The little white bugs on your tomato plants are almost certainly whiteflies, tiny sap-sucking insects about 1/16 inch long that cluster on the undersides of leaves. If you shake or brush a leaf and a cloud of tiny white specks flies up, that confirms it. Whiteflies are the most common white insect pest on tomatoes, though white-colored aphids and mealybugs (which look cottony rather than winged) occasionally show up too.
How to Confirm They’re Whiteflies
Whiteflies look like miniature white moths and tend to gather on the undersides of leaves, especially newer growth near the top of the plant. When you disturb them, they flutter up in a brief swarm before settling back down. Two species target tomatoes most often: the greenhouse whitefly and the sweetpotato whitefly. Both look nearly identical to the naked eye.
Flip a leaf over and you may also notice tiny, flat, oval scales stuck to the surface. These are whitefly nymphs, the immature stage. After hatching from eggs, nymphs insert their mouthparts into the leaf and stay in one spot for about a month, feeding through four stages of development before becoming winged adults. The full lifecycle from egg to adult takes 21 to 36 days depending on temperature, which is why populations can explode quickly in warm weather.
The Damage They Cause
Whiteflies feed by piercing leaf tissue and sucking out plant sap. This weakens the plant, causing leaves to yellow, curl, and eventually drop. As they feed, they excrete a sticky substance called honeydew that coats leaves with a shiny film. A black fungus called sooty mold often colonizes this honeydew, making leaves look darkened and dirty. The mold itself doesn’t infect the plant, but it blocks sunlight and reduces the leaf’s ability to photosynthesize.
Sweetpotato whiteflies cause an additional problem: uneven fruit ripening. Tomatoes may develop blotchy color patterns that make them unmarketable or unappealing, even if the fruit is otherwise healthy.
The most serious risk is viral. Whiteflies transmit Tomato Yellow Leaf Curl Virus (TYLCV), which causes stunted growth, a bushy appearance from shortened stems, crumpled and curled leaves, and flower drop that dramatically reduces your harvest. An adult whitefly can pick up the virus after just 15 to 30 minutes of feeding on an infected plant, then spread it to new plants for weeks afterward. Symptoms take up to three weeks to appear, so by the time you notice the damage, the virus may have already spread. There is no cure for infected plants.
Insecticidal Soap and Neem Oil
For most home gardeners, insecticidal soap is the first line of defense. These products use potassium salts of fatty acids to break down the soft outer coating of whiteflies on contact. The key word is “on contact”: the soap only works while it’s still wet on the insect, so thorough coverage matters more than anything else.
Mix insecticidal soap at a 1 to 2% concentration, which works out to roughly 2.5 to 5 tablespoons per gallon of water. Spray both the tops and undersides of leaves, paying special attention to the undersides where whiteflies congregate. Apply in early morning or late evening, never in full sun or when temperatures exceed 90°F, because the soap can burn leaf tissue under those conditions. Slower drying time actually improves effectiveness, giving the spray more contact time with the insects.
Because insecticidal soap has no lasting residual action, you’ll need to reapply every 4 to 7 days until the population is under control. Neem oil works similarly and can be found blended with insecticidal soap for a combined effect. Both options are considered organic-friendly and break down quickly in the environment.
Companion Planting With Marigolds
Interspersing French marigolds among your tomato plants genuinely repels whiteflies. Researchers at Newcastle University confirmed in a 2019 study that marigolds release a compound called limonene, a citrus-scented chemical that whiteflies find repellent. The smell slows the insects down and discourages them from settling on nearby tomato foliage.
You don’t need a complicated arrangement. Simply planting marigolds between or alongside your tomato plants is enough for the scent to disperse through the foliage. The researchers also found that adding other companion plants the whiteflies dislike didn’t increase or decrease the marigold’s repellent effect, so marigolds alone do the job. For greenhouse growers, small containers of pure limonene hung among tomato plants produced a similar result.
Yellow Sticky Traps for Monitoring
Yellow sticky traps are useful for tracking whitefly populations, though they won’t solve an infestation on their own. Place each trap so its bottom edge sits level with the top of your plant canopy. As the plants grow, move the traps up to keep pace. In a greenhouse, aim for roughly one trap per 1,000 square feet of growing area for whitefly-susceptible crops, with additional traps near doors and vents where insects migrate in.
A spike in trap catches tells you the population is growing, but high numbers on traps don’t automatically mean you need to spray. Always combine trap data with direct inspection of the plants, checking the undersides of leaves for nymphs, eggs, and live adults before deciding on treatment.
Biological Control With Parasitic Wasps
If you’re growing in a greenhouse or high tunnel, tiny parasitic wasps can provide long-term whitefly suppression. These wasps are barely visible to the naked eye and completely harmless to people. A single female parasitizes about 5 whiteflies per day and kills an additional 3 by feeding on them directly, averaging around 8 whiteflies eliminated daily over her roughly two-week lifespan.
Releases work best when started early, while whitefly numbers are still low (just a few per plant). Waiting until the population is large makes biological control far less effective, because the wasps can’t reproduce fast enough to catch up. You can order these wasps from biological control suppliers and release them directly onto infested plants.
Preventing Whiteflies Before They Arrive
Whiteflies are easier to prevent than to eliminate. A few practical steps reduce your risk significantly:
- Inspect transplants carefully. Whiteflies often arrive on nursery starts. Check the undersides of leaves before bringing new plants into your garden or greenhouse.
- Remove heavily infested plants. A plant covered in whitefly nymphs acts as a nursery for the next generation. If a plant is badly infested and showing virus symptoms like stunting and leaf curl, pulling it out protects the rest of your crop.
- Clean up at season’s end. Whiteflies overwinter on plant debris in mild climates. Removing old tomato plants promptly after harvest reduces the population that carries over to next year.
- Use reflective mulch. Silver or aluminum-colored mulch disorients whiteflies and reduces landing rates on young plants during the early weeks of the growing season.
Whitefly populations build fastest in warm, calm conditions with plenty of new leafy growth to feed on. Catching them early, when you notice just a handful of adults fluttering off a leaf, gives you the widest range of effective options before the population spirals and virus transmission becomes a real threat.