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WHITEFLIES
LINK TO BIOTYPING INFORMATION
Click here to view the "Recommendations for Management of Whiteflies, Begomovirus, and Insecticide Resistance for Florida Vegetable Production"
(Including new efficacy information and other updates!) Final Agenda for 3 April Q Biotype Technical Advisory Group Meeting (contains links to PDF files for each presentation)
Don't forget that management doesn't end once you ship! You should make very sure that you take appropriate actions to eradicate any residual whitefly populations while you have the chance!
Whiteflies have long been considered a major pest of ornamental crops. Until 1986, the primary pest species was the greenhouse whitefly (GHWF), Trialeurodes vaporariorum (Westwood). In 1986, Bemisia tabaci (Gennadius) was found attacking an array or ornamental plants in Florida greenhouses. Scientists in Florida soon realized that this species was causing damage different than any ever attributed to whiteflies. This damage caused many plants to show signs of being infested because the plants turned yellow, white or silver depending on the specific host plant. Because various squash species tuned silver when infested with the B-biotype (also known as Bemisia argentifolii Bellows & Perring) this biotype was given the common name, silverleaf whitefly (SLWF). Throughout the following discussion we will refer to this whitefly as either the B-biotype of Bemisia tabaci or SLWF.
The Q biotype--In March 2005, Dr. Tim Dennehy (University of Arizona) reported the detection of the Q-biotype of Bemisia tabaci. This detection came after testing whiteflies collected from poinsettia in a retail outlet. These whiteflies were collected in December of 2004 as part of a pesticide resistance monitoring program in Arizona. Most of the samples had been from cotton and other crops, poinsettia and ornamental crops were not the primary focus of this monitoring program. Drs. Judy Brown, Tim Dennehy (University of Arizona) and Frank Byrne (University of California) independently verified the whitefly as being the 'Q-biotype'. This is the first time this particular strain has been found in the United States. The Q-biotype is thought to have originated from the Mediterranean region and has been associated with whitefly control problems. Dr. Dennehy determined that the strain of whiteflies collected from the poinsettia (Poinsettia-04) can be characterized as being virtually immune to the IGR pyriproxyfen (Distance), having strikingly reduced susceptibility to the IGR buprofezin (Talus) and a reduced susceptibility to the neonicotinoids insecticides imidacloprid (Marathon or Merit), acetamiprid (TriStar) and thimethoxam (Flagship).
At this point in time, this strain has not been tested for susceptibility to the other materials registered for whitefly control in ornamentals. It must be noted, that this information is generated using laboratory bioassays and that no field efficacy work has been conducted to determine how these data relate to controlling the Q-biotype in the field, at least in the United States. This biotype is known to have resistance to pyriproxyfen (Horowitz et al. 2003), buprofezin and reduced susceptibility to the neonicotinoid insecticides imidacloprid and acetamiprid in other regions of the world.
Description and Biology The greenhouse and silverleaf whiteflies are the primary whitefly pests of greenhouse crops. Banded-winged and citrus whiteflies are also found in greenhouses but usually do not reproduce and develop damaging populations. Whiteflies, when compared to other pests of ornamentals, have a long life cycle, ranging from 2.5 to 3 weeks up to 2 months under cooler conditions. Adults are moth-like and covered with white, waxy powder. Adult female whiteflies are about 1/16 of an inch in length and deposit about 50 eggs in cool environments and up to 400 eggs at higher temperatures. Consequently a whitefly population can reach very high levels in a few generations at higher temperatures. Eggs are inserted on end upon a short stalk on the underside of leaves. The eggs are whitish to light beige but darken to a dark blue or purple before hatching. The immature stages resemble miniature scale insects. They are flat and oval, glassy to opaque, light yellowish or greenish, and often provided with a fringe of wax filaments. Older immatures tend to be darker and either cream or yellow. Newly hatched immature crawlers move around on the leaf for only a few hours and then insert their mouthparts and begin to feed. The remainder of the immature development is sessile. Whiteflies feed exclusively on leaves, nearly always occurring on the undersurface. They suck juices from the plants and also excrete large quantities of honeydew in which sooty mold grows. Feeding Damage and Symptoms Whiteflies