Among consumers worldwide, there is a common belief: “Since citrus fruits are harvested in winter, they must have been sprayed more heavily in summer to maintain quality out of season.” Or: “If summer vegetables like tomatoes and cucumbers are grown in greenhouses during winter, more pesticides are used to control pests and diseases; therefore, pesticide residues must be higher in winter.”
These perceptions are not entirely baseless; however, the determining factors of pesticide residues (the “intensity” at harvest) are not simply the product’s natural season. The actual determinants of residue levels are:
- The timing of pesticide application and the interval before harvest (Pre-Harvest Interval – PHI)
- The type of pesticide used (systemic or surface-acting contact)
- The conditions of the production environment (greenhouse/open field, temperature, light, humidity)
- The Integrated Pest Management (IPM) strategies applied by the producer.
Comparing Seasonal Perceptions with Reality
For instance, citrus fruits harvested in winter are not necessarily more “treated” in summer. Spraying usually takes place in spring and summer, while harvest occurs in winter. Residue levels are determined by the interval between application and harvest, as well as the chemical properties of the pesticide.
Similarly, greenhouse production is more complex than often assumed. In some cases, pesticide use may indeed be higher for summer vegetables grown in winter greenhouses, since enclosed environments have high humidity, pest pressure continues year-round, and low light and cooler temperatures slow down pesticide photodegradation (light-induced breakdown). However, in modern greenhouses, biological control and integrated methods can reduce this need. With proper practices, climate control, and natural pest management, it is even possible to use fewer pesticides in protected environments. Therefore, the idea that “greenhouse in winter = more residues” is also not always true.
Main Factors Determining Pesticide Residues
Application Timing and Pre-Harvest Interval (PHI): The waiting period before harvest (PHI) is the most important determinant of pesticide residues. If adhered to, particularly surface residues from contact pesticides can largely degrade naturally.
Pesticide Type: Contact pesticides remain on the surface of fruits and vegetables; washing and environmental factors can reduce them quickly. Systemic pesticides, on the other hand, penetrate plant tissues and reach the fruit interior, persisting longer.
Environmental Conditions and Degradation: Sunlight, temperature, and microbial activity accelerate pesticide degradation. In greenhouses (with less UV light and more humidity), half-lives can be longer, meaning residues may be detectable for longer. In addition, post-harvest and storage methods affect the breakdown of pesticides.
Production Method and Integrated Pest Management (IPM): Both open-field and greenhouse farming can reduce pesticide use through biological control, natural enemies, and monitoring systems. Especially in modern greenhouse farming, dependence on chemical pesticides is decreasing.
Situation in Turkey and Worldwide
According to annual reports by the European Food Safety Authority (EFSA) and FAO/WHO, most food samples are below legal maximum residue limits (MRLs), but a small proportion of violations are detected. These violations often cluster in specific products and seasons.
In Turkey, official monitoring programs are in place, and for exports, the EU’s Rapid Alert System for Food and Feed (RASFF) occasionally reports pesticide-related alerts. These tend to increase during peak harvest and export periods.
Seasonally Higher-Risk Products in Turkey
Spring (April–June):
- Strawberries: Turkish strawberries in May–June are subject to frequent and diverse spraying. Their thin skin and rapid consumption make them more prone to residues compared to other fruits.
- Cherries: Due to short harvest periods and export intensity, pre-harvest insecticide use increases residue risks.
Summer (June–August):
- Grapes, blueberries: Fungicide applications against fungal diseases increase in summer, making residues more likely in grapes and other summer fruits.
- Tomatoes, peppers, eggplants: High pest pressure in hot months leads to increased insecticide and fungicide use.
Autumn (September–November):
- Apples, pears, pomegranates: Post-harvest storage treatments and certain fungicides determine residue risks in these fruits.
Winter (December–February):
- Citrus fruits (oranges, mandarins, lemons): Spraying usually occurs in spring–summer; since harvest is in winter, if PHI is observed, residue levels are generally within legal limits.
Season Alone Is Not a Reliable Indicator
Seasonality by itself is not a dependable measure of pesticide residues. The citrus example shows why the assumption “winter fruits are more treated in summer” is flawed: residue levels depend on application timing, pesticide type, and PHI.
For consumers, the key considerations are the producer (or distributor) and the nature of the food itself. Early-season harvests of strawberries and other soft fruits are especially critical, as they face both diverse pesticide use and rapid consumption. Such early-season products, marketed at high prices due to low competition, may actually pose higher risks. Furthermore, the structure of the fruit or vegetable affects pesticide penetration and thus residue levels.
Greenhouse production can slow residue breakdown due to enclosed conditions, but good management can reduce pesticide needs.
While official monitoring reports in Turkey and globally show relatively good compliance, civil society reports highlight risks of multiple residues and emphasize the particular sensitivity of children.
Conclusion
Consumers and policymakers must go beyond “season” and consider scientific criteria. Pesticide testing and reporting should be expanded, and risk assessments should be made based on evidence. Incorporating seasonal fluctuations into monitoring systems would improve effectiveness. Targeted inspections during peak harvest and export seasons would strengthen market control. Educating farmers on PHI compliance and integrated pest management provides essential tools to reduce overall pesticide use. Most importantly, sharing monitoring results with the public strengthens good-faith information flow. Transparency increases consumer trust and protects public health.
References
European Food Safety Authority (EFSA). Annual Report on Pesticide Residues in Food, 2023. EFSA Journal 2025;23(1):e20010. Available at: https://www.efsa.europa.eu/en/efsajournal/pub/20010
World Health Organization (WHO) & Food and Agriculture Organization (FAO). Joint FAO/WHO Meeting on Pesticide Residues (JMPR) – 2023 Evaluations. Available at: https://www.fao.org/pest-residues-jmpr
European Commission – RASFF (Rapid Alert System for Food and Feed). Annual Summary Report of 2024 on pesticide-related violations and alerts among EU countries. Available at: https://food.ec.europa.eu/safety/rasff_en
Republic of Türkiye, Ministry of Agriculture and Forestry. Monitoring Program Results for Pesticide Residues in Plant Products, 2023. Available at: https://www.tarimorman.gov.tr
Republic of Türkiye, Ministry of Agriculture and Forestry – General Directorate of Food and Control. Annual Activity Reports on Food Control Services. Available at: https://www.tarimorman.gov.tr/GKGM
