The challenge of controlling aphids often begins with managing the ants that protect them. This relationship is a form of mutualism: aphids feed on plant sap and excrete honeydew, a sugary waste product. Ants actively harvest this honeydew as a primary food source and, in return, aggressively defend the aphid colonies from natural predators like ladybugs and lacewings. This protection means that eliminating aphids requires first disrupting the protective shield provided by the ant population.
Disrupting Ant Access
The first step is to isolate the aphid colonies by preventing ants from reaching the infested plants. For trees and woody shrubs, a sticky barrier applied to the trunk creates an obstacle the ants cannot cross. Products like Tanglefoot are applied to a protective band wrapped around the trunk, which prevents the sticky material from staining the bark. The barrier should be checked periodically to ensure the surface has not become bridged by debris, allowing ants to cross.
A chemical barrier can be established at the base of plants using food-grade diatomaceous earth (DE). This fine powder is composed of fossilized diatoms, which are microscopically sharp and lacerate the waxy exoskeleton of ants, causing fatal dehydration. Diatomaceous earth should be applied as a dry, continuous ring around the base of the plant or any entry points, as its effectiveness is greatly reduced when it becomes wet.
Strategic placement of slow-acting ant baits is an effective, long-term method for reducing the entire ant colony. Liquid baits, typically a mixture of sugar and a low-concentration toxicant like borax, are attractive to ants who consume the sweet liquid. The ants then carry the poison back to the nest and share it, which eliminates the queen and the rest of the colony over time. These bait stations should be placed along known ant trails, away from the plant itself, to draw the foragers’ attention away from the aphid honeydew.
Eradicating Aphids
Once the ants are no longer actively protecting the aphids, the focus shifts to direct treatment of the infested plant material. For light infestations on sturdy plants, a forceful jet of water is a simple and immediate solution. The strong spray physically dislodges the aphids from the leaves and stems, and they are unable to climb back onto the plant.
For heavier infestations, contact sprays provide a treatment option. Insecticidal soaps work by dissolving the protective outer layer of the aphid, leading to dehydration and death. When applying, thoroughly cover the entire insect, paying particular attention to the undersides of leaves and new growth where colonies tend to congregate. A solution can be made using one to two teaspoons of mild liquid soap per quart of water.
Neem oil, derived from the neem tree, provides another organic treatment option that acts as both a contact killer and a systemic product. When applied as a foliar spray, the oil suffocates the aphids by blocking their breathing pores. As a systemic treatment, the active compound, azadirachtin, is absorbed by the plant and disrupts the feeding and growth cycles of any remaining pests that ingest the sap.
Neem oil must be mixed with water and a mild soap emulsifier to ensure the oil is evenly distributed in the spray solution. Application should be done during the cooler parts of the day, such as early morning or late evening, to prevent the risk of leaf burn, known as phytotoxicity. Repeat applications every seven to fourteen days may be necessary until the aphid population is completely eliminated.
Sustainable Prevention Strategies
Long-term management involves creating a garden environment that naturally resists large-scale aphid outbreaks. Encouraging beneficial insects is a key strategy. Ladybugs and lacewings are particularly effective predators; the larval stage of the lacewing, often called the “aphid lion,” can consume hundreds of aphids during its development.
These beneficial insects can be attracted by planting nectar-rich companion plants like dill, fennel, cosmos, and sweet alyssum in the garden. Providing suitable habitat and avoiding the use of broad-spectrum insecticides helps ensure these natural controls remain active and ready to manage small outbreaks.
Controlling the type and amount of fertilizer used is important for prevention. High-nitrogen fertilizers promote a flush of tender, succulent new growth that is highly attractive to aphids. This rapid growth contains high concentrations of soluble nitrogen, which enhances aphid reproduction and population expansion. Using organic or slow-release fertilizers, which release nitrogen more gradually, helps to maintain steady, healthier plant growth that is less appealing to sap-sucking pests.