Managing the Sweet Potato Whitefly:
Bastiaan M. Drees, Professor and Extension Entomologist
Department of Entomology
Texas A&M University System
College Station, Texas
The sweetpotato/silverleaf whitefly has become a widespread difficult-to-control pest of Texas greenhouse and nursery crops since 1987. It is the number one pest of poinsettia grown in Texas nurseries, and control failures have been reported for most conventional insecticides. Management of whiteflies in the greenhouse must be thought of as a program through the entire production cycle. Repeated use of some insecticides can lead to whitefly populations that are resistant to that class of insecticides. Therefore, care needs to be taken to plan a strategy and to use products with different modes of activity over time to preserve their effectiveness.
Biology: Description. The sweetpotato/silverleaf whitefly (Bemisia tabaci/Bemisia argentifolii) adult is small, about 0.9 to 1.2 mm in length and holds its solid white wings roof-like over a pale yellow body while at rest. Immature stages begin with a pointed oblong yellow egg (0.2 mm) which darkens at the apex just before hatching. The first instar or crawler stage (0.2 - 0.3 mm) settles down on the underside of leaves and goes through three more molts as a sessile, flattened yellow oval nymph. The last instar is often referred to as a pupa (0.7 - 0.8 mm) and has distinct eye spots.
Life cycle. The duration from egg to adult may be 18 days under warm temperatures, but may take as long as 2 months under cool conditions. The number of eggs produced is greater in warm weather, but typically range from 50 to 400 eggs (avg. 160, of which about 2/3 are female. This explains in part how whitefly populations can build up so rapidly.
Distribution on plants. Sweetpotato/ silver-leaf whitefly adults often concentrate on younger leaves where oviposition is highest. Following the development of the plant, larger nymphs are typically more numerous on older leaves.
Damage. Direct crop damage occurs when whiteflies feed on plant phloem in high enough numbers, excreting honeydew which promotes sooty mold and reduces plant quality. Chlorosis of leaves and/or bleaching of parts of poinsettia plants has been associated with whitefly infestation. Consumers find whitefly presence on and around plants objectionable. The Texas Floral and Nursery Law mandates that nursery crops be "pest free." Production facilities found to harbor heavy infestations of sweetpotato whiteflies have received Stop Sale Orders from Texas Department of Agriculture inspectors until control has been obtained.
Cultural practices: The most important whitefly management practices include good horticultural practices. Certain varieties (ie. 'Marble') are known to be more susceptible to heavy whitefly infestation, making variety selection a part of whitefly management. Avoid using whitefly infested cuttings. Plant structure and spacing are important because coverage of plants with non-systemic insecticide sprays is difficult on lower leaves and within thick canopies. Heavily infested plants and leaves can be removed and destroyed to reduce whitefly breeding areas. Weeds and other plants frequently serve as alternate hosts and harborage areas for whiteflies. Removal of weeds and "pet" plants from in and outside of the greenhouse will reduce these sources of infestation.
Biological control: A number of natural enemies of whiteflies have been identified. The common green lacewing (Chrysoperla carnea) and a lady beetle (Delphastus pusillus) have been shown to be effective predators when released in high numbers. On potted hibiscus, one to five Chrysoperla larvae per plant have been shown to provide economic control of whiteflies. Parasitic wasps (Encarsia formosa) have also been released in greenhouses to affect whiteflies by host feeding and/or parasitism. Feasibility of conducting a biological control program is dependent upon size of operation, desired level of control, cost of natural enemies and available labor to apply natural enemies. Fungal pathogens of whiteflies such as Beauveria bassiana (Naturalis ) have shown promise as microbial insecticides for whitefly suppression but require registration by the Environmental Protection Agency.
Scouting/monitoring: Direct observation and use of yellow sticky traps are useful methods for monitoring whiteflies. These techniques are useful for early detection of whiteflies and documentation of relative whitefly abundance over time. However, economic injury levels based on these monitoring methods have not been developed. Results of monitoring efforts provide documentation on the need for and success of suppression efforts applied.
