Ants rarely cause problems directly. Instead, they make trouble by driving away creatures that might eat or parasitize aphids and whiteflies--two pests that do considerable damage. These pests excrete honeydew, a sugary sap ants like to eat. Some ants invade houses during rainy weather. Outdoors, their nests often grow larger when temperatures are mild--a nuisance for the gardener. Fire ants, found in the Southwest, inflict a painful sting and often build their mounds in lawns or compost heaps.
To reduce food sources for ants, keep aphid and whitefly populations under control. These pests sometimes live in tree canopies high above the ground; the only sign of their presence is a trail of ants marching up and down the tree trunk. To keep ants from reaching their goal, encircle tree trunks with sticky bands for several weeks. Pyrethrin kills ants on contact. Fire ants may relocate if their nests are repeatedly drenched with boiling or soapy water.
For chemical control, place baited ant traps sold for indoor use in the garden. Spray with carbaryl or diazinon to kill large infestations around patios. Drench or dust fire ant mounds with chlorpyrifos or diazinon.