In 1997 over $7 million-worth of potted flowering plants were sold in the United States, and poinsettias accounted for fully 32 percent of the total with $2.22 million in sales—making poinsettias the leading revenue-generating crop for commercial growers (USDA National Agricultural Statistics Service 1998). For many years Trialeurodes vaporariorum, the greenhouse whitefly (GWF), was the most destructive pest affecting poinsettias, but today Bemisia argentifolii, the silverleaf whitefly (SLWF), causes more damage.
Until the late 1980s, T. vaporariorum was the major pest of poinsettias. In 1986, an apparently new strain of B. tabaci, ‘strain B,’ caused substantial damage to poinsettias in Florida, and by 1991 had spread throughout the United States where it caused $500 million in damage (Brown et al. 1995). Prior to affecting poinsettias, B. tabaci, commonly called the tobacco, cotton, or sweet potato whitefly, was a common pest of agricultural crops (Byrne et al. 1990). In 1994 B. tabaci ‘strain B’ was identified as a separate species, B. argentifolii, the silverleaf whitefly. The SLWF was given its name because when SLWF feed on the leaves of squash plants, characteristic silvering symptoms are produced (Powell and Lindquist 1997). Another type of whitefly, the bandedwinged whitefly, is occasionally found on sticky traps in poinsettia production areas, but is seldom a problem on the crop (Sanderson 1996).
Although this publication focuses on the impact of GWF and SLWF on poinsettia crops, it is also important to note that both whiteflies are vec-tors for a variety of plant diseases. The SLWF is a vector of geminiviruses, which have been described as “some of the most devastating diseases of vegetables such as tomato, bean, and squash and of field crops such as tomato, beets, tobacco, and corn” (Agrios 1997).
Biology, Life Cycle, and Behavior
Whiteflies land on the top surface of plant leaves and immediately walk around to the shaded lower side to feed and lay eggs (van Lenteren and Noldus 1990). All life stages develop on the undersides of the leaves. The first instar of the nymph is called the crawler. The crawler emerges from the egg, moves a short distance, and begins to feed.
The developing whitefly remains immobile (sessile) for three more nymph instars then molts to become a mobile adult (Sanderson 1998). In experiments with SLWF on a poinsettia crop, the timing of each stage at 72F is as follows (Hoddle 1998a):
The rate of whitefly development is determined primarily by temperature, but host plant preferences play an important role. Experiments with GWF demonstrate that the rate of egg laying (oviposition), egg number laid per female, female longevity, total development time from egg to adult, and the mortality rates for all life stages are directly related to host plant nutrition (van Lenteren and Noldus 1990). Moderate greenhouse temperatures (60–75F) favor the GWF, higher greenhouse temperatures (above 75F) favor the SLWF, but both thrive on poinsettia (Powell and Lindquist 1997). If both the GWF and the SLWF are present in a poinsettia crop, the SLWF will out-compete and exclude the GWF in 50–60 days (Hoddle 1998a).
The GWF and SLWF have wide host ranges: there are over 275 plant species affected by GWF (Byrne et al. 1990) and over 500 species affected by SLWF (Brown et al. 1995). Both the GWF and SLWF have a life cycle with four developmental stages—the egg, nymph, pupa and adult stage (Figure 1).
The SLWF and GWF can be distinguished in any life stage using the characteristics listed in Table 1.
Whiteflies damage poinsettias in two ways; by direct feeding (chewing on the leaves) and by indirect feeding (tapping into the phloem to extract sugars). No matter which feeding method is used, relatively small populations of whiteflies make poinsettias unsalable.
Direct feeding damages poinsettias by causing bracts and stems to become chlorotic and bleached. Indirect feeding damages poinsettias when whiteflies use their piercing-sucking mouthparts to penetrate phloem, causing the whiteflies to excrete honeydew onto the leaves. Honeydew is sticky, full of undigested sugars, and is frequently invaded by sooty molds (nonparasitic fungi). The resultant “accumulations of sooty mold reduce the aesthetic quality and marketability of the crop, even though the plants are not directly injured” (Daughtrey et al. 1995).
The simple presence of whiteflies, even in the absence of other types of damage, reduces the aesthetic value of plants offered for retail sale.
Integrated Pest Management (IPM) of Whiteflies
There are a number of challenges in managing pests in poinsettias (Parrella 1995).
