Insecticides are chemicals that kill insects. Farmers reach for them when caterpillars strip a cabbage patch, aphids clog a wheat head, or potato beetles defoliate a field faster than anything else can respond.

The category covers a wide range of compounds. Organophosphates and carbamates, developed in the mid-20th century, attack the insect nervous system. Pyrethroids, modelled on a compound found in chrysanthemum flowers, are common in both agriculture and household sprays. Neonicotinoids, introduced in the 1990s, move through plant tissue and are taken up by insects feeding on sap. Bacillus thuringiensis, a soil bacterium, is a biological insecticide that kills specific larvae without harming most other organisms.

Problems and precautions

• Resistance develops quickly in short-generation pests
• Broad-spectrum products kill pollinators and natural enemies
• Runoff can contaminate streams and groundwater
• Applicators need protective equipment and careful timing

Spraying at dusk, when bees have returned to the hive, is a common workaround. So is scouting fields and treating only when pest numbers cross a threshold.