Calls to most county extension offices have been reporting about tiny whitish flies that have invaded tomatoes in greenhouses.
Although I have not done a site visit to verify, the farmers I have spoken to have described the insects as tiny white moths with powdery wings and triangular in shape. After listening to downhearted farmers describe symptom after symptom, I came to a conclusion that the insects should be the dreaded ‘white flies’.
In greenhouses, white flies can be devastating. They congregate on the underside of leaves, where they lay eggs, suck out the plant’s fluids and inject toxic saliva.
The depleted leaves spot, turn yellow, silver or white, then fall off. Given sufficient infestation, the plant dies. As they feed, white flies secrete honeydew, which develops a dark, sooty mold that disrupts normal photosynthesis and further starves the plant.
Besides siphoning food and effectively starving the plants, white flies transmit plant viruses which represents an even greater danger. The ease with which white flies can spread disease makes the insect a serious threat to tomato production worldwide. Unfortunately, this insect is also proving increasingly adaptive and resistance to methods of control.
Management
Today, I will highlight integrated practices that will help in management of white flies in greenhouses. White flies infestations can devastate entire crops and ruin the plants we work so hard to maintain. Take precautions early enough to avoid losses. Scout for these insects and remove infested leaves or hose down using water sprays.
In many situations, natural enemies will provide adequate control of white flies; outbreaks often occur when natural enemies are disrupted by insecticide applications, dusty conditions, or interference by ants. Avoid or remove plants that repeatedly host high populations of whiteflies.
Avoiding use of insecticides that kill natural enemies is an important aspect of white fly management. Products containing carbaryl, pyrethroids, or imidaclo¬prid (especially as a foliar application) can be particularly disruptive. Control of dust and ants, which protect white flies from their natural enemies.
Traps
Yellow sticky traps can be displayed around the garden to trap adults. Such traps won’t eliminate damaging populations but may reduce them to manageable levels. When using traps, take note that white flies do not fly very far, so many traps may be required. You may need as many as one trap for every two plants. Place traps with the sticky side facing plants but out of direct sunlight. Essentially traps are most useful for monitoring and detecting white flies rather than controlling them.
Hand removal of leaves or plants heavily infested with the non-mobile nymphal and pupal stages may reduce populations to levels that natural enemies can contain. Remove and destroy whitefly-infested plants. As a precaution, always inspect new plants for white flies and nymphs before introducing them in the greenhouse. Water sprays may also be useful in dislodging adults. Watering can also reduce the hot, dry dusty conditions that favor white flies and inhibit their natural enemies.
Reflective mulches can also be used. Reflective mulches can repel white flies from greenhouse, and yellow sticky traps can be used to monitor or, at high levels, reduce white fly numbers. Use of insecticides, insecticidal soaps or oils such as neem may reduce but not eliminate populations.
Systemic insecticides may be more effective but can have negative impacts on beneficial insects and pollinators.
When putting reflective mulch in your garden, first remove all weeds. Place the mulch on the plant beds and bury the edges with soil to hold them down. After the mulch is in place, cut 3- to 4-inch diameter holes and plant several seeds or single transplants in each one. You may be able to furrow irrigate or sprinkle your beds if you use coated construction paper or other porous mulch if it is sturdy enough to tolerate sprinkling. Plastic mulches will require drip irrigation.
Crop loss
In addition to repelling white flies, the mulch will enhance crop growth and control weeds. Reflective mulches have been shown to deter pests that transmit viruses in commercial vegetable crops, perhaps helping to reduce disease incidence and crop loss.
White flies can be difficult to control with insecticides. Most less-toxic products such as insecticidal soaps, neem oil, or petroleum-based oils control only those white flies that are directly sprayed.
Therefore, plants must be thoroughly covered with the spray solution, and repeat applications may be necessary. Be sure to cover undersides of all infested leaves; usually these are the lowest leaves and the most difficult to reach.
If your greenhouse consistently host high populations of white flies, remove all host plants from the greenhouse for at least two weeks (and assuring that no white flies can enter from outside) may eliminate problems.
The writer is an expert on sustainable agriculture and agricultural solutions