There are four potential sources of whitefly infestations in greenhouses, as follows (Parrella 1995):
Putting up screens excludes whiteflies and prevents whitefly migration. Screens are also cost effective. Growers in Europe, North America, and Israel who have installed screens report their use of pesticides declined by 50–90% (Robb and Parrella 1995).
Several publications produced by the National Greenhouse Manufacturers Association (1996a and 1996b) contain recommendations about screens. For example, “Greenhouse Insect Screen Installation: Considerations for Greenhouse Operators” provides a discussion of screen materials and construction methods, and “Standards for Ventilating and Cooling Greenhouse Structures” contains the necessary engineering formulas to compensate for the presence of screens. The North Carolina Commercial Flower Growers Association has published a detailed discussion of available screen materials, a comparison of their efficiency, and a list of screen manufacturers (Bell 1997). The article by Bell is available on the Internet at www2.ncsu.edu/unity/lockers/project/flor/iculture/www/NCCFGA.
Examples of the calculations needed to expand the ventilation surface area to compensate for the reduced airflow through screens are available from the North Carolina Cooperative Extension Service (Baker and Shearin 1998); they are also available from the Web site listed above. Baker and Shearin have developed a PC-compatible program to guide you through the airflow/surface area calculations. The program is available from James Baker, Entomology Extension, Box 7613 NCSU, Raleigh, NC 27695. The cost is $10, payable to the “North Carolina Agricultural Foundation.”
Good crop sanitation involves inspecting incoming poinsettia cuttings for the presence of whiteflies, followed by routine inspections (monitoring). Routine inspections allow you to identify whitefly infestations early and take appropriate corrective actions. All of the information in this “Cultural Controls” section is taken from Sanderson (1996).
A recommended practice is to thoroughly inspect, one month prior to the arrival of any new poinsettia cuttings, all the poinsettia plants currently growing in the greenhouse. If whiteflies are found, thoroughly spray the plants with insecticide to reduce the whitefly population, if possible, to zero. Three weeks later (one week prior to the arrival of the new cuttings), examine existing plants for the presence of whiteflies. If whiteflies are found, take appropriate control measures.
Purchased cuttings should be carefully inspected for all life stages of the whitefly. Where possible, inspect newly purchased cuttings in a holding area separate from the poinsettia production areas. Sanderson recommends that “each shipment and cultivar should be inspected individually, because whitefly levels can differ by cultivar and propagator.” Focus the inspection on the undersides of the three oldest (that is, lowest) leaves of the cuttings because lower leaves are more likely to harbor the immature life stages. Adult whiteflies prefer the upper leaves.
The best way to monitor greenhouses for the presence of whiteflies is to use yellow sticky cards. The minimum number of cards recommended for successful monitoring is one yellow sticky card per 1,000 square feet of greenhouse floor space (Powell and Lindquist 1997), but one card per 250 square feet is more effective.
Use of sticky cards in poinsettia was pioneered by the New York State Poinsettia IPM Program in 1989–92. The program combines sampling adults using yellow sticky cards (3x5 inch) with leaf sampling to detect immature stages. Key features of the New York State Program include (Sanderson 1995)
Sequential sampling for the New York State Poinsettia IPM Plan is shown in Table 2 and Examples 1–3. Each PMU is sampled weekly, and control measures, if necessary, are determined for each PMU. There are three thresholds based on the acceptable number of immatures on leaf surfaces. In each PMU, plants are randomly selected and six leaves are inspected per plant. The minimum number of plants inspected is 14 for the low threshold (0.1 nymph/sample unit), 10 for the moderate threshold (0.6 nymph/sample unit), and 6 for the high threshold (3.0 nymphs/sample unit). The cumulative number of each life stage of whitefly is recorded. If the cumulative number of nymphs in the PMU equals the maximum boundary (for the selected threshold), sampling is stopped and control measures are necessary that week for that PMU. If the minimum number of plants are inspected in the PMU and the cumulative number of whitefly nymphs is below the minimum boundary for the threshold, inspection is stopped, control actions are not necessary and that PMU is not inspected again until the following week.
Sequential sampling reduces inspection time and the cost of inspection, yet provides a high level of assurance that whiteflies are being controlled. The effectiveness of the sequential sampling plan was verified with the cooperation of commercial growers. Growers who used sequential sampling achieved their target control levels and reduced their insect scouting costs by 40 percent (Sanderson et al. 1994).
