all outdoor uses in 2018. Field evidence on effects at the colony and population level is mixed for honey bees and more consistently negative for wild bees in some studies. Pollinator declines have multiple causes, including habitat loss, disease and climate, and pesticides are one of them.
Aquatic organisms are exposed through drift, runoff and drainage. Toxicity is expressed as the LC50 (concentration lethal to half the test organisms over a set time, such as 96 hours for fish or 48 hours for Daphnia) or EC50 (concentration causing an effect such as immobilisation or growth inhibition). Pyrethroids are extremely toxic to fish and aquatic invertebrates even though their toxicity to mammals and birds is low. Herbicides are most toxic to algae and aquatic plants. Organochlorines such as DDT bioaccumulate and biomagnify in food chains; DDE caused eggshell thinning and population declines in fish-eating and raptorial birds such as peregrine falcons, bald eagles and ospreys, which recovered after DDT was banned in many countries.
Soil organisms, including earthworms, springtails, mites, fungi and bacteria, can be harmed by some fungicides, fumigants and insecticides, with effects on decomposition and nutrient cycling. Earthworm toxicity is tested in standard tests, and soil microbial function is tested by nitrogen and carbon transformation. Birds can be poisoned directly by granules or treated seed, and indirectly by loss of insect food; herbicides that remove weeds also remove food plants for insects and birds. Indirect, food-web effects are often larger than direct toxicity but are harder to assess in regulatory tests. Pesticide self-poisoning is also a major public health problem in some low- and middle-income countries; bans on the most hazardous pesticides in Sri Lanka and elsewhere were followed by falls in suicide deaths without loss of agricultural output, according to published studies.
Persistent organic pollutants (POPs) are organic chemicals that persist in the environment, bioaccumulate in fatty tissues, biomagnify in food chains, travel long distances and harm health and the environment. Many early POPs were organochlorine pesticides. Their combination of persistence and semi-volatility leads to the 'grasshopper effect' or global distillation: they evaporate in warm regions, travel in the atmosphere, condense in cooler regions and repeat the cycle, accumulating in polar regions far from where they were used. Arctic peoples and wildlife carry high body burdens of POPs as a result.
The Stockholm Convention on Persistent Organic Pollutants was adopted in 2001 and entered into force in 2004. The original twelve POPs, the 'dirty dozen', were the pesticides aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, mirex and toxaphene, the industrial chemicals hexachlorobenzene (also a pesticide) and polychlorinated biphenyls (PCBs), and the unintentional by-products polychlorinated dibenzo-p-dioxins and dibenzofurans. Chemicals are listed in Annex A (elimination), Annex B (restriction, with specific exemptions; DDT is listed here because it may still be used for disease vector control in line with WHO guidance) or Annex C (unintentional production, to be minimised). Further pesticides have been added over time, including lindane and the other hexachlorocyclohexanes in 2009 and endosulfan in 2011.
New chemicals are evaluated by the POPs Review Committee using the screening criteria in Annex D: persistence (half-life greater than two months in water or six months in soil or sediment, or other evidence); bioaccumulation (bioconcentration or bioaccumulation factor greater than 5,000, or log Kow greater than 5, or other evidence); potential for long-range environmental transport (for example, half-life in air greater than two days, or monitoring data in remote areas); and evidence of adverse effects.
The Rotterdam Convention on the Prior Informed Consent Procedure for Certain Hazardous Chemicals and Pesticides in International Trade, adopted in 1998 and in force since 2004, requires exporters of chemicals listed in its Annex III to obtain the informed consent of importing countries; it lists many pesticides and severely hazardous pesticide formulations. The Basel Convention governs transboundary movement of hazardous wastes, including obsolete pesticide stocks. Together these are often called the BRS conventions, sharing a secretariat. The FAO and WHO International Code of Conduct on Pesticide Management provides voluntary standards for the whole life cycle of pesticides.
India is a party to the Stockholm and Rotterdam conventions. DDT is still produced in India for vector control under the convention's exemption. Endosulfan was banned in India by order of the Supreme Court in 2011, following health concerns in Kasaragod district of Kerala, where it had been sprayed aerially on cashew plantations for many years, and its global listing under the Stockholm Convention in the same year. Obsolete pesticide stocks remain a problem in many countries in Africa, Asia and eastern Europe, and their safe disposal is a long-running FAO programme.