feed on more than 500 species of host plants. Greenhouse-grown ornamentals such as poinsettia, hibiscus, ivy, gerbera daisy, lantana, verbena, garden chrysanthemum, salvia and mandevilla are especially susceptible to whitefly damage. Whiteflies feed on plant phloem by injecting enzymes and removing the sap, reducing the vigor of the plant. Honeydew secretions from the whitefly promote the growth of sooty mold which also significantly reduces plant quality. The most obvious whitefly feeding damage symptoms are stem blanching, chlorotic spots, leaf yellowing and shedding and, at high population levels, plant death. In many crops the damage caused by Bemisia tabaci is indirect. This species of whitefly is responsible for transmitting many devastating viruses. Detection and Sampling Monitor whitefly population levels by trapping winged adults on sticky cards and inspecting leaves for the presence of feeding immatures. Strategically place yellow sticky cards throughout the greenhouse, especially near doors and among new plants to provide information about the presence and movement of whiteflies. Detect whiteflies on plants by randomly selecting 10 plants per 1,000 square feet of greenhouse space and thoroughly examining these plants on the underside of leaves, using a 10X hand lens, for the presence of whitefly adults, nymphs and eggs. Determination of biotypes is accomplished in the laboratory by using sophisticated biochemical techniques. Therefore, growers will have to send preserved whiteflies to one of the participating labs. Currently, this pest is not rated as a pest with quarantine status which means there are no legal requirements for these labs to report their findings. Irrespective of which whitefly biotype you have it's prudent to monitor for whiteflies and develop a Resistance Management Program (RMP). Management Chemical Control. Insecticides are the primary method used to control whiteflies. Many compounds have been used, but the systemic Marathon (imadacloprid) has been used most frequently over the past few years. The newer chemically related compounds Celero (clothianidin), Flagship (thiamethoxam), Safari (dinotefuran), and TriStar (acetamiprid) are also effective. Endeavor (pymetrozine), Aria (flonicamid), and the insect growth regulator (IGR) Distance (pyriproxyfen) are new chemical classes that have activity as well. There are several other effective IGRs: Azatin/Ornazin/AzaDirect (azadirachtin), Enstar II (kinoprene), Pedestal (novaluron), and Talus (buprofezin). The newest material to be registered for whitefly control is Judo (spiromesifen). Tank mixes of orthene (acephate) with a pyrethroid are synergistic, providing better control than either alone. A few other general insecticides, aerosols and soaps or oils can also be used. The newly hatched crawlers and the adults are most susceptible to chemicals, but the waxy covering on the larger immatures makes them more difficult to cover thoroughly with spray material. Resistance is a PROBLEM and every effort should be made to rotate chemicals each time an application is made don’t rely on any one product or chemical class for whitefly control.
Exclude whiteflies from the greenhouse. Whiteflies are very small. Screens with a hole size of <0.19mm are required to exclude adult whiteflies. Be proactive, find them first. Scout greenhouses by using sticky cards, leaf inspections or random sampling techniques. Use this information as a basis for decisions for chemical applications. Start any control practice early, when the first whiteflies are detected. Control is difficult once populations have increased to high levels. Biological Control. Several biological agents are available including predators (i.e. Orius, Delphastus, lacewing larvae, etc.), parasitoids (i.e. Eretmocerus, Encarsia, etc.) or pathogens (i.e. Beauveria bassiana, etc.). Check with suppliers on compatibility with chemicals and environmental requirements such as temperature, humidity, and daylength. In Florida, Bemisia tabaci is effectively managed on ornamentals and vegetables grown in greenhouses with Encaria transvena. Other potential management tools that you probably are not currently using. One area where nothing is currently being done to manage pests is during transit or shipping of plants. Drs. Lindquist, Oetting, and Osborne obtained significant whitefly reduction by treating whitefly infested cuttings and plants with insect pathogens just prior to boxing and shipping. A large grower could easily produce their own insect pathogens that would be used throughout the growing season in conjunction with all of the other pesticides being used. A couple of the largest growers in Florida are currently doing this. With fungi, it is not a typical case of using either chemical or biological control. YOU CAN USE BOTH ECONOMICALLY.
WHAT CAN YOU DO?
