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.
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 lead to the males producing sex hormones and milt. The sense of smell in the males is inhibited by pesticides. They therefore do not respond as effectively to the female pheromones, and production of hormones and milt is affected. Atrazin affects the sense of smell of male salmon at concentrations of only 0.04 µg/litre, which is in line with the concentrations 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 a depletion of the genetic material.