Ants and Aphids: A Pesky Partnership

Convergent lady beetle feasting on an aphid

Late spring is prime time for one of our “favorite” plant pests: aphids. Temperatures are averaging in the 70s, and plants are still flush with tender, new growth from an unusually wet winter. Turn over a leaf, and you might see itsy-bitsy insects in a variety of colors, from lime green to brown, red or black, depending on the species. They are generally less than a quarter-inch in length, and difficult to see in detail without a microscope.

How could something so small damage your plants? Their power lies in numbers; females can give birth to 12 live young per day during the growing season. These newborn nymphs can become reproducing adults in as little as 7 days, meaning colonies can grow quickly when conditions are right. All these little mouths require feeding, which is achieved by a long, slender mouthpart that pierces plant tissue and sucks out the sap.

While aphid populations are low, their presence often goes unnoticed, but larger parties can cause more serious damage. Typical damage looks like yellowing, distorted foliage at the tender growth of shoots where the aphid’s favorite food resides. Aphids secrete the excess sugars as a liquid called “honeydew,” which grows a fungus called black sooty mold, creating a harmless but unsightly mess. Their feeding can also transmit viruses. Vegetables such squash, lettuce, beets, and potatoes are especially vulnerable.

Ants are particularly fond of this honeydew, “milking” the aphids and taking it back to their nests. In return, ants fend off predators such as ladybeetles and parasitoid wasps and reduce fungal diseases by removing infected bodies. One experiment conducted in Japan blocked ants from tending eight aphid colonies; all the colonies perished except for a lone survivor in one colony; the others were all eaten by natural enemies!

To keep ants out of trees, place sticky tape (e.g. Tanglefoot) around the trunk to trap them. Be sure to prune back other access routes such as where branches contact the roof, fences, or other plants. Check the sticky tape regularly, as dead ants stuck on the tape can create a morbid, but functional bridge to the other side for their living counterparts.

Sticky tape works best on single-trunk trees. For everything else, use a bait station with slow-acting poison to manage ant populations. Foraging ants collect the bait and take it back to their colony to share death with everyone, including the queen. For minor ant issues, ant stakes or small bait containers will do. For serious invasions, refillable bait station may be necessary. When selecting a bait, remember that ants are attracted to various foods depending on their species and the time of year. Baits are generally carbohydrate, protein, or fat-based; if ant identification eludes you, try different products to see which they prefer. Consult the UC Integrated Pest Managament page on ants to see who likes what: <>

Besides shooing the ants away, you can encourage their natural enemies to take up permanent residence in your outdoor home, providing faster relief for your plants. Avoid using broad spectrum pesticides, which kill everything (pests will always return first if predators are gone). You can also provide nectar plants for the beneficials, which need food when aphids are not around. Aphids predators are small, so their forage should be correspondingly small. Asters, Yarrow, Buckwheat, and Coreopsis are all good choices. Provide water by lightly sprinkling plants in the morning.

You can also create a less hospitable home for aphids. Avoid overwatering and high applications of nitrogen fertilizer, which causes plants to put out a lot of lush growth that aphids love. Aphids can be knocked off plants with a spray of water; infested plant parts can also be pruned off. If you absolutely must use a chemical, try neem oil or insecticidal soap, which have a short residual time and are less likely to kill non-target organisms (i.e. beneficials) than broad-spectrum pesticides. Thorough coverage of the foliage is necessary for good control, and repeat application may be needed until populations are tolerable.

If you have a gardening related question you can contact the UC Master Gardeners at 209-953-6112. More information can be found on our website:

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