Aphids are small, pear-shaped insects that feed on the sap of tomato plants. Although aphids grow quickly, they are easy to control. In most cases, a gardener needs a spray bottle, insecticidal soap and water.
Remove aphids manually
The simplest way to kill aphids is to pinch off and dispose of aphid-infested leaves. If you wish to save the leaf, however, scrape the aphids off with a toothpick. Pay attention to the underside of leaves too.
Spray aphids with water
Using a spray bottle or water hose, spray aphids with cold water. The force of the water dislodges aphids from the tomato plant. Repeat this process every three or four days until the aphids are gone.
Apply insecticidal soap to the plant
In tougher cases, insecticidal soap can remove the waxy coating of aphids, causing them to dry out and die. If insecticidal soap is unavailable, combine 1 teaspoon of dish soap with 1 quart of water. Avoid applying soap solutions when temperatures rise above 90 degrees Fahrenheit or if the tomato plant is water-stressed, as this may damage the plant.
Keep the areas around tomato plants free of weeds. Many weeds carry aphids as well as other insects and diseases that may damage tomato plants.