Pesticides are a vital tool in safeguarding the world’s food supply. The need to protect crops from pests, disease, and weeds has existed for centuries, with the Roman’s having used ash, crushed cypress leaves and diluted urine to protect their crops. Nowadays, pest control methods and the chemicals used are a great deal more sophisticated. It is estimated that without crop protection, which includes the use of pesticides, half of the world’s food crops would be lost.
Aside from deterring insects from eating the plants, the use of chemical pesticides is essential with a view to protecting human health, which can be compromised when crops are contaminated by rats, mice, flies and other insects. However, dialogue surrounding human health and pesticide use often tends to focus on the capacity of pesticides to harm human health rather than protect it. Given the potential harm that chemical pesticides may cause, they are naturally subject to strict regulation. These regulations have a major impact on the agricultural industry, in many cases limiting crop production and reducing income for farmers. In 2021, the UK government introduced new “Maximum Residue Levels”. Consequently, food growers are forced to perform a balancing act, using as little pesticide as possible, while maintaining crop yield.
Problems facing the pesticide industryThere are many factors that make the use and development of pesticides difficult and contentious. These include:
- Consumer opposition to foods produced using pesticides, owing to both the reported health and environmental implications.
- Many weeds, insects, and fungi have been shown to have developed a resistance to existing herbicides, which have remained largely unchanged in terms of their mode of action for nearly 30 years.
- Invasive species impact crops and require new and multiple approaches. Fall armyworm, for example, are reported to have colonized one-third of the world within the course of just one year.
- The half-lives of some pesticides are relatively long and may have the dire potential to induce adverse effects when released into the soil, terrestrial and aquatic systems.
- Regulatory hurdles are becoming increasingly strict. More than half of the active ingredients used in pesticides prior to 1991 have since been withdrawn from the EU on account of pesticide regulation.
All of these issues place an increasing burden on the food industry and highlight the need for research into the development of new, safe pesticides if we are to ensure sustainability within modern architecture. In this sense, the use of zebrafish in the testing of pesticides represents a major and important breakthrough in this field as brings reliable toxicity results in a cost-effective manner.
Pesticide toxicity testing with zebrafish
Zebrafish offer many advantages with regard to the testing of pesticides to ensure that food treated with the chemicals in question is safe for human consumption. The fish can be maintained with lower costs compared with mammals and reproduce in large numbers. Zebrafish also share a large proportion of their genetic code with humans (around 70-75% homology), meaning that test results are readily translatable to humans.
Moreover, another advantage that zebrafish offer over other IN VIVO models is that they are transparent during their embryonic and larval phases, which enables the direct observation of the internal physiological effects and organ damage in response to various chemicals and toxins. Recently developed fluorescence-based assays have used fluorescent probes to monitor the physical response to pesticide chemicals and toxicity mechanisms based on the level of fluorescence produced by zebrafish larvae and the assessment of biochemical biomarkers.
Besides, zebrafish embryos represent a New Alternative Model (NAM) for IN VIVO toxicity testing as they have a fast organ development with most of the organs being physiologically functional at 5 days post fertilization. This allows multiple testing options in this stage, even behavioral and neuronal studies, when the animal is still an embryo. This significantly reduces the ethical concern compared with adult vertebrates and is aligned with the 3Rs principle of animal care.
Pesticide exposure has been shown to cause both acute and long-term health problems in humans, including harm to the reproductive system. One benefit offered by zebrafish in this regard is that they can be genetically modified to produce green and red fluorescent proteins in the male and female gonads respectively. It has been shown that exposure to toxicity can cause a higher proportion of larvae to develop into male adult fish. Observing the color of the fluorescence produced by the larvae under exposure to pesticides enables quick determination of the sex of the larvae, with a view to assessing whether certain chemicals disrupt oogenesis, thus causing a higher percentage to develop as males. Another example is the toxicity to the reproductive system shown by Deltamethrin a pesticide commonly used as an insecticide and weed killer on lawns, in gardens, and on golf courses. Exposure to this pesticide caused reduced egg and sperm production in the zebrafish. Therefore, such studies may provide valuable evidence and serve as effective tools for identifying safe pesticides.
There is a wide variety of toxicology assays based, principally, on zebrafish embryos and work evaluating the toxicity of multiple chemical compounds including candidates for being new pesticides and agrochemical compounds. The toxicity evaluation of these compounds includes studies of teratogenicity and thyroid disruption IN VIVO by transgenic zebrafish expressing the fluorescence protein mCherry under the thyroglobulin promoter, the thyroid hormone precursor. Besides, reproductive toxicity assays are also being developed.
Even in the event that a completely harmless, yet effective pesticide were to be found, the development of resistance in unwanted plants and insects means that the search for new and innovative pesticides is set to continue indefinitely. With profit margins already tight, and faces with ever-tightening regulations, agricultural producers are in desperate need of new pest control methods that are effective, safe, and inexpensive to produce.
The use of the zebrafish model as a NAM in agrochemical research offers numerous advantages and promising avenues for development, being a reliable and cost-effective IN VIVO model. The fish’s genetic similarity to humans makes it a valuable preclinical testing tool, with many ground-breaking studies having already been carried out using a wide range of advanced technologies. Zebrafish may well hold the key to overcoming the problems facing the pesticide industry.
- Why do we need pesticides? (Irish Department of Agriculture, Food, and the Marine), (https://www.pcs.agriculture.gov.ie/aboutus/aboutpesticides/whydoweneedpesticides/)
- Importance and benefits of pesticides - (Pesticidefacts.org) (https://pesticidefacts.org/topics/necessity-of-pesticides/#:~:text=Without%20crop%20protection%2C%20including%20pesticides,to%20insects%2C%20diseases%20and%20weeds.&text=Pesticides%20are%20important.,as%20raising%20productivity%20per%20hectare.)
- Five challenges facing the pesticides industry (CHAP Solutions), (https://chap-solutions.co.uk/blogs/five-challenges-facing-the-pesticides-industry/)
- Akhlaq Hussain et. al., Multiple Screening of Pesticides Toxicity in Zebrafish and Daphnia Based on Locomotor Activity Alterations (2020), Biomolecules Journal, (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7564125/)
- Mirna Velki et. al, Pesticides diazinon and diuron increase glutathione levels and affect multixenobiotic resistance activity and biomarker responses in zebrafish (Danio rerio) embryos and larva (2019), Environmental Sciences Europe, (https://enveurope.springeropen.com/articles/10.1186/s12302-019-0186-0)
- Zebrafish as a model to study the toxicity of different pesticides – a case study (2021), Bionomous, (https://bionomous.ch/articles/zebrafish-model-toxicity-pesticides/)
- Jaka, O., et al., Screening for chemicals with thyroid hormone-disrupting effects using zebrafish embryo (2023). Reproductive toxicology (https://www.sciencedirect.com/science/article/abs/pii/S0890623823001375?via%3Dihub)