An Ant Infestation on the Orchids

Ants and Honeydew

• There are more than 12,000 kinds of ants worldwide. Many of them have a mutually beneficial relationship with insects such as aphids, scale insects and mealybugs that suck plant sap and excrete a sweet honeydew that ants like to eat. These sucking insects can weaken, stunt or disfigure orchid plants. The ants protect and tend their honeydew producers. As a result, pest insects can increase in abundance and be spread from plant to plant by ant attendants. Ants can become aggressive in defending their source of sweets.

Source of Ants

• Ants live in a communal nest, usually in soil. Worker ants go out from the nest to find food, laying a scent trail when they're successful. Other workers follow the trail to the food. The ant nest need not be close to the food. If a worker ant crawls through a small crack or crevice and finds honeydew producers on your orchid plant, other workers will follow. Sometimes ants move into orchid potting mix when orchids are moved outside for a while. These ants are looking for structural elements to support their nest, and some kinds of ants prefer the sort of open, well-drained substrate orchid potting mix provides.

Honeydew Producers

• Aphids are small insects 1/16 to 1/8 inch long. They gather in groups on tender plant growth such as new shoots, flower stalks and flower buds. Adults are winged but juveniles can't fly. They can be green, yellow, or pink. Honeydew appears at the end of the abdomen.

  Mealybugs don't move around much. They're concentrated in protected parts of the plant such as leaf folds and the undersides of leaves. Mealybugs are covered with white waxy filaments that look mealy or cottony.

  Scale insects move around only in the crawler stage when they're very young. Once they attach themselves with their mouthparts to the plant, they stay put and grow a hard scale on top of their bodies. All these sucking insects can carry plant pathogens such as viruses from plant to plant.

Control

• The best control for an ant infestation in orchids is to eliminate the insects from which they take honeydew. For slight infestations, remove aphids, mealybugs and scale insects physically with cotton swabs dipped in denatured alcohol or a mild detergent mixed with water. For serious infestations, use insecticidal soap or a general purpose insecticide. William Cullina, in "Understanding Orchids," advises using a systemic insecticide for mealybugs on the roots of orchids. For ants that have nested in a rooting medium, put on gloves, take the plant back outside, unpot it and remove the potting medium. Wash off the roots, plant and pot, then repot the orchid using fresh orchid mix.