For a copy of the sequential poinsettia inspection plan contact Media Services Distribution Center, Cornell University, 7 Business and Technology Park, Ithaca, NY 14850 (607-255-2080). The document title is “New York State IPM Program, publication No. 403,” 1993, Cornell Cooperative Extension; the cost is $10.
Many commercial growers raise multiple crops, use continuous crop cycles, and need to control many pests simultaneously (Heinz and Parrella 1994). In contrast, poinsettias are often the only crop in the greenhouse, have a single growing cycle, and are affected by only one pest, the whitefly. This makes poinsettias an excellent candidate for the use of biological control methods, which should be used as part of an overall IPM program.
Example 1: Low Threshold. The number of nymphs are less than lower limit, therefore no action required this week. IPM scout randomly samples a pest management unit (PMU); the sampling plan calls out that at least 14 plants will need to be inspected before reaching the conclusion that no action is required. No nymphs were found after the completion of the required sample. Since zero is less than the lower limit of one, no action required this week.
Example 2: Low Threshold. The number of nymphs reach upper limit, management action required this week. IPM scout sets out to sample the PMU. If one nymph is found in any of the first 8 plants, the upper limit has been reached and a management decision is required. Similarly, if two nymphs are found on the 10th through 14th plant, a management decision is required. The scout found no nymphs until the 6th plant when 1 nymph was found, which meant that the upper limit was reached. A management decision is required this week for this PMU.
Example 3: Low Threshold. After initial target sample, the number of nymphs found is between the upper and lower limits. The sample is expanded until the number of nymphs is below the lower limit or equal to the upper limit. IPM scout randomly samples; no nymphs were found until the 14th plant on which a single nymph was found. The scout must continue sampling until a total of 30 plants have been inspected with no additional nymphs found. A single nymph in 30 plants is less than the lower limit of 2 nymphs, thus, no action required this week in this PMU. Note that if the number of nymphs reaches the upper limit for any of the steps in the expanded sample, a management decision is required.
Entomopathogenic fungi, also known as mycopesticides or mycopathogens, are fungi that prey on insects. Entomopathogenic fungi are a useful component of any IPM program because they are relatively host specific, inexpensive to produce, able to function in a wide range of greenhouse environments, and safe to humans (Brownbridge et al. 1994). One type of entomopathogenic fungus, Beauveria bassiana, is very effective when whitefly populations are low. Three to five sprays typically eliminate whiteflies in the greenhouse (Sanderson 1996).
Current research shows that three other entomopathogenic fungi, Paecilomyces fumosoroseus, Metarrhyzium lecani, and Verticillium lecani are effective at controlling whiteflies (Sanderson 1996). These organisms, however, are either not available commercially or are not labeled for use in greenhouses.
Entomopathogenic fungi, however, do not offer stand-alone pest-control capabilities and are best used in conjunction with a program of conventional insecticides or insect growth regulators (Sanderson 1996).
Research continues into controlling whiteflies in poinsettia production greenhouses using natural enemies of the whitefly. Encarsia formosa, a parasitoid, is effective against GWF but not SLWF (Sanderson 1996). A predator, Delphastus pussillus, combined with the parasitoid, Encarsia luteola, was effective in trials against SLWF in commercial greenhouses, but controlling white flies with these organisms costs five times more than conventional pesticides (Parrella 1995). The parasitic wasp, Eretmocrus eremicus, when used in conjunction with insect growth regulators (IGRs), was effective against SLWF in experimental and commercial greenhouses (Hoddle 1998b). The advantage of using parasitoids in combination with IGRs is that fewer IGR applications are necessary, which reduces the probability that whiteflies will develop resistance to the IGRs (Hoddle 1998b).
Imidacloprid, a member of a new class of synthetic insecticides called chloronicotinyls, has proven extremely effective against whiteflies in poinsettia crops (Hoddle 1998b). Marathon® 1G, which is labeled for ornamental and greenhouse crops, is a granular formulation of imidacloprid.
Imidacloprid is systemic, has a low mammalian toxicity, and is also effective against aphids (Sanderson 1996). However, because imidacloprid is so effective and so long lasting, one application per crop can induce the whiteflies to develop resistance (Parrella 1995). To prevent imidacloprid resistance from developing, use other pesticides in alternate years in different areas of your greenhouse (Parrella 1995).
Pesticides labeled for greenhouse use against whiteflies are shown in Table 3. Please note that pesticide classes are included in the table to help you plan pesticide rotations.
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