Exposure and environmental impact

Indirect effects can be difficult to detect

A pesticide does not need to have 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 a 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.