Direct observation of whiteflies is accomplished by examining the under surfaces of sets of randomly-selected leaves on a number of plants in each production area. The number of leaves to examine is rather arbitrary, although accuracy of estimating whitefly infestations is improved as more leaves and plants are sampled. The number of adult and/or immature whiteflies can be estimated on each leaf or part of leaf sampled. In cotton, a single 3.88-cm 2 disc from the base of the second sector (area between major veins near the middle of the leaf) of the fifth main stem node leaf has been determined to be the most efficient sample unit for estimating egg and nymph densities. This method is time consuming and is hindered when whitefly infestations are spotty. Conversely, the number of adult and/ or immature whitefly infested leaves can be recorded. This method is less time consuming so that more plants can be examined. For instance, if 100 leaves are sampled from 10 plants (10 leaves per plant) and 20 leaves are found to harbor immature whitefly stages, then the plants have 20 percent of their leaves infested. By repeating this sampling method, an average percent infested leaf value can be calculated. In Florida, state guidelines specify that no more than 20 percent of the leaves may be affected by whiteflies, with up to five pupae per leaf.
Yellow sticky cards are best used by hanging them just above the canopy of the crop, near vents and doors and even outside the greenhouse. The number of cards to hang over each bench has been suggested at 1 per 1,000 sq. ft. Cards hung horizontally over the canopy, with one sticky surface on the bottom, may collect more whiteflies than cards hung vertically. Cards should be removed periodically (ie. weekly) and whitefly adults counted from the whole card or from randomly selected subsample areas on each card (ie. 1 sq. inch from three locations on each card) to save time. Whitefly numbers are recorded as number of adults/card sample area/ day. Data generated can document the relative abundance of adults trapped over time.
Insect Growth Regulators: Several types of insecticides are considered to act as insect growth regulators (IGR's). These products do not kill adult whitefly stages, instead they prohibit or delay insect development. Products containing azadirachtin, derived from neem tree seeds, act by preventing insects from molting properly and are often called "chitin synthesis inhibitors." Another group of IGR products contain ingredients like kinoprene or fenoxycarb. These chemicals, referred to as "juvenoids", mimic the effects of an insect's hormone called the juvenile hormone. When whiteflies are treated with these products, immature stages (eggs, nymphs, pupae) fail to develop into mature adults. Use of IGR insecticides requires understanding of the mode of action. Migration of untreated whiteflies into a treated area may mask the effects of the treatments. However, their use may reduce or delay population growth, delaying the need for conventional insecticides. "Juvenoid" IGR-treated populations may also be more susceptible to other insecticide.
Resistance management:
Resistance management is the theoretical approach used to preserve pesticide-susceptible insect pest populations. Use of products with similar modes of action on the insectsą nervous system can lead to a population becoming resistant/ tolerant to that group of products. Various approaches have been suggested, such as; 1) use of high insecticide rates; 2) use of the lowest effective rates; 3) use of product mixtures; 4)rotation of pesticide classes during a life cycle; and, 5) rotation of pesticides between approximate life cycle periods called, "long rotation." Use of the lowest effective insecticide rates reduces selection for resistance. Resistant populations, after a period of time without selection pressure and with the introduction of susceptible individuals, may regain susceptibility to insecticides to which they had developed resistance.
Different groups or classes of insecticides have different modes of action. For the sweetpotato/silverleaf whitefly, use of nerveactive insecticides should be delayed as long as possible in a production cycle. When necessary, however, long rotation of nerve active pesticide classes between the 18 to 30 day life cycle of the whitefly is believed to help maintain their susceptibility to insecticides. Therefore, when nerveactive pesticides are used, products from one class are regularly applied (on 3 to 7 day intervals depending on infestation level) for 18 to 30 days (depending on temperature conditions). Then products from another class are used until the end of the next approximate generation. Use of the lowest effective rates of products or product mixtures is encouraged.
