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.

MATERIALS AND METHODS

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 density of female scales present was estimated according to the following scale: 0: 0 scales; 1: 1–20 scales; 2: 21–40 scales; 3: 41–60 scales; 4: 61–80 scales; 5:81–100 scales; 6:101–120 scales; 7: 121–140 scales; 8: 141–160 scales; 9: 161–180 scales, 10:181–200+ scales. (2) Mortality was calculated by examining 10 randomly chosen female scales remaining on each plant after the postspray assessment. Each scale was examined with a hand lens to determine if it was alive or dead. Female scales were used in both assessments because males tend to cluster, making them difficult to count.

TABLE 1.

PRE-SPRAY AND POST-SPRAY QUALITY RATINGS (RANGE = 1–10) OF ORCHIDS USED IN THE PHYTOTOXICITY STUDY.

Two separate trials were conducted. In the first trial, 2% (v:v) petroleum oil 435 (Growers 435, Growers Fertilizer Co., Lake Alfred, FL) was used and in the second trial 1% (v:v) Prescription Treatment Ultra-Fine Oil (Whitmire Micro-Gen Research Laboratories, Inc., St. Louis, MO) was used. Thirty Cattleya orchids were selected as described above and used in each trial (10 water controls, 10 oil treatments and 10 oil + 0.05% (v:v) Silwet treatments). Because Silwet L-77 is not labeled for use as a pesticide, it was only used in combination with petroleum oil. Plants were sprayed to run-off using a 3.8-L pump sprayer (H. D. Hudson Manufacturing Company, Hastings, MN). Plants were treated within 1 h of the pre-spray assessment. Plants were kept in a greenhouse at 18.0 to 27.0°C and 60–80% RH. Plants in each trial were treated once a week for 2 weeks and the post-spray assessment was done 1 week after the second treatment. Each trial was performed twice.

RESULTS

DISCUSSION

The results show that the addition of 0.05% Silwet L-77 to petroleum oil 435, Prescription Treatment Ultra-fine oil, or Prescription Treatment Ultra-pure oil significantly increased the efficacy of these oils against the Boisduval scale and the flat mite. Previous research has demonstrated that Silwet L-77 alone is toxic to several other important arthropod pests (Tipping et al. 2003; Cowles et al. 2000; Hollingsworth 2005; Wood et al. 1997; Liu and Stansly 2000; Imai et al. 1995; Purcell and Schroeder 1996; Srinivasan et al. 2008; Cocco and Hoy 2008). This study demonstrates, for the first time, that Silwet L-77 could be an important management tool to increase the efficacy of oil against Boisduval scale and the flat mite on orchids. The reason for the increased efficacy is not clear. Silwet L-77 greatly reduces the surface tension of water, allowing it to infiltrate plant leaf stomata (Neumann & Prinz 1974) and hydathodes (Zidack et al. 1992). It is plausible that this reduction in surface tension of water around the body of an arthropod may allow an infiltration of water into the spiracles, drowning the arthropod (Cowles et al. 2000; Shapiro et al. 2009).

In the present study, three petroleum oils were used. After the initial work with the 435 light horticultural oil, the study was repeated with oil labeled for use on orchids, the Prescription Treatment Ultra-Fine oil. Upon completion of the experiments with the Ultra-Fine oil, we were informed by the manufacturer that it would no longer be available, but a replacement, Prescription Treatment Ultra-Pure oil, was available. The addition of 0.05% (v:v) Silwet L-77 significantly increased the efficacy of all 3 oils tested against the Boisduval scale on Cattleya orchids or the Ultra-Pure oil against the flat mite on Dendrobium orchids when compared to the water control.

There was no significant difference between the oil alone and the oil + Silwet L-77 treatment when treating Grammatophyllum orchids for flat mite infestations, perhaps due to the fact that Grammatophyllum orchids have large, thin leaves that show very little pitting as a result of feeding by the flat mite. In contrast, Dendrobium orchids have smaller, thicker leaves that produce deep pitting from flat mite feeding, and flat mites are often located in these pits. Because pitting of the Grammatophyllum orchid foliage was very superficial, the mites were easily removed by both oil or oil + Silwet, whereas the addition of the Silwet L-77 to oil appeared to better penetrate into pits containing flat mites on the Dendrobium leaves, removing them. Due to the lack of available Grammatophyllum orchids infested with mites, the experiment was only performed once and should be repeated.

TABLE 2.

EVALUATION OF SILWET L-77 (0.05%) IN COMBINATION WITH 2 OILS FOR CONTROL OF BOISDUVAL SCALE INFESTATIONS IN CATTLEYA MERICLONE ORCHIDS.

TABLE 3.

EVALUATION OF SILWET L-77 (0.05%) IN COMBINATION WITH PRESCRIPTION TREATMENT ULTRA PURE OIL FