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The sly whitefly is a tricky tomato pest

  • James Campbell
  • The scent of tomato leaves wafting through the air reminds me there is nothing quite like the taste of a freshly picked, vine-ripened tomato, a highlight of my summer. I was so disappointed last year when I was picking my lackluster tomato crop. I had a whitefly infestation; I had no idea what a tricky pest the whitefly was.

    Despite being called whiteflies, Aleyrodidae are not true flies at all since they are more closely related to aphids, scales, and mealybugs. They use their piercing mouthparts to enter the vascular tissue of a plant so they can suck sap from the phloem. Phloem transports the sugars and other metabolic products created in photosynthesis to where the plant needs nutrients.

    Whiteflies will damage your tomato plant in a couple of ways. By sucking on the phloem, they will divert the nutrients from your tomato plant, which will quickly become weak and may be unable to carry out photosynthesis. Leaves will wilt and turn pale or yellow. Plants will become  more prone to disease, growth will be stunted, and the production of tomato fruit will suffer. Like aphids, whiteflies excrete honeydew, a sugary substance that can lead to a fungal infection of your plants called black sooty mold. Left unchecked, black sooty mold can obscure sunlight from plant leaves turning them yellow. The honeydew may also attract ants which will interfere with the activities of natural predators of the whitefly.

    When a plant is under attack it releases a combination of chemicals through the air. Some of the chemicals warn other parts of the plant, and other plants nearby, to ramp up defenses in preparation. In very basic terms, plants have two main defense systems: one for microbes and another one for insects. When a plant is infected it produces salicylic acid, a hormone that triggers a plant to produce antimicrobials or tough molecules that barricade a plant’s cells from the microbial siege. When plants are bitten by insects, the plant releases jasmonic acid, a hormone that triggers the production of insecticidal toxins.

    Whiteflies are particularly devastating because they can hijack the plant's warning system. When they bite a plant, they cause it to send out the message that the attack is microbial, causing the plant under attack and nearby plants to invest in the wrong kind of defense.

    How do you prevent and detect whitefly infestation?  Silver or aluminum-colored reflective mulch helps repel whiteflies. I am not a fan of plastic mulches, but if you go this route, make sure your drip irrigation is placed under the mulch. Avoid pesticides that will kill natural predators of the whitefly, such as ladybugs, lacewings, minute pirate bugs, and big-eyed bugs. Interplanting your tomatoes with marigolds really does work. Research has shown that marigolds release limonene which repels and slows down whiteflies. Check the undersides of your tomato leaves where you may find whiteflies in clusters. They are very small and hard to see if they are on their own so check around the veins of the leaf.  Even if you cannot see them, run your finger along the leaf and feel for the stickiness of honeydew. Whiteflies are active during the daylight and if they are feeding, they will fly off the leaf in a swarm making it pretty obvious.

    If you have whiteflies do not ignore them. Remove infested leaves as soon as you detect them. Hose off your tomato plants with water which will remove the adults. You can also set out yellow sticky traps to reduce numbers. Insecticidal soaps or oils, such as neem, will help keep the population in check, but will not eliminate them altogether. Make sure you fully cover the plants with insecticidal soap or oil, including the underside of the leaves.

    For more information on pest management in your home garden visit http://marinmg.ucanr.edu/Pests_-_Problems/