Risk assessment combines hazard (the intrinsic potential to cause harm, measured by toxicity) with exposure (the amount that organisms actually encounter). A highly toxic compound used in small amounts in a closed system can pose less risk than a moderately toxic compound sprayed widely. Environmental risk assessment compares a predicted environmental concentration (PEC), estimated from application rate, fate properties and scenario models, with toxicity endpoints. The toxicity-exposure ratio (TER) is the toxicity endpoint divided by the exposure; for aquatic organisms under EU rules, an acute TER of at least 100 for fish and invertebrates, and a chronic TER of at least 10, is required unless refined assessment shows acceptable risk. For example, a fish LC50 of 0.5 mg/L and a PEC of 0.002 mg/L give TER = 250, which passes the acute trigger of 100; a Daphnia EC50 of 10 micrograms per litre with PEC 0.5 micrograms per litre gives TER = 20, which fails.
For bees, EU guidance has used a hazard quotient: application rate in grams of active substance per hectare divided by the bee LD50 in micrograms per bee, with a value above 50 triggering higher-tier assessment. For example, a product applied at 100 g/ha with a contact LD50 of 0.05 micrograms per bee has HQ = 2,000, far above 50, while one at 50 g/ha with LD50 of 2 micrograms per bee has HQ = 25.
Assessments are tiered: they start with simple, conservative estimates and move to more realistic and costly studies (semi-field, field, monitoring) only if the lower tier indicates possible risk. Mitigation measures such as buffer zones, timing restrictions and drift-reducing equipment can be required as conditions of approval.
In the European Union, active substances are approved at EU level under Regulation (EC) 1107/2009, with scientific assessment by the European Food Safety Authority, and products are authorised by member states. The regulation includes hazard-based cut-off criteria: substances classified as carcinogenic, mutagenic or toxic to reproduction in the highest categories, endocrine disruptors, and persistent organic pollutants or PBT substances are generally not approved, regardless of exposure. The Sustainable Use Directive 2009/128/EC requires national action plans, training, inspection of equipment, restrictions on aerial spraying and promotion of integrated pest management. Several pesticides widely used elsewhere, including atrazine and paraquat, are not approved in the EU.
In the United States, the Environmental Protection Agency registers pesticides under the Federal Insecticide, Fungicide, and Rodenticide Act, with residue tolerances under the Federal Food, Drug, and Cosmetic Act as amended by the Food Quality Protection Act of 1996, and reviews registrations periodically. In India, pesticides are registered by the Central Insecticides Board and Registration Committee (CIB&RC) under the Insecticides Act of 1968, which requires data on chemistry, bio-efficacy, toxicity, residues and packaging; use is approved crop by crop, and use on unregistered crops ('label claim') is not permitted. Reviews by expert committees have led to bans or restrictions on a number of older pesticides. A Pesticide Management Bill intended to replace the 1968 Act has been under consideration since 2020.
Total quantity of active ingredient applied is a poor measure of risk, because modern products are applied at grams per hectare while older ones were applied at kilograms, and toxicity differs by orders of magnitude. Risk indicators combine use data with hazard and sometimes exposure to track progress. They are used by governments to set targets, by researchers to compare systems and by food companies and advisers to choose products.
The Environmental Impact Quotient (EIQ), developed at Cornell University by Kovach and colleagues in 1992, combines ratings for toxicity to farm workers, consumers and various non-target organisms with persistence and other properties into a single number per active ingredient; the field EIQ multiplies the EIQ by the percentage of active ingredient in the product and the rate applied. It is simple and widely used, but has been criticised for arbitrary weightings and for combining incommensurable effects.
The treatment frequency index (TFI), used in Denmark and France, counts the number of standard doses applied per hectare per season: for each application, the dose applied divided by the standard (registered) dose, summed over applications. For example, a full dose of one product plus two half doses of others gives TFI = 1 + 0.5 + 0.5 = 2.0. It reflects intensity of use but not hazard. Denmark's pesticide load indicator, introduced in 2013, adds hazard: it estimates load on human health, environmental fate and ecotoxicity for each product, and Denmark taxes pesticides according to their load, making the most hazardous products more expensive. The EU uses harmonised risk indicators that weight quantities sold or emergency authorisations by hazard group, with heavier weights for candidates for substitution and unapproved substances.
Several governments and international agreements have adopted reduction targets. The Kunming-Montreal Global Biodiversity Framework of 2022, Target 7, calls for reducing the overall risk from pesticides and highly hazardous chemicals by at least half by 2030, including through integrated pest management. The EU Farm to Fork Strategy proposed a 50 percent reduction in the use and risk of chemical pesticides and in the use of more hazardous pesticides by 2030; a proposed regulation to make this binding was withdrawn in 2024 after opposition. France's Ecophyto plans since 2008 aimed to halve pesticide use but made limited progress for many years, illustrating how hard reductions are without changes in incentives and systems. Denmark's tax and Sri Lanka's bans on highly hazardous pesticides are examples of policies with measurable effects. Studies such as that of Lechenet and colleagues on French arable farms suggested that many farms could substantially reduce pesticide use without loss of productivity or profitability, although results vary by farm type.
