Environmental impacts of pesticides

Last changed: 27 May 2016
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Use of pesticides in Sweden is low compared with that in many other countries, but there are great regional differences within Sweden. For example, use of pesticides in the county of Skåne represents over 50% of the total amount used in Sweden. Therefore a lower environmental impact can be expected in Sweden as a whole, but there can be a high impact at local level. Our knowledge of the biological effects that can be caused by normal use of pesticides in the field is limited.

 

Lack of knowledge about the impact on natural environments

Most of what we know about the effects of modern pesticides (pesticides designed to be easily broken down) is based on laboratory studies carried out under controlled conditions (for example a high temperature of 25°C), often with one experimental organism and one pesticide at a time. It is difficult to translate the results obtained in the laboratory to possible consequences in natural environments where a more complex system exists. In general, there are also few studies investigating toxicity (how poisonous a pesticide is to organisms) or of the effects of pesticide breakdown products under field conditions.

What is known is that several different pesticides are often used together within agriculture. Environmental monitoring has shown that several pesticides often occur together in agricultural streams. Two or more chemicals can sometimes interact so that the effect is more than doubled. We still know very little about how pesticides interact or, in the best case scenario, counteract each other in aquatic environments.

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Examples of environmental impact in aquatic ecosystems

Only a few field studies have investigated how the normal use of pesticides within agriculture can affect aquatic ecosystems. One example where negative ecological effects have been reported is a German study of a stream where 8 of 11 common benthic animal species completely disappeared and the remaining three species decreased drastically as a result of the use of insecticides (parathion-ethyl and fenvalerate) on neighbouring agricultural land. Insecticides (deltamethrin and permethrin) were also highly likely to have been involved in extensive fish kill-off that particularly affected eels in Lake Balaton in Hungary. Insecticides had been used in the area to control mosquitoes. 

Difficult to detect ecological effects in the environment

In the examples above from Germany and Hungary, the effects of pesticide use are very clear. However, it is not always as easy to detect ecological effects. One possible reason for this is that there is no clear relationship between spraying of a certain compound and the effects in the environment. Several pesticides can e.g. interact and give rise to unexpected effects. Moreover, water in the agricultural landscape is often exposed to different types of impact, such as high concentrations of nutrients or dredging and clearing. Changes in the environment can also take place slowly, making them difficult to detect. The effects may not be detectable for a long time after the exposure. Because of the close interactions between different organisms in aquatic environments, the effects can also influence completely different species or groups of species than the pesticide was originally intended for. It can sometimes be the breakdown products and not the parent compound that cause ecological effects, which further complicates the identification of cause and effect. CKB is working in different studies to assess the ecological effects of water in the agricultural landscape.

Mixing substances can result in interactive effects

In nature pesticides seldom occur alone, but in a mixture of different substances. These substances can affect each other so as to produce an additive or synergistic effect. An additive effect is when the observed effect corresponds to the sum of the toxicities of the individual substances. Substances can also interact so that the toxic effect is greater than the sum of effects of the individual substances, i.e. they have a synergistic effect.

Chironomid. Foto: SLU

Examples of interactive effects

Some laboratory studies have been carried out on the effects of low concentrations of pesticides on aquatic organisms. These clearly show that there is a risk of biological effects from pesticide use, even though it can be difficult to detect these effects. In a trial with mosquito larvae, researchers were able to show an interaction between atrazine and chlorpyrifos, two completely different pesticides. Atrazine is a herbicide that is not toxic to mosquito larvae even at high concentrations. Atrazine, which is a component of e.g. Totex strö, was banned from sale in Sweden in 1989, but is still detectable in aquatic environments. Chlorpyrifos is an approved insecticide that affects e.g. the swimming behaviour of mosquito larvae even at low concentrations. The product had limited use in Sweden and was banned in 2008, but is still approved for use within the EU. When mosquito larvae are exposed to atrazine and chlorpyrifos together, the atrazine affects the breakdown of chlorpyrifos in the larvae. This leads to the formation of breakdown products that are actually more toxic than the parent substance, which results in chlorpyrifos being more toxic at lower concentrations than if it were acting alone. Similar effects can also be obtained with other insecticides belonging to the same group as chlorpyrifos (e.g. methylparathion, diazinon, malathion and trichlorfon). This type of interaction is difficult to predict in view of the fact that the pesticides are directed at completely different target organisms. The risk of interactive effects can increase with the number of pesticides used within an area. There is also a risk of interactive effects between a pesticide and another environmental toxin, e.g. metals or persistent organic compounds such as PCB and brominated flame retardants. CKB is working on various studies for assessing possible interactive effects between different substances. 

Chronic effects emerge much later

The effects of a pesticide may emerge a long time after exposure, or after repeated or prolonged exposure, which is another difficulty when assessing the risks of pesticides. These are usually referred to as chronic effects and can cause e.g. cancer, reduced growth, impaired immune defence and lower reproductive ability. These factors can affect population size and, in the long term, the entire ecosystem. In pesticide testing, both acute and chronic effects are studied. Acute effects include mortality and changes in enzymes and movement patterns arising within a few hours (often 24-48) after exposure. Chronic effects are normally studied over 7-21 days. The results from such studies can be difficult to translate to nature, where the organisms are often exposed to low concentrations over a long period, in some cases the entire growing season. In order to uncover gradual changes in natural populations as a result of pesticide exposure, there is a need for long-term environmental monitoring programmes where communities of organisms are sampled.

