Pesticide accumulation (SQA National 5 Biology): Revision Note

Accumulation of toxins

• Pesticides contain toxins designed to kill target pests, but the same chemicals can also harm non-target organisms, such as pollinators, predators and aquatic life

Accumulation within organisms

• When organisms consume plant material that has been sprayed with pesticides, toxins can build up inside the tissues over time

  • This is known as bioaccumulation

• While the toxins may not be harmful to non-target organisms in small doses, they can become toxic at higher concentrations

Accumulation through food chains

• Toxins can also accumulate as they pass between the levels in a food chain

  • This is known as biomagnification

• As toxins are passed up the food chain from one level to the next, they can become more concentrated due to the decrease in total biomass of organisms at higher trophic levels

  • I.e. the smaller organisms at the bottom of the food chain will each consume a small volume of toxin, and then the organisms at the top of the food chain will consume many smaller organisms and receive a much larger dose

• Toxicity increases at higher levels of the food chain and can reach lethal levels

Example: DDT

• DDT was a widely used insecticide in the mid-20th century that was found to have harmful effects on top predators, such as birds of prey

  • When DDT was sprayed on crops, it would leach into waterways and eventually enter freshwater and marine ecosystems

  • DDT would then enter food chains via plankton and accumulate in the bodies of fish

  • These fish would then be eaten by birds, which would accumulate higher concentrations of DDT

Alternatives to pesticides

• Biological control and GM crops can reduce the spraying of pesticides by preventing or suppressing pests in other ways

Biological control

• Biological control uses living organisms, e.g. predators, parasitoids, or disease-causing microbes, to reduce pest populations, instead of chemical sprays

• Examples include:

  • ladybirds and their larvae predate on herbivorous aphids

  • parasitic wasps lay eggs inside caterpillar eggs, killing the pest before it hatches

  • predatory mites hunt red spider mites on crops like tomatoes and cucumbers

GM crops and pest control

• GM crops are crop plants in which the DNA has been deliberately altered to give useful traits, such as resistance to insects or disease

• They can be used to reduce the need for pesticides in several ways, e.g.:

  • Bt maize / cotton produce a highly specific Bt protein that is toxic to certain insect larvae

  • A GM soybean plant produces an enzyme inhibitor that blocks digestive enzymes in insect guts

  • Crops may be engineered to release insect sex pheromones, which confuse pests and reduce mating, lowering pest numbers