Physical barriers, such as fine mesh screens, can be used in greenhouse production to reduce the potential for infestation. Under field conditions, there are several types of barriers that can reduce whitefly problems. These include reflective mulches that tend to repel whiteflies, oil-coated yellow mulches that act as a trap for whiteflies, floating row covers (generally made out of a light fiber mesh and placed over newly planted crops) that exclude whiteflies during the vegetative growth of the crop, and trap crops. Floating row covers work exceptionally well for early-season protection, but can be expensive and often have to be removed at flowering for proper pollination to take place. Other barriers may be only partially effective and should be considered in conjunction with other control tactics.
Another way to avoid or reduce whitefly infestations is to adjust planting dates to avoid heaviest insect migration periods. Highly susceptible crops such as cucurbits, crucifers and tomatoes should not be planted when whitefly migration is expected (such as at the end of cotton season). In Weslaco, Texas, cucumbers planted in mid-June 1991 could not withstand the high numbers of migrating adults despite twice weekly applications of insecticides. However, cucumbers planted in August after migrations had diminished were much less affected. An option to not planting anything during this period is to plant a less susceptible crop such as peppers.
In Immokalee, Florida tomatoes planted early in the "spring" season (early January), are targets fo whiteflies migrating from fall crops. Even a delay of 3 weeks can make a difference between 100% virus infection or an acceptable yield.
Early planting also can be an effective way to avoid whiteflies because they reproduce more rapidly under hot, dry conditions. Thus, early planting of spring and summer crops allows them to mature before conditions are favorable for rapid population increases.
Field location also can affect the potential for whitefly infestation. The earliest and heaviest whitefly infestations most often occur in fields located near crops with prior or current infestations. This has been observed in spring melon crops located near infested cabbage, and in summer cotton located near infested cabbage or cucurbit fields. The greatest threat to tomato crops in south Florida are previous tomato crops which provide a source of both whiteflies and geminivirus. Susceptible crops should not be planted near infestation sources. To combat the problem of whitefly-transmitted viruses, all infected plants should be removed. Susceptible crops should not be grown continuously because they provide an ever-increasing source of inoculum.
Since crop residues can harbor whiteflies and virus inoculum, they should be rapidly and completely destroyed after the final harvest. The subsequent planting of susceptible crops should be avoided until migration has ended. This practice can reduce whitefly infestation as well as carryover of viral inoculum.
Establishing a host-free period by careful choice of planting site and date is now a commonly accepted recommendation for reducing whitefly populations in many areas of the southern U.S. that are severely affected by this pest. This practice requires regional cooperation to be effective.
Although these practices may not completely eliminate whitefly problems, they can help to reduce pest populations and damage to manageable levels. These practices should be modified only to preserve known populations of natural enemies of whiteflies.
Host plant resistance (HPR) is one of the preferred methods for minimizing the damage caused by whiteflies and associated viruses, because it does not require the complete elimination of the pest to be effective. HPR protects the crop by making it less suitable for the pest or by making the crop tolerant to the pest. The result is less crop damage. Examples of this are (1) the use of smooth-leaf (glabrous) cotton rather than hairy-leaf cotton to reduce the impact of whiteflies on yield, and (2) resistance to cotton leaf crumple virus in the 'Cedix' cotton variety. Possible resistance to the sweetpotato whitefly also has been developed in certain tomato, peanut, squash and pepper varieties.
Commercial producers should note that the resistance of a crop can be affected by the level of pest infestation, with even some resistant cultivars damaged by large whitefly populations. Also, crops resistant to one pest may not be resistant to another. For example, smooth leaf cottons are less attractive to sweetpotato whitefly but more attractive to cotton fleahopper. Finally, resistant commercial cultivars take time to develop and are not currently available for many crops.