Whitefly on basil plants is a not a death sentence, but gardeners must be persistent with whichever control method they choose. Biological, mechanical and chemical methods for ridding your basil of this pest exist. Each of these has benefits and drawbacks.
Biological Control of Whitefly
Many people are aware that beneficial insects which prey on whiteflies are a form of biological control. The problem is that whiteflies reproduce so fast that predator wasps and others can't kill them fast enough. Still, avoid broad-spectrum pesticides that will kill wasps, because every whitefly they eat helps.
Few people know that fungus can be a helper too. Beauveria bassiana is a fungus that enters insects through their skin, produces toxins and destroys vital nutrients. It is mixed with water, per the manufacturer's directions, and applied immediately. Beauveria bassiana is killed by sunlight, so it should only be applied at night all over leaf surfaces.
Mechanical solutions for killing whiteflies on basil include horticultural oil, insecticidal soap and whitefly traps.
Unlike poisons, horticultural oil works by smothering the whitefly. It must be sprayed on all plant surfaces on a regular basis to keep the population in check. Horticultural oils are temperature rated. This is important information. If applied in hot sun some oils will burn basil leaves. No matter the time of year, users should avoid applying oil at midday when the sun is at its strongest.
Insecticidal soap also works by smothering. It should be applied whenever whiteflies are a problem. It is a good idea to make insecticidal soap part of your regular garden maintenance. This way, when more whitefly eggs hatch another round of spray will soon be there to nab them.
Whitefly traps are sticky yellow cards that attract whiteflies and kill them. They can be applied directly to basil plants or placed on stakes nearby. The downside of this type of trap is that it is nondiscriminatory. You may catch some good bugs too. It is easy to make whitefly traps. Simply spray a yellow index card with spray adhesive or cooking oil and place it near your basil.
Killing whiteflies on basil with chemical pesticides requires careful consideration. Users must thoroughly read insecticide labels to make sure they are safe for edible crops. A commonly used one for basil is pyrethrin. It is the active ingredient in many brand-name products and is often seen mixed with insecticidal soap for an added punch.
Spinosad is another insecticide available for edible crops like basil. It is sold in ready-to-use or hose-end-spray applicators, so even those with large amounts of basil will find it effective.
Both pyrethrin and Spinosad are safe for killing whitefly on basil, because they don't remain active for long periods. Pyrethrin may be applied in the evening when beneficial bugs are not present, and it will be gone by the next morning. Spinosad is only active while it's wet. This is why gardeners can safely apply these chemicals frequently.
How They Hurt
Whiteflies are harmful to basil and other plants because they suck the sap from the leaves. This weakens the plant, and a weak plant produces less of the prized leaves.
Whiteflies and other leaf-sucking insects excrete a substance like sugar water called honeydew. Sooty mold grows on this honeydew and turns the leaves black. If too much sooty mold forms, the leaves drop because they can't get enough sunlight.
Whitefly, if left untreated, can spread throughout an entire garden. This makes careful inspection and treatment vital in keeping it under control. In severe cases systemic chemical controls are used on nonfood crops.
Companion planting for the control of whitefly is not effective if used by itself. It can be joined with other methods and certainly won't hurt. Marigold and oregano are two plants that repel whiteflies. Rather than planting a row of basil in your garden, alternate it with one of these two plants. Instead of planting a huge container of just basil, add marigold and oregano with it. You will not only reduce the appeal for whiteflies, you will also have a beautiful arrangement.