Synergistic product mixtures can be used to lower the required dose needed for effective control, thereby reducing the selection for resistance. Conversely, mixtures of products of different modes of action can also contribute to the selection of "multiple resistance" whereby a pest becomes resistant to all products in the mixture. Two of the more effective product mixtures identified for sweetpotato/silverleaf whitefly suppression have been Enstar® II plus Pentac® and Orthene® plus a pyrethroid such as Tame®. However, reduced effectiveness of the later mixture has been reported after multiple applications.
Since sweetpotato/silverleaf whiteflies are reported to have become tolerant/resistant primarily to nerve-active insecticides, the use of insect growth regulators, insecticidal soaps and horticultural/paraffinic oils has become increasingly important. These products are not nerve-active insecticides. However, since these products are non-systemic good coverage is essential. Insect growth regulators are best used early in a production cycle for delaying and suppressing population growth, delaying the need for nerve-active pesticides. However, their use can be maintained throughout the production cycle. Insecticidal soaps and horticultural oils can be used at any time to help suppress populations without using nerve-active insecticides. If biological control programs are being conducted, a soap application can be used prior to. Insecticida release of natural enemies l soaps can also be added as tank mixtures to nerve-active insecticides to increase penetration of the insecticide into the pests, improving control. However, repeated use of soaps or oils can lead to phytotoxicity or leave residues.
Insecticides vary in their effect on adult versus immature sweetpotato/silverleaf whiteflies. Insect growth regulators, for instance, have no effect on adults. On the other hand, endosulfan (Thiodan®) and nicotine (not specifically labeled for whitefly) are known to kill adult whiteflies. Horticultural oils have been shown to repel egg laying adults after treatment. Such product attributes can be useful during the production cycle as the need for selective use arises.
Phytotoxicity. Particularly after bracts begin to show color, care must be taken to select products that are not known to cause plant damage. Very few foliarly-applied pesticides are safe for use at this time. Abamectin (Avid®), though not registered for whitefly control on poinsettia, has been safely used during this growth stage. Fumigants containing sulfotep can also be used at that time. Contact the product representatives if you are uncertain whether a product can be safely used after bracts begin to show color. Whenever using a new product or product mixture including adjuvants) for the first time, spray a few plants and observe them for possible phytotoxic reactions such as yellowing or puckering of leaves, leaf burn or leaf drop.
Systemics. Recently available imidacloprid 1G (Marathon®) will change whitefly management programs significantly. Media treatments are most effective when applied to vegetatively growing plants. Trials conducted on vegetatively growing poinsettia (Sept. 3) using a liquid formulation of imidacloprid provided over 30 days of suppression. Generally, systemic products are less effective when plants are flowering or fruiting. Use of systemic insecticides as preventive treatments will delay the onset of infestation, but also may increase selective pressure for resistance. Few foliarly applied products have systemic activity. Acephate (Orthene®) and avermectin (Avid®, not registered for whitefly control) do have some systemic properties that can improve coverage and provide some residual control.
Coverage. Adopt spraying methods that improve coverage, especially to the undersides of the leaves where whitefly populations develop. Immature whiteflies are not mobile (except crawlers) and will not move about to contact insecticides; therefore, insecticides must be placed directly on immatures to obtain maximum benefits. Use of higher spray volumes for conventional foliar applications and use of different application methods over time can help improve coverage.Hypothetical treatment programs - A pest management plan: With the above
information as "rules of thumb", many possible use sequenses of products and/or product mixtures are possible for a sweetpotato/silverleaf whitefly management plan. No single program may be best for all situations. Pesticide use history in a greenhouse or from the source of the cuttings/plants affects the susceptibility of a whitefly population to insecticides. On-site experimentation with various treatments can provide valuable information to the grower. However, theoretical programs or management plans can be proposed for control of greenhouse grown poinsettia pests.
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