Orchid popularity has experienced continual growth in the United States, and orchids are the second most popular potted plant behind poinsettia (Jerardo 2007). Orchids are susceptible to numerous arthropod pests, which can be difficult to control. Common arthropod orchid pests that are frequently encountered by hobbyists are the boisduval scale (Diaspis boisduvalii) and several mites, most commonly the twospotted spider mite (Tetranychus urticae) and several flat mites (Tenuipalpidae), including the orchid mite (Tenuipalpus orchidarum), the phalaenopsis mite (T. pacificus), and the oncidium mite (Brevipalpus oncidii) (Johnson 2008, 2010).
Most orchid hobbyists use synthetic organic pesticides when treating plants for arthropod infestations. However, many orchids are sensitive to chemical pesticides, and damage to the plants and flowers can occur. Also, health concerns and the development of pest resistance to many pesticides make less toxic arthropod control options desirable, especially for hobbyists or small commercial growers.
UF/IFAS researchers evaluated the ability of Silwet® L-77, a surfactant used to increase the spread and wettability of pesticides, to improve the efficacy of horticultural oils in controlling boisduval scale and mite infestations in orchids. Initial experiments were performed to determine if Silwet® L-77 alone caused damage to orchids under greenhouse conditions. Representatives of seven genera were tested: Dendrobium, Oncidium, Doritaenopsis (Phalaenopsis hybrids), Paphiopedilum, Epidendrum, Cymbidium, and Cattleya. No phytotoxicity was observed to foliage, roots, stems, buds, or flowers on any of the orchids tested at temperatures between 21.0ºC and 35.0ºC (69.8ºF–95.0ºF) with relative humidity between 45% and 95% (Figure 1).
After initial tests indicated that the tested plants were not susceptible to phytotoxicity, researchers obtained plants from several commercial growers in Florida that were infested with either boisduval scale (Figure 2) or flat mite. Oil + Silwet® L-77 reduced scale density more so than water control or oil alone (Figures 3–5), while mortality rate of scales that were not removed by the spray increased from 39% to 47% with oil alone and from 86% to 93% with oil + Silwet® L-77. Similarly, when treating Dendrobium orchids that had flat mite infestations, the number of mites significantly decreased when using oil + Silwet® L-77 as compared to using oil alone. (To view the complete paper, visit https://journals.flvc.org/flaent/article/view/76056.)
Results indicate that the addition of Silwet® L-77 to horticultural oils significantly increases its efficacy over oil alone when treating plants for boisduval scale or flat mite infestations, and it may work well for other arthropods. Directions for using oil + Silwet® L-77 are given below using measurements suitable for homeowners.
Mixing Oil + Silwet® L-77
Mix horticultural oil with water according to directions.
Add Silwet® L-77 at 0.05%. A syringe or other device to measure such small amounts is helpful when measuring this amount of liquid. See Table 1 for assistance with measurements.
Applying Oil + Silwet® L-77 to Plants
Although no phytotoxicity was observed on the orchids used in this study, other genera and intergeneric hybrids may exhibit phytotoxicity symptoms after treatment. Care should be taken when treating plants for the first time. It is recommended that an individual plant be tested before spraying large groups of similar plants.
One hundred percent coverage is crucial. All aerial parts of the plants should be treated, both upper and lower leaf surfaces, all the way down to the potting mix. Roots also can be treated if pests are present, such as mealybugs that sometimes can be found on roots.
Because no residual chemicals are left on the plants, it is important to monitor pest populations after treatment and re-treat when necessary, according to the oil manufacturer's recommendations.
Due to the sensitivity of some orchids, it is recommended that plants be sprayed early in the day when it is cooler. Although no phytotoxicity was observed on the orchids in this study, heat increases the risk of damage when using oils.
There are reports in the literature of increased incidence of bacterial diseases when using Silwet® L-77. Therefore, if the growing area has a history of bacterial diseases or if it is suspected that bacterial diseases may be present, use with caution (i.e., use early in the day to minimize leaf wetness, remove all symptomatic plants, and treat for bacterial diseases).
Jerardo, A. 2007. "Floriculture and nursery crops yearbook." Economic Research Service, US Department of Agriculture. Accessed March 2022. https://downloads.usda.library.cornell.edu/usda-esmis/files/5m60qr88b/tt44pr36t/n009w541n/FLO-yearbook-10-04-2007.pdf
Johnson, P. J. 2008. "Mites on Cultivated Orchids." Accessed March 2022. https://staugorchidsociety.org/PDF/Johnson-Mites.pdf.
Johnson, P. J. 2010. "Boisduval Scale on Orchids." Accessed March 2022. https://staugorchidsociety.org/PDF/Johnson-Scale.pdf.
Measurement chart for oil + Silwet® L-77 using measurements suitable for orchid hobbyists.