Pesticides and Fertilizers: A toxic relationship that is stressful for frogs

Reference: Van Meter, R.J., Adelizzi, R., Glinski, D.A., and Henderson, W.M. 2019. Agrochemical mixtures and amphibians: The combined effects of pesticides and fertilizer on stress, acetylcholinesterase activity, and bioaccumulation in a terrestrial environment. Environ. Toxicol. Chem. 38 (5): 1052-1061.

Image demonstrating the application of pesticide and fertilizers in large scale agricultural productions: Source: Photograph taken by Lite-Trac.

Modern civilization would not be possible without the manifestation of agricultural practices thousands of years ago. It had a modest beginning of gathering wild grains to plant and harvest to sustain one’s own family.  Today, large-scale industrial agriculture is widespread. In an effort to increase crop productivity and yield, farmers routinely use chemicals. Some of these chemicals are used to provide plants the nutrients they need to grow (i.e. fertilizers) and others are used to control the presence of organisms such as insects and weeds that damage crops (i.e. pesticides). Pesticides and fertilizers are frequently combined to form a mixture that is sprayed onto the land during the growing season. Since the 1960s, the use of chemicals in agriculture (agrochemicals) has dramatically increased. This trend will continue to rise in response to a growing human population. While these types of chemicals have given us a great advantage, they have also come at a cost to the environment.

Containment fields only exist in SciFi
Image of breeding pond for amphibians surrounded by cropland. Source: Photograph taken by Roland zh

An invisible force field does not surround a piece of land preventing objects or organisms from entering or exiting. The spraying of pesticides and fertilizers onto crops cannot be contained to a specific area. There is a risk that the chemical mixture will be transported by wind and water to an unintended site as well as cause harmful effects on wildlife that are unintentionally exposed. Due to factors related to space and time, amphibians are a particularly vulnerable group within the agricultural landscape. Amphibians will travel to and from breeding ponds by crossing vast areas of croplands. In northern latitudes, most amphibians breed in the spring, which is also a common application window for agrochemicals. Understanding the interaction between pesticide/fertilizer mixtures and amphibians is vital to helping sustain populations that are dramatically declining worldwide.

The black hole
Image of a crop duster spraying pesticides on a field. Source: USDA Photograph taken by Charles O’Rear.

Scientists know that specific agrochemicals inflict harmful effects on amphibian populations throughout their development from egg to adult. Research has shown that pesticides and fertilizers are linked to deformities, stunted growth, altered feeding behaviour, increased disease susceptibility, hormonal imbalances as well as death in amphibians. However, it becomes increasingly difficult to gain a foothold on the potential effects when thousands of agrochemicals are currently on the market and new ones are introduced each year. There is no doubt that amphibians are being exposed to multiple chemicals at a time, but what is unknown is how they interact with each other in the environment as well as how they behave simultaneously within the amphibian body.

Chemicals within a mixture may interact with each other causing an increase or decrease of harmful effects to amphibian body systems. (A similar concept: Your doctor and pharmacist review your current medications to ensure new medications will not interact and cause side effects. Now imagine that they have no idea how they will interact in your body because the drugs were originally developed to treat your dog). Here lies the black hole.


Shedding light in a dark corner

Van Meter and colleagues wanted to shed some light on this particularly dark area of scientific knowledge. The goal of their study was to better understand the uptake and resulting effects of two pesticides (i.e. atrazine and alachlor) and one fertilizer (i.e. urea) separately and as mixtures on young adult Southern Leopard frogs. The frogs were divided into 8 different groups that covered every exposure scenario (i.e. no chemical, atrazine only, alachlor only, urea only, atrazine and alachlor combined, atrazine and urea combined, alachlor and urea combined as well as alachlor, atrazine and urea combined). Each exposure group consisted of 18 frogs. Each frog was placed into a bowl containing soil that was treated according to one of the chemical scenarios listed above. After an 8-hour exposure period, samples were collected to measure stress hormone levels, nervous system activity as well as how much of each chemical was detected in amphibian tissues before and after a 1-hour cleansing period.

A glimmer of a conclusion
Image of a Southern Leopard frog. Source: Photograph taken by Stephen Friedt

Van Meter and her fellow researchers observed higher pesticide tissue levels, elevated stress hormones and altered nervous system activity in Southern leopard frogs after skin contact with a common pesticide-fertilizer mixture. The interaction between the chemicals and amphibians became illuminated. They found that fertilizers such as urea can actually reduce the uptake as well as slow down the removal of the pesticide atrazine by frogs. By interfering with a frog’s ability to remove atrazine from its body, it actually builds up over time and can cause more serious health effects even though less of it is absorbed through the skin. In addition, pesticides like atrazine increased the uptake of other pesticides (i.e. alachlor) into frog tissues. Frogs exposed to both pesticides showed a marked increase in their stress response as well as changes to their nervous system activity. Consequently, pesticide and fertilizer mixtures have the potential to negatively impact reproduction and survival in amphibians by improving pesticide uptake and slowing its removal by amphibians.

These nuggets of information are critical to providing insight and guidance when regulating the usage of these chemical mixtures. Continuing scientific investigations into the most commonly used agrochemical mixtures and their effects on amphibians is vital. With further research, recommendations to prevent the mixing of certain agrochemicals that are known to be detrimental to amphibians could be created.

How can you help?

Only use pesticides and fertilizers in your home and garden when absolutely necessary. Often natural methods such as introducing beneficial insects or adding nutrients to soil through composting are highly effective. These natural alternatives are possible for large-scale farm operations as well (check out this story). If you do decide to use a pesticide or fertilizer, it is IMPORTANT that you follow the instructions for safe use, storage and disposal. Not following these instructions could result in you unknowingly exposing yourself, your family and of course frogs to toxic chemicals!

Reviewed by:

Whitney Kroschel

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Anita Masse

Anita is currently a research manager/administrator for the University of Saskatchewan (Canada) branch of the EcoToxChip project. In 2016, she graduated with a MSc in Aquatic Ecotoxicology focusing on the reproductive and developmental effects of elevated dietary selenium on amphibians. She looks forward to imparting a "bite" of scientific knowledge that will empower readers to engage in discussions that can inspire change.

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