How to Control Hydroponic Pests

These pests are frequent invaders of grow rooms and hydroponic plants. Here’s what you can do to protect your plants and control the pests when you need to.



Description and damage: The tiny, powdery white insects suck plant juices in both their nymph (wingless) and adult (winged) stages. As they feed, they excrete a sweet fluid, referred to as honeydew, that breeds sooty mold, a fungus as appealing as it sounds. You may notice the mold before you see the whiteflies.

Prevention: In a greenhouse or large grow room, you can release parasitic wasps, which prey on whitefly nymphs. Also scrape off the tiny eggs from the bottoms of leaves.

Solutions: Start by placing a few sticky traps that lure and capture adults around your plants. Neem oil sprays disrupt the pests’ growth and feeding patterns, keeping nymphs from turning into adults that lay more eggs.

Key: Spray the undersides of leaves and stems where the nymphs cluster.

Spider Mites

Description and damage: Not surprisingly, spider mites look like spiders (to which they are related), but they’re only the size of a pinhead. Adults can be reddish, pale green, or yellow, while nymphs are paler green or yellow. Unlike spiders, which are beneficial because they feed on pests, spider mites weaken plants by sucking juices from the stems. Yellow speckles on leaf tops are often the first visible evidence of spider mites, and you may also see very fine webbing on leaves and new shoots.

Prevention: Spider mites thrive in very dry conditions, so keep the humidity level in your grow room as close to 50 percent as possible, which is ideal for your plants and not for spider mites.

Solutions: Pyrethrin (pronounced pie-wreath-rin) is a substance naturally occurring in chrysanthemums—and it is approved for use by certified organic growers. It paralyzes bugs and stops spider mites from feeding at all stages. Be careful not to mistake pyrethrin with pyrethroids, synthetic compounds that are harmful to aquatic life and may be toxic to pets.

Key: Spider mites migrate from plant to plant on their webbing. Isolate infested plants as soon as you find them.


Description and damage: Another plant-juice sucker, thrips can be yellow, brown, or black, and the tiny insects tend to show up in herds. Flowers (including unfertilized buds) are the prime targets for thrips; the petals and leaves they feed on become dark and brittle. Greenhouse thrips bore holes and insert their eggs into leaves and stems.

Prevention: Gently shaking plant stems stirs up thrips. The unhatched eggs look like a tiny pimple or a dab of white glue on leaves—lightly scrape them off with your fingernail and crush them.

Solutions: Sticky traps capture thrips so you can dispose of them. Potassium salts of fatty acids (the active ingredient in insecticidal soap) wash off the protective coating on the soft-bodied bugs’ exteriors.

Key: Thrips have a short life cycle—they live for only a few weeks. You may need to repeat applications of insecticidal soap three or four times, at 10-day intervals, to control both adults and nymphs.


Description and damage: Weak or stressed plants attract aphids—soft, pear-shaped insects that can be green, yellow, black, gray, or even pink. Some have wings, others don’t. They suck sap from all parts of the plant, including the buds, and excrete sticky honeydew, which attracts ants that feed on it. Worst of all, aphids spread viral diseases from one plant to the next.

Prevention: Overfed plants—such as those raised on high-nitrogen synthetic fertilizers—are especially vulnerable to aphid infestations. Stick to organic plant food. Aphids fall off plants easily when they’re doused with water.

Solutions: Insecticidal soap is as effective at controlling aphids as it is other pests.

Key: Aphids are found in many outdoor environments, including gardens and lawns. To keep aphids and their eggs away from your plants, avoid bringing tools or anything else from outside into your grow room.

Fungus Gnats

Description and damage: Fungus gnats look similar to whiteflies but are dingy gray rather than white. The adult gnats do not feed on or harm plants, but the larvae—small, pale, translucent maggots—devour plants’ roots. The maggots feed on algae, too, and thrive in the dark, dank, conditions around the base of hydroponic plants.

Prevention: Use a moss- and algae-killing treatment to clear your system of the algae the maggots may feed on. Maggots are especially attracted to rock wool, but no matter what kind of growing media you use, let it dry out after watering and feeding to deprive the pests of essential moisture.

Solutions: Capture flying adults, which lay eggs at the base of plants, with sticky traps. Use a combination spray of neem oil, insecticidal soap, and pyrethrin to eliminate adults.

Key: Discard any plant that has a maggot infestation—rescue is nearly impossible.

Remember: Insects lay eggs by the hundreds. A problem ignored is a problem multiplied exponentially.