A contact pesticide works only on pests or pathogens it physically touches, and usually has little or no residual action. A systemic pesticide is absorbed by leaves or roots and carried through the plant's vascular tissue, protecting new growth and reaching pests that feed on sap. Translaminar products sit in between: they move from one leaf surface to the other, but not throughout the plant.
In practice
Soaps, oils, many botanical extracts and some microbials are contact products, so coverage, especially of leaf undersides, decides whether they work, and repeat applications are needed. Systemic products persist in plant tissue, and with a crop that is smoked or concentrated that means a real risk of residues in the final product. Many jurisdictions permit very few or no systemic actives on cannabis, and test for residues; the conazole fungicide myclobutanil, which forms toxic combustion products, has been behind well-publicised cannabis recalls. Check what is registered for cannabis locally, and never assume a product legal on vegetables is legal on cannabis.
Don't confuse with
- Residual activity: how long a product keeps working; contact products can still have some.
- Biological control: living agents, not chemicals.
- Pre-harvest interval: the waiting time before harvest set on a label.