Integrated pest management (IPM) combines prevention, monitoring and intervention to keep pests below damaging levels while minimising risks to people and the environment. The concept of integrated control was set out by Stern and colleagues in 1959, who defined the economic injury level (the pest density at which damage equals the cost of control) and the economic threshold, or action threshold (the density at which control should be applied to prevent the pest reaching the injury level). IPM uses, in order: prevention through resistant varieties, rotation, sanitation, planting dates and habitat for natural enemies; monitoring with scouting, traps and forecasting; non-chemical interventions such as biological control and mechanical methods; and selective pesticides only when thresholds are exceeded, choosing products with lower risk to non-target organisms and rotating modes of action to delay resistance.
The EU Sustainable Use Directive lists eight general principles of IPM that all professional users must apply, and the FAO promotes IPM through national programmes. In India, IPM became a cornerstone of national plant protection policy in 1985, with central IPM centres conducting farmer field schools and surveillance, and the National Centre for Integrated Pest Management of ICAR supporting research.
The best-known example is rice in Indonesia. In the 1980s, heavy, subsidised insecticide use killed natural enemies and caused outbreaks of the brown planthopper. A presidential instruction in 1986 banned 57 insecticides on rice, subsidies were phased out, and a national programme trained farmers through farmer field schools, in which groups of farmers learn by observing pests and natural enemies in their own fields through the season. Insecticide use fell sharply while yields were maintained. Farmer field schools spread to many countries and crops; evaluations show improved knowledge and lower pesticide use in many cases, though effects vary and costs per farmer are significant.
Reviews have found that IPM programmes often reduce pesticide use and increase yields or profits, but adoption of full IPM remains limited. Barriers include the knowledge and time needed for monitoring, risk aversion, advice from pesticide sellers, lack of reliable thresholds for many pests, and low prices of pesticides that do not reflect their external costs. Policies that address these, such as independent advice, taxes or bans on the most hazardous products, and support for biological control, help IPM reduce risk.
Repeated use of pesticides with the same mode of action selects for resistant pests, weeds and pathogens. Resistance raises use, costs and environmental load as farmers spray more or switch to more hazardous products. IPM reduces selection pressure by using thresholds, non-chemical methods and rotation of mode-of-action groups, and by maintaining refuges, as required for insect-resistant crops. Examples of resistance include brown planthopper to several insecticides, bollworms to pyrethroids, weeds such as Phalaris minor in wheat to isoproturon in north-western India, and many weeds to glyphosate.
Biopesticides are pest control agents derived from living organisms or natural materials. Main categories are: microbial pesticides, containing bacteria (Bacillus thuringiensis against caterpillars and some other larvae; Pseudomonas fluorescens against diseases), fungi (Trichoderma against soil-borne pathogens; Beauveria bassiana, Metarhizium anisopliae and Lecanicillium against insects), viruses (nuclear polyhedrosis viruses and granuloviruses, specific to certain caterpillars) and nematodes; botanicals, plant extracts such as neem (azadirachtin) and pyrethrum (pyrethrins); semiochemicals, especially pheromones used for monitoring, mass trapping and mating disruption; and biochemicals such as spinosad, derived from fermentation of a soil actinomycete. Natural enemies released for biological control, such as Trichogramma, are often grouped with biopesticides in policy, though they are not pesticides in the strict sense.
Biopesticides generally have narrow target ranges, low toxicity to mammals and many non-target organisms, short persistence and few residue problems, and they fit IPM and organic systems (see organic-farming-certification). Their limitations include slower action, sensitivity to sunlight, temperature and humidity, short shelf life, the need for precise timing, variable quality of products, and higher cost per treatment in some cases. Natural does not always mean safe: some botanicals, such as rotenone and nicotine, are toxic to fish or mammals, and spinosad and pyrethrins are toxic to bees when sprayed, so risk assessment is still needed.
Quality control is a major issue in many countries, including India, where surveys have found microbial products with low counts of viable organisms or contamination with synthetic pesticides. Biopesticides in India must be registered under the Insecticides Act; registration requirements are lighter than for synthetic chemicals in some respects but still demanding for small producers.
Biopesticides account for a small but growing share of the global pesticide market. Growth is driven by residue concerns among retailers and consumers, bans on older synthetic products, resistance, organic demand and policy support for IPM. In the EU, low-risk active substances and basic substances have simplified approval routes. Research priorities include formulation for field stability, combining microbial agents with reduced doses of chemicals, and producing agents locally at low cost.
Spray drift is the movement of pesticide droplets or vapour away from the target area during or after application. Particle drift, the movement of small droplets by wind at the time of spraying, is the main form; vapour drift arises from volatilisation after application. Drift harms neighbouring crops, especially when herbicides drift onto sensitive broadleaf crops, contaminates water and organic fields, and exposes people and wildlife.