Foto: Hans Lundqvist, SLU

Chronic effects mean that there is a slow change or that the effects become detectable only a long time after exposure. Examples of chronic effects are e.g. effects on reproduction, development and behaviour. Adult individuals perhaps show no signs of exposure, and it is only in the next generation that the exposure has an impact. This has been shown e.g. for salmon.

Examples of chronic effects - salmon reproduction

Several different pesticides have been shown to give rise to chronic effects at low concentrations through disrupting salmon reproduction. This applies for example for the insecticides cypermethrin, diazinon and carbofuran (breakdown products from carbosulfan) and the herbicide atrazin. Despite their dissimilarities as pesticides, these are able to cause similar reproductive disruptions in salmon. Before breeding, female salmon secrete pheromones that stimulate the males to produce sex hormones and milt. The sense of smell in the males is inhibited by pesticides and therefore they do not respond as effectively to the female pheromones, and production of hormones and milt is affected. Atrazine affects the sense of smell of male salmon at concentrations of only 0.04 µg/L, a concentration previously detected in Swedish streams. An additional effect of pesticides is that both salmon roe and milt are negatively affected, which leads to decreased fertilisation of the roe. There is a risk of breeding being unsuccessful and the consequences could be a slow decline in the salmon population, and/or depletion of the genetic material.

Indirect effects can be difficult to detect

A pesticide does not need to exert a direct toxic effect on an organism or group of organisms in order to have an impact. If other organisms living in community with it are affected, this can have an indirect effect on the primary organism. An example of an indirect effect is when an insecticide is spread to an aquatic environment and causes a decrease in the number of insect larvae and small crustaceans. Algae and bacteria present in the water are then exposed to lower grazing pressure and can instead increase their growth. This causes shading of larger aquatic plants, which decrease in biomass because of the increased competition. That in turn changes the living environment for insects and crustaceans, which can lead to further decreases in their populations and an algae-dominated community. This can create a negative trend that is difficult to break. The result of exposure to a relatively low concentration of insecticide can thus be similar to eutrophication effects. This makes it more difficult to uncover the reason for the ecological effects.

Researchers in Lund have shown that both insecticide (cypermethrin) and herbicide (metsulfuron methyl) can cause the same type of ecological effects in aquatic environments through indirectly affecting large groups of organisms. In trials, the exposure to both compounds led to a more algae-dominated community. This in turn resembles the effects seen in eutrophication of surface waters.

Impact on the biological diversity of terrestrial species

A large number of species that live in and around fields are dependent on the agricultural landscape for their survival. In recent decades the biological diversity in these environments has decreased. One of the reasons for this is considered to be use of pesticides. These may kill species living in the field and directly affected by spraying and those with their habitat in neighbouring areas, which are affected by wind drift of pesticides. The effect can be immediately fatal, but non-fatal effects that disrupt reproduction or animal behaviour may also arise. In addition, there is an indirect impact so that other species are affected apart from to those directly affected by the pesticide.

Historical use

Those products used in the ‘infancy’ of plant protection products, such as arsenic, mercury, DDT and organophosphates hit hard and more or less directly on a number of species, such as birds and pollinating insects. Many died or experienced serious damage, while the reproduction of other species such as sea eagles and peregrine falcons failed as the compounds accumulated in the environment. The effects were obvious, easy to demonstrate and resulted in many compounds being banned.

Direct and indirect impact

Modern pesticides are more specific but can also have a direct impact on species closely related to the target organism. For example, wild plants may be affected by herbicides and a number of insect species apart from the target insect can be eliminated in an area. However, many of the effects of modern pesticides used today are indirect. The birds in the agricultural landscape are affected by some of their food sources disappearing through insecticide use. Bumblebees, wild bees and other pollinators find it more difficult to find food if many of the plants they use as pollen and nectar sources disappear due to herbicide use. Conversely, plant communities are affected if some species of pollinating insect disappear due to the use of insecticides. For example, wind pollinated plants may come to dominate over insect pollinated plants, while plant species that are dependent on a certain insect species for their pollination may disappear.

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Photo montage of biological diversity in a summer field by Monica Kling. 

More knowledge needed

It is difficult to determine and confirm how strongly pesticides contribute to changes in biological diversity, since a number of different changes occur continually in the cultivated landscape. In areas with very intensive pesticide use, a number of factors in the environment are also affected by the intensive farming within which heavy pesticide use occurs. There would therefore need to be a monitoring programme in which biological diversity and the use of pesticides are documented in a number of areas throughout the country.   

More knowledge needed on breakdown products in the environment

Pesticides are broken down, more or less rapidly, via chemical, physical or biological processes to one or more breakdown products. Relatively little is known about the breakdown products formed, how they are spread and how they act in the environment. For many of the breakdown products there is absolutely no available ecotoxicological information, i.e. information on the toxicity of the substances for different organisms. A similar lack of information exists for physico-chemical properties, i.e. the properties that determine e.g. the spread to aquatic environments and the degradability. There are few published scientific investigations on the effects of breakdown products in the environment. 

Of course, it is difficult to make a risk assessment and predict possible ecological effects when the information available is so inadequate. In some cases, the breakdown products may be more or less toxic than the parent compound, but based on the data available today most breakdown products appear to be clearly more toxic than their parent compound. British researchers carried out a study in 2003 on almost 40 different pesticides and their breakdown products and showed that for those which were more toxic than their parent compound, this toxicity could be explained by one or more of the following four factors: 1) The structure of the active ingredient persisted in the breakdown product, 2) the breakdown product was the active ingredient in the pesticide, 3) the breakdown product accumulated more strongly than the parent compound and 4) the breakdown product was a more potent pesticide. 


Contact
Page editor: mikaela.gonczi@slu.se