Keeping whiteflies out of the greenhouse

Credit: Flickr / Eran Finkle

Q: Last summer, I had a whitefly infestation on my greenhouse tomatoes. I treated them with Safer’s soap, and eventually removed the tomatoes from the greenhouse. This winter, I have been growing arugula in my greenhouse, and have noticed a few whiteflies. I am reluctant to spray the greens with insecticidal soap, because we want to eat them.

Whiteflies are tiny insects with powdery-white wings that colonize in large numbers on the undersides of leaves, feed on plant juices and excrete sticky honeydew. Infested leaves become pale or discoloured; plants may wilt and lose leaves.

The reason for insect problems stems from stressed plants. It may be that your tomatoes were too wet, too dry, rootbound, in need of fertilizer, or that the greenhouse was too hot or poorly ventilated.

Check to see what the problem is and correct it and I think the whitefly problem will go away. Inspect seedlings and plants from nurseries before purchase to ensure they are free from whiteflies.
Placing homemade sticky traps near plants to capture whiteflies is very effective. Apply insecticidal soaps to control as a last resort, but do not eat the plants immediately afterwards. Allow them to outgrow the sprayed leaves before harvest. You should not need to decontaminate the greenhouse, unless all plants inside are under attack, in which case I would choose introducing Encarsia formosa as a solution.