Neonics persist – particularly in soil – for months and in some cases years and environmental concentrations can build up. This effectively increases their toxicity by increasing the duration of exposure of non-target species.
The metabolities of neonics (the compounds which they break down into) are often as or more toxic that the active ingredients.
The classic measurements used to assess the toxicity of a pesticide (short-term lab toxicity results) are not effective for systemic pesticides and conceal their true impact. They typically only measure direct acute effects rather than chronic effects via multiple routes of exposure. In the case of acute effects alone, some neonics are at least 5,000 to 10,000 times more toxic to bees than DDT.
The effects of exposure to neonics range from instant and lethal to chronic. Even long term exposure at low (non-lethal) levels can be harmful. They are nerve poisons and the chronic damage caused can include: impaired sense of smell or memory; reduced fecundity; altered feeding behaviour and reduced food intake including reduced foraging in bees; altered tunneling behaviour in earthworms; difficulty in flight and increased susceptibility to disease.