First of all, don't panic! Growers that have the Q-biotype in other parts of the world are still producing crops. Go Back To The Basics And Fine Tune Your Whitefly IPM Program! Scout- Determine the extent of any whitefly problems. Make sure everyone’s training is up to date and that employees are sensitized to the fact that not all whiteflies are the same. If any populations exist and the numbers or even their presence seems the least bit unusual FIND OUT WHAT YOU HAVE. Studies have shown that in a population that has both Q and B biotypes, increasing pesticide applications will only increase the proportion of the population that are the Q biotype. Prevention- Don’t let new whiteflies into your operation. You may not be able to prevent movement into your nursery from outside sources but reduce your risk by inspecting new plant material in a secure place so that whiteflies don’t escape. Quarantine all new plant material introduced into your nursery. Sanitation- Remove sources of infestation that might carry over populations from one season to the next. Cultural- Grow plants so as to facilitate good pesticide coverage. If possible, try to have a crop free period so as to break any cycling within your nursery. Physical- Screening, weeding… Resistance management - a numbers game? The greater the number of whiteflies present when an application is made the greater the chance that at least one individual might posses the ability to survive the treatment. The more frequently a given pesticide or mode of action is used the greater the potential for developing a problem. Along those same lines, the longer the residual activity the greater the “selection” pressure. If no virus is involved in the system, then the ultimate goal is not zero whiteflies throughout the production cycle but zero whiteflies on the plant material leaving the facility. If this statement can be agreed to, then we have a much better chance managing resistance in whiteflies. Older recommendations stated that “Insecticides should be applied a minimum of two times at a five to seven day interval to allow for egg hatch between applications so that both adults, nymphs and individuals that hatch from eggs are killed. This is not appropriate for many of the new pesticides that have residual activity of one week or greater. If the insecticide is properly applied and is not providing control, change to another material with a different mode of action because whitefly populations have the propensity to develop resistance. This is why scouting weekly and especially after a pesticide application is critical. Table 1 includes several examples of insecticides listed according to their chemical classification. Growers must learn from experience which chemicals, when correctly applied, fail to give satisfactory control, and to then try other materials in a different classification. Often, a severe pest problem occurs on plants in the retail shop because a few eggs, nymphs or adults survived even the best and most conscientiously followed control program during production. It is imperative, therefore, that your most effective pesticide be applied to major whitefly host plants as close to the date of shipment as possible.
There are a number of ways to deal with this issue but the bottom line is; the fewer applications one makes of materials from a given class, the smaller the potential for resistance developing. To that end, what can be done? First off, I recommend you develop a list of all the pesticides that are legal to use for whitefly control on the crop you are growing. Next, I suggest that each be evaluated under your particular situation for phytotoxicity. When you are finished you will have a list, hopefully not too short, from which you can develop a management program. The next problem is to review the labels to find restrictions/limitations on how often a material can be applied to a given crop. The plan you put together should be based on all of these points and the fact that growers will have to apply materials to manage other pests. Scouting is essential to the success of any pest management program.
Click here to go to the "Management Program for Whiteflies on Propagated Ornamentals with an Emphasis on the Q-biotype"
This is under consideration and open to discussion. Please send any comments to Lance Osborne From a resistance management perspective it would seem prudent to treat with those materials highlighted with yellow when the whitefly pressure and numbers are the greatest. Once the population have been reduced then treatment with those materials highlighted with green could be used. The theory behind this is simple: DON'T USE YOUR BEST MATERIALS AGAINST LARGE POPULATIONS BECAUSE THE LARGER THE POPULATION THE GREATER THE CHANCE OF TREATING A RESISTANT WHITEFLY. The Mode of Action information was obtained from: Insecticide Resistance Action Committee Mode of Action Classification v 5.1 (2005) Revised and re-issued, September 2005 (LINK)Olympic Horticultural Products Chemical Class Chart (Chemical Class Chart Vol, VII)
LINKS:
The Biology and Management of Silverleaf Whitefly, Bemisia argentifolii Bellows and Perring (Homoptera: Aleyrodidae) on Greenhouse Grown Ornamentals. (http://www.biocontrol.ucr.edu/bemisia.html) European Whitefly Studies Network (www.whitefly.org) Bemisia Newsletter (http://www.tropicalwhiteflyipmproject.cgiar.org/bemisianewsletter.jsp) IRAC Neonicotinoid Insecticide Resistance Management Guidelines, September 2004
Contributors in alphabetical order: James Bethke Luis Canas Joe Chamberlin Ray Cloyd Tim Dennehy Jeff Dobbs Scott Ferguson Dan Gilrein Richard Lindquist Scott Ludwig Ron Oetting Lance Osborne Cristi Palmer John Sanderson |
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