What Are Those Little White Insects on My Tomato Plants?

On their way to a bumper crop and the envy of all who pass, your tomato plants have more than jealous neighbors as admirers. Even the healthiest tomatoes (​Solanum l​​ycopersicum​), perennials only in U.S. Department of Agriculture plant hardiness zones 10 and 11, attract tiny white insects intent stealing their nutritious sap. The bugs' presence is unmistakeable: At the slightest disturbance, they swarm from the leaves in clouds. Fortunately, toxic chemicals aren't required to send them packing.

Like the pushiest of relatives, whiteflies make themselves completely at home. They come for dinner, make a mess and stay to raise their families. They resist eviction by chemical insecticide and may not leave until they've drained all the sap the plants have to offer.

Three kinds of whiteflies — greenhouse, sweet potato and bandedwing — target tomatoes, but greenhouse whiteflies are by far the nastiest. Measuring about 1/16 inch long, they feed in groups and lay eggs on the backs of the leaves. By transmitting the tomato infectious chlorosis virus, greenhouse whiteflies are capable of seriously damaging an entire tomato crop.

Whiteflies also excrete undigested sap as sticky, transparent honeydew. Honeydew attracts sooty mold spores capable of burying the leaves beneath layers of black fungus.

Inspect the tomatoes daily and prune lightly infested leaves to remove larvae and eggs. Use clean, sharp stem pruners disinfected between cuts in 70 percent rubbing alcohol, and dispose of the leaves in sealed plastic bags.

Rinse the adult whiteflies from the plants with a strong spray of hose water that reaches the backs of the leaves. The University of California Integrated Pest Management Program reports weekly use of this syringing technique controls whiteflies at least as well as chemical insecticides.

If rinsing fails, suction adults off with a battery-operated vacuum early in the morning while they're lethargic. Freeze the dust cup overnight before emptying its contents into a sealed plastic bag and placing it in the trash.

A host of beneficial bugs, including pirate bugs, lacewings and ladybugs, prey on whiteflies. To lure them to your tomato patch, layer several shallow dishes with pebbles partially submerged in water and place them around the plants. The predators come to drink and remain to eat.

Keep the friendly bugs even happier by mixing some pollen- or nectar-producing herbs such as annual dill (​Anethum graveolens​) or perennial fennel (​Foeniculum vulgare​, zones 4-9), in with your tomatoes.

A persistent whitefly problem calls for organic, ready-to-use neem oil. Lethal to the whiteflies and eggs when wet, it's harmless to beneficial insects once dry. Water the tomatoes well and spray until until all their surfaces drip with the oil, making sure to coat the backs of the leaves.

Repeat weekly, or at the label's recommended rate, until the plants are whitefly free. Wear protective clothing, waterproof gloves, safety goggles and a respirator mask, and always heed the label's precautions when working with the oil.