The safety of insecticides is predominantly evaluated based on lethal effects on other animals. However, new findings from Eawag support the notion that also lower concentrations can be problematic, because they affect the nervous systems of fish and cause changes in their behaviour. This can indirectly affect the survival of fish populations and may be one of the reasons for the fish decline that we are observing in Switzerland. Researchers are uncovering the mechanisms of action and suggesting how these effects could be taken into account in the assessment and evaluation processes of insecticides.
Insecticides have a bad reputation. Most of them are designed to target the nervous system of fruit and crop pests (such as the aphid), but they often affect the nervous systems of other organisms as well. These include bees, aquatic insects – and vertebrates, such as fish or even humans. In some cases, this already occurs in very low concentrations. “Many fundamental aspects of the nervous system have changed very little over the course of evolution“, explains Ecotoxicologist Sarah Könemann, who researched the effects of insecticides in her doctoral thesis at the aquatic research institute Eawag and EPFL. For example, some insecticides target certain molecules in the nervous system of insects, but we have almost exactly the same molecules in our nervous system. This is why insecticides can have an effect on humans, too.
In most cases, the concentrations in which insecticides are found in the environment are not high enough to be acutely toxic, i.e. lethal, for vertebrates. ”However, there is a very wide range between a lethal effect and no effect“, explains Könemann. ”This is where I wanted to take a closer look.”
Fish can smell certain insecticides
For this reason, the researcher examined how six common insecticides affect the movement patterns and neuronal activity in the brains of zebrafish larvae. She discovered that these fish perceived the insecticides imidacloprid and diazinon as stress when they were exposed to a high concentration of them in a short time. And: Könemann was able to demonstrate that the fish larvae smelled these insecticides and subsequently avoided the substances. ”At first glance, this seems like a sensible reaction“, says the researcher. ”They flee from the insecticide and thus prevent chronic effects.”
Yet, such a change in behaviour can have negative consequences as well, e.g., if the fish avoid certain habitats as a result. This could lead to them no longer finding sufficient sexual partners or abandoning areas with a particularly rich food supply. “Such effects may therefore be another factor negatively impacting fish populations, which are already challenged by other stressors.”