Why Are There Ants on My Tomato Plants?

Ants scrambling over tomato plants are most likely worker ants on a mission to gather food. The good news is that ants rarely eat tomato fruit or plant parts. What they do like to eat is the sweet honeydew produced by aphids, scale insects, and mealybugs.

According to Zach Schumm, insect diagnostician with Iowa State University, if you're seeing ants on your tomato plants, “There could be aphids or some other sap-feeding insect resulting in honeydew production,” he says. The available honeydew is what draws the ants, not your tomatoes.

Ants as Pest Indicators

Ants racing up and down the stems and leaves of tomato plants may be annoying, but they're simply a sign of pest presence. It's the pests that leave behind the honeydew that you need to monitor. “Ants on tomato plants are a good reason to scout for sap-feeding insects (aphids being the prime one) and to determine if that population is high enough to warrant management,” Schumm says.

How to Scout for Aphids

Aphids are the most common sap-feeding plant pest. These tiny, soft-bodied insects feed on the leaves of many garden vegetables. Look for them in groups on the underside of leaves. Extensive aphid infestation can cause plant growth to slow significantly and leaves to curl downward.

Aphids are attracted to tender, new growth, making them most abundant in late spring and early summer when tomato plants are growing vigorously. Wet conditions or high temperatures will decrease the population. Aphid populations can rise quickly in cool, dry conditions. 

When to Take Action

Because natural predators, such as lady beetles and lacewings, can quickly bring an aphid outbreak under control and changing weather conditions can knock out a population in short order, chemical control is rarely needed to combat aphids and ants that get attracted to tomato plants because of aphid presence.

If it appears your plant is suffering lasting damage, a strong spray of water to the leaves and stems will dislodge most aphids from a plant. You can also prune away infested leaves and stems without worrying about fruit production—tomatoes push up new growth quickly during the growing season.

Most aphids are heat-intolerant and will disappear by midsummer.

Tips for Pest-Free Tomatoes

Keep your tomato plants healthy and pest-free with these care tips.

Don’t overfertilize. Too much fertilizer on your tomatoes will spur plants to put on lots of tender, new growth that can attract aphids and subsequently ants. Use a slow-release fertilizer product labeled for vegetables to encourage steady, moderate growth. 

Water regularly. Tomatoes need about 1 inch of water a week for strong, healthy growth. Apply too much or too little water and the plant will be more susceptible to pests—such as aphids—and disease infestations. 

Spread mulch. Blanket the soil around tomatoes with a 2-inch-thick layer of dry grass clippings, shredded leaves, pine needles, or straw around tomato plants to prevent soil-borne fungal diseases from splashing onto plant foliage. Mulching your tomato plants also helps the soil retain moisture and maintain a moderate temperature.