A pesticide is a chemical substance designed to kill or control pests that damage crops, spread disease, or invade homes and buildings. Farmers spray pesticides on their fields to protect wheat, corn, and vegetables from hungry insects, destructive fungi, and unwanted weeds. Gardeners use pesticides to keep aphids off roses or prevent beetles from destroying tomato plants.

The word combines pest (an unwanted creature or plant) with the suffix -cide (meaning “killer”). Different types of pesticides target different problems: insecticides kill insects, herbicides kill weeds, and fungicides kill harmful fungi and molds.

Pesticides have dramatically increased the world's food supply by protecting crops from destruction. Before modern pesticides, farmers regularly lost huge portions of their harvests to insect swarms and plant diseases. However, pesticides require careful use because they can harm helpful creatures like bees and butterflies, and some can pollute water or soil if used improperly.

Scientists constantly work to develop pesticides that target only harmful pests while leaving beneficial insects and the environment unharmed. Organic farmers often use natural pesticides made from plants or minerals instead of synthetic chemicals, though these can sometimes be less effective at protecting crops from damage.