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1 March 2010 Silwet L-77 Improves the Efficacy of Horticultral Oils for Control of Boisduval Scale Diaspis boisduvalii (Hemiptera: Diaspididae) and the Flat Mite Tenuipalpus pacificus (Arachnida: Acari: Tenuipalpidae) on Orchids
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Pest management of insects and mites on orchids has been based on the use of synthetic organic pesticides. However, less-toxic chemical control is needed by hobbyists and small growers. Orchids from 8 genera were treated with Silwet L-77 alone and no evidence of phytotoxicity was seen. Subsequently, 3 petroleum oils in combination with Silwet L-77 were tested to determine efficacy in suppressing Boisduval scale Diaspis boisduvalii Signoret (Hemiptera: Diaspididae) and the flat mite Tenuipalpus pacificus Baker (Arachnida: Acari: Tenuipalpidae). The results indicate Silwet L-77 increased toxicity of the oils and increased removal of the scales and mites from foliage over oil alone, yet no phytotoxicity was observed in any of the plants treated. Petroleum oil Silwet L-77 is considered low-toxic, may be less expensive than some pesticides, and may conserve natural enemies, making it a potential addition to an integrated pest management program for orchids.

The Orchidaceae is believed to be the largest family of flowering plants (Bechtel et al. 1992; Dressler 1993; Tsavkelova et al. 2008) with approximately 19,000 named species (Atwood 1986; Dressier 1993). Orchids are the second most economically important flowering plant produced in the United States and sales continue to grow (Jerardo 2006). As orchid popularity continues to grow, so will the desire for pest and disease information, as illustrated in the question and answer section of Orchids magazine published by the American Orchid Society, which frequently contains questions submitted by hobbyists and commercial growers concerning the identification and control of orchid pests and diseases. Common arthropod pests of orchids that can be difficult to control include the Boisduval scale Diaspis boisduvalii Signoret (Hemiptera: Diaspididae) and several species of mites.

Boisduval scale is one the most important orchid pests and can be difficult to control with traditional chemical methods because females possess a hard covering (Hamon 2002; Johnson 2009). Boisduval scale is commonly introduced into an orchid collection on an infected plant and scales can move quickly to a variety of orchids (Johnson 2009). Mites also can be a major problem on cultivated orchids (Johnson 2008). Mite species that are known pests of orchids include the two-spotted spider mite (Tetranychus urticae Koch, Arachnida: Acari: Tetranychidae) and several flat mites (Tenuipalpidae) such as the orchid mite (Tenuipalpus orchidarum Parfitt), the phalaenopsis mite (Tenuipalpus pacificus Baker), and the oncidium mite (Brevipalpus oncidii Baker) (Johnson 2008).

Synthetic organic pesticides historically have been the primary tactic used in pest control on orchids, but many orchids, some of which are highly valuable or rare, exhibit phytotoxicity when treated with pesticides (Johnson 2008). Therefore, alternatives to traditional pesticides are desirable, especially for hobbyists and smaller orchid producers. In order to preserve as many natural enemies as possible and have the least impact on the environment and human health, more alternatives to traditional pesticides are needed. One alternative to traditional pesticides is Silwet L-77, an organosilicone surfactant.

Surfactants commonly are used in agriculture to increase the wettability and spread of herbicides or pesticides on plants, and thus improve control of the target pest or weed (Tu et al. 2001). Silwet L-77 has been shown to increase the effectiveness of limonene for the control of mealybugs (Pseudococcus longispinus Targioni-Tozzetti) by reducing the surface tension around their wax-covered bodies (Hollingsworth 2005). Tipping et al. (2003) demonstrated that Silwet L-77 alone is toxic to Pacific spider mite eggs (Tetranychus pacificus McGregor), grape mealybug crawlers (Pseudococcus maritimus Ehrhorn), western flower thrips (Frankliniella occidentalis Pergande), and cotton aphid (Aphis gossypii Glover). In addition, Silwet L-77 alone is toxic to nymphs of the Asian citrus psyllid (Diaphorina citri Kuwayama) and can significantly increase mortality of D. citri eggs when applied with one-fourth the label rates of imidacloprid or abamectin and mortality of adults when applied using one-fourth or one-half the label rate of imidacloprid (Srinivasan et al. 2008). Silwet L-77 also can increase the efficacy of fungicides. Silwet L-77 or another surfactant (Kinetic) improved the activity of the protectant fungicide maneb using potato early blight or dry bean rust as model systems (Gent et al. 2003). Thus, the addition of Silwet L-77 as an adjuvant to chemical pesticides has been shown to increase their efficacy, even at lower-than-label rates.

The objectives of this study were to (1) determine if Silwet L-77 can be used without causing phytotoxicity to 8 commonly cultivated orchid genera, and (2) determine if Silwet L-77 can increase the effectiveness of 3 horticultural oils against Boisduval scale (Diaspis boisduvalii) and the flat mite (Tenuipalpus pacificus) on orchids.


Phytotoxicity Study

Representatives of 7 commonly cultivated orchid genera obtained from commercial orchid growers in south Florida were used: Doritaenopsis Luchia Lip ‘Sog. F714’ X Han-Ben ‘Girl RL’ in 10.2-cm pots, Cymbidium Golden Elfin 15.2-cm pots, Dendrobium cultivars Blue Sampran, Burana Stripe, Lady Pink, Lena Pink, Pegasus, and Salaya Fancy, all in 10.2-cm pots, Epidendrum mericlones in 10.2-cm pots, Oncidium KBR in 10.2-cm pots, Paphiopedilum Maudiae types in 10.2-cm pots, and Cattleya mericlones in 10.2-cm pots. Plants received pre- and post-spray ratings based on overall vigor, color, and presence/absence of spots or necrotic areas on leaves, stems, flowers, or flower buds. Plants received a quality rating on a scale from 1 to 10, with 10 being undamaged and 1 being so severely damaged that the plant could not be sold. Plants were watered and fertilized as per commercial practice for each type of orchid.

Plants were treated within 1 h of the prespray assessment. Treatments consisted of a water control or Silwet L-77 (99.5% polyalkyleneoxide modified heptamethyltrisiloxane, Helena Chemical Co.) at 0.05% (v:v) and the number of replicates of each orchid type is shown in Table 1. Plants, including inflorescences, were sprayed twice at 7-d intervals with hand sprayers until completely covered and assessed for phytotoxicity damage 1 week after the second spray. Plants (pre- and post-spray) were kept in a greenhouse at the Department of Entomology and Nematology, University of Florida, Gainesville, FL at temperatures ranging from 21.0 to 35.0°C and 45 to 95% RH and 16L:8D photoperiod from the time the plants were obtained until the end of the experiment. Statistical analysis was performed with the PROC GLM procedure in SAS (SAS Institute 2002) to compare the pre-spray rating with the control, and the post-spray rating with the control. Levene's test for homogeneity of variance was performed (Hill & Lewicki 2006).

Silwet + Oil Efficacy Trial (Boisduval Scale)

A commercial Cattleya orchid grower in central Florida with a heavy Boisduval scale infestation participated in the efficacy trial. Thirty Cattleya orchid hybrids and mericlones in 15.2-cm pots were selected based on size and severity of scale infestation (200+ female scales per plant and nymphs and males were so abundant that much of the foliage appeared white). Pre- and post-spray assessments were performed 2 ways: (1) The den