By Phyllis Gilreath, Dave Schuster and Phil Stansly
Biotype Q of the sweet potato whitefly, a recent immigrant from the Mediterranean region where it is the most prevalent biotype of the sweet potato whitefly, is threatening vegetables and other crops in the United States.
Biotype B, also known as the silverleaf whitefly, is the whitefly pest of most concern to greenhouse and field-grown vegetable growers in the southern United States and has been since the late 1980s. Biotype B damages plants not only directly -- by feeding and inducing disorders, such as silverleaf of squashes and irregular ripening of tomatoes -- but also indirectly by transmitting damaging plant viruses, particularly tomato yellow leaf curl virus in Florida.
Biotype Q has plagued greenhouse-grown crops in southern Spain for years, increasing insecticide costs. The Q biotype was first identified in the United States in March 2005 by scientists at the University of Arizona and the University of California on poinsettia plants in Arizona that originated from a nursery in California. Since then, the biotype has been identified on ornamental plants in greenhouses and in retail outlets in at least 22 states, including Florida.
Biotype Q could move to greenhouse and field-grown vegetables as a result of adults escaping infested greenhouses and nurseries and the relocation of infested ornamental plants and vegetable transplants. Of special concern are vegetable transplants produced in close association with ornamental plants.
The Q biotype is visually indistinguishable from the B biotype. The two biotypes can only be identified by analyzing enzymes, protein or DNA. Biotype Q reproduces and develops at least as rapidly as the B biotype on many of a wide range of host plants (more than 500 plants from 74 families), including many important vegetable crops such as tomatoes and cucurbits. Biotype Q can transmit tomato yellow leaf curl virus at least as well as biotype B but causes little or no silverleaf of squashes or irregular ripening of tomatoes.
The major problem facing vegetable growers is that biotype Q is resistant or tolerant to many insecticides commonly used for managing whiteflies, including pyrethroids (many products), the nicotinoids (Admire Pro, Platinum and Assail) and insect growth regulators (Knack and Courier). Whiteflies that are resistant to nicotinoids also are cross resistant to pymetrozine (Fulfill). Resistance to endosulfan, which commonly is used to control whitefly adults, is uncertain. Unfortunately, unlike the B biotype in the United States, resistance to these insecticides in biotype Q appears to be stable and does not diminish over time. However, biotype B appears to out-compete biotype Q; that is, in the absence of conventional insecticides, biotype B predominates.
What should a vegetable grower do?
A grower should probably do nothing different from what he already does if his whitefly management program is working as usual. However, early detection is the key to making the necessary adjustments, so growers should maintain good scouting activities.
Keep in mind that if both biotypes are present, applications of tolerated insecticides will select for the Q biotype. Therefore, insecticides should be sprayed only as needed. Good resistance management practices also should be followed, including rotation of chemicals in different insecticide groups and proper use of nicotinoids -- only once per crop per season.
Spraying of field perimeters is not recommended because doing so will increase unnecessary exposure of the whitefly population to insecticides and may kill natural enemies such as parasitic wasps, which can be helpful in controlling whiteflies -- especially biotype Q.
In order to decrease exposure of whiteflies to crucial nicotinoid products, growers also are urged to refrain from using the insecticides on crops when they aren't necessary.
While biotype Q is resistant to many insecticides, recent greenhouse trials on ornamental plants in Georgia, California and New York have indicated that spriomesifen (Oberon), and abamectin (Agri-Mek) combined with bifenthrin (Capture) still are effective. Another nicotinoid, dinotefuran (Venom) has been shown to be more effective on biotype Q than other nicotinoids. Spiromesifen is particularly effective against whitefly nymphs and could be substituted for insect growth regulators; however, although it kills adults, it is slow-acting. Soaps, oils and other products like Prev-Am should still be useful but require excellent coverage. Potassium bicarbonate (MilStop) has been effective in some trials but not in others; however, it is registered only for disease control.
Paramount to any resistance management program is a strict adherence to cultural practices that reduce overall whitefly populations. The most important cultural manipulation is the inclusion, when possible, of a two to three month crop-free period in the production cycle. This forces whiteflies to move into noncrop, nonsprayed host plants where virus innoculum decreases, mor-tality is high and competition would favor nonresistant B over Q or resistant B.
To quickly kill whiteflies, crops should be destroyed immediately after final harvest using a burn-down herbicide combined with oil. Burn-down sprays used in the field should be timed to avoid windy days and should be applied block-by-block as harvest terminates rather than waiting to destroy entire fields at one time.
U-picking or pin-hooking of old plantings should be avoided because these activities prolong the presence of whiteflies and viruses, and adequate control is difficult because of the short re-entry intervals required.
Only transplants free of whiteflies should be used. Transplants should not be grown in facilities shared with ornamental plants, especially those brought in from the outside. If this is not possible, ornamental plants should be isolated or grown as far from vegetable transplants as possible, and the two should not be moved, shipped, etc. at the same time. People working in ornamental production sections should not enter vegetable transplant production sections.
Whitefly adults are attracted to the colors yellow and green and can be transported into production facilities on yellow or green clothing or utensils. Where feasible, production facilities should be fitted with such protections as insect-proof screening or UV-blocking films to reduce adult penetration into facilities.
Yellow sticky traps placed around greenhouse openings can be useful in detecting low, immigrating adult whitefly populations. In addition, plants should be thoroughly inspected at least twice weekly.
It remains to be seen whether biotype Q can become established under field conditions -- especially while competing against biotype B. If it does, however, vigilance and preparedness can help lessen the impact. CVM
Phyllis Gilreath is a University of Florida Institute of Food and Agricultural Sciences, Gainesville, extension agent based in Palmetto, (941) 722-4524, ext. 229. Her e-mail address is firstname.lastname@example.org.
Dave Schuster is a professor at the UF/IFAS Gulf Coast Research and Education Center, Wimauma, (813) 633-4124. His e-mail address is email@example.com.
Phil Stansly is a professor at the UF/IFAS Southwest Florida Research and Education Center, Immokalee, (239) 658-3400. His e-mail address is firstname.lastname@example.org.