Droplet size is the main factor that applicators can control. Spray quality is classified by standards into categories from very fine to ultra coarse, according to the volume median diameter (VMD, the diameter at which half the spray volume is in smaller droplets). Small droplets, often taken as those below about 100 to 150 micrometres, stay airborne longer and drift further; large droplets drift less but may give poorer coverage. Other factors include wind speed and direction, boom height, travel speed, air temperature and humidity (small droplets evaporate quickly in hot, dry air, becoming even smaller) and temperature inversions, stable air conditions often in the early morning or evening in which fine droplets can hang and move long distances.
Drift-reducing measures include: low-drift and air-induction nozzles, which produce coarser droplets containing air bubbles and can reduce drift substantially; lower boom heights and travel speeds; spraying only within suitable wind speeds, avoiding both calm conditions that may indicate inversions and strong winds; no-spray buffer zones next to water bodies and sensitive areas, which regulators may require as a condition of approval; vegetated windbreaks and hedges, see agroforestry-trees-on-farms; shielded sprayers; and drift-reducing adjuvants. In tree crops, air-assisted sprayers should be adjusted to the canopy to avoid blowing spray over the trees.
Aerial application, by manned aircraft and increasingly by drones, raises particular drift concerns. The EU Sustainable Use Directive prohibits aerial spraying in general, with limited derogations. In India, the government issued standard operating procedures in the early 2020s for spraying pesticides by drone, with crop-wise interim approval of products; drones reduce operator exposure and water use but require attention to drift and calibration.
Correct calibration ensures the intended dose is applied, avoiding both under-dosing (poor control and resistance) and over-dosing (residues, costs and environmental harm). For a boom sprayer, the application volume in litres per hectare is: L/ha = (nozzle output in L/min x 600) / (forward speed in km/h x nozzle spacing in m). For example, nozzles delivering 1.2 L/min at 6 km/h with 0.5 m spacing give 1.2 x 600 / (6 x 0.5) = 240 L/ha. If the label rate is 1 L of product per hectare, each 240 L of spray mixture must contain 1 L of product. Worn nozzles increase output and change spray quality, so nozzles should be checked regularly; EU rules require periodic inspection of application equipment.
Safe practice also includes appropriate personal protective equipment, mixing and loading away from water, triple rinsing of containers, safe storage and disposal, and record keeping. Many acute poisonings of farm workers in developing countries are linked to inadequate protective equipment in hot climates, leaking knapsack sprayers and re-entry into treated fields too soon.
Learn the main chemical groups and their modes of action, the WHO and Indian toxicity classes, and the fate parameters Kd, Koc, Kow and GUS. Practise first-order decay, half-life, ADI, TER, hazard quotient, treatment frequency index and sprayer calibration calculations until they are routine. Know the Stockholm Convention's dirty dozen, annexes and screening criteria, and the Rotterdam and Basel conventions.
As an adviser, residue analyst, regulatory scientist or sustainability officer, you interpret labels and safety data, recommend lower-risk products within IPM, calibrate sprayers and set buffer zones, investigate residue exceedances and drift complaints, and compile risk indicators for supply chains. Judgement is needed to distinguish an MRL exceedance from a health risk, to weigh hazard against exposure, and to give advice independent of product sales.
At senior level you shape national pesticide policy, registration decisions, retailer residue standards or corporate risk-reduction targets. Decisions include phasing out highly hazardous pesticides, choosing indicators and targets, designing taxes or incentives, supporting biopesticide registration and quality control, and balancing trade, farmer livelihoods and environmental protection.
Research questions include the fate of pesticides and metabolites under field conditions and climate change, sublethal and mixture effects on pollinators and aquatic communities, landscape-scale exposure modelling, validity of risk indicators, effectiveness of IPM and policy instruments in reducing risk, and formulation and field performance of biopesticides. Rigorous work combines field monitoring, controlled experiments, modelling and policy evaluation.
Toxicity classes, MRLs, triggers and approval status differ between countries and change with reviews; values quoted are from the named systems and should be checked against current rules. Half-lives and Koc values vary with soil and climate and are not listed compound by compound. The status of individual pesticides in India (bans, restrictions and pending reviews) changes frequently and is not listed in detail. Human health epidemiology of chronic pesticide exposure, residue analytical methods and remediation technologies are treated briefly or not at all. Evidence on colony-level effects of neonicotinoids and on the effectiveness of some policy instruments is mixed and is presented as such.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
These notes are free to read here, with no account.
What the Laisha app adds:
Free for ten days, then a subscription. We would rather write that here than have you find it later.