#66 Translocation of applied chemicals (pesticides) throughout the plant

People who grow crops for food sometimes need to use chemicals called pesticides. Pests such as insects that eat the crop plants, or fungi that grow on them, can greatly reduce the yield of the crop. Pesticides are use to kill the insects or fungi.

Some pesticides kill only the insects or fungus that the spray touches. They are called contact pesticides. They can be very effective if they are applied properly, but they also kill insects and pests that are useful to the plant.

Systemic pesticides are more effective because when sprayed onto the leaves of the plant, they are absorbed by it through the cuticle or stomata and into the phloem tubes. They move through the plant in the phloem (translocation)  and are taken in by any insect eating the plant or sucking up phloem sap. 

So any insect feeding on the plant, even if it was hidden under the leaf where the spray could not reach it, will eventually end up feeding on pesticide. The same is true for fungi; no matter where they are growing on the plant, the pesticide will eventually reach them. Once an insect has ingested enough pesticide it will die, meanwhile the harmless insects remain safe.

The disadvantages of systemic pesticides are that they may accumulate in the food chain. 

Systemic pesticides may need to be taken up by roots or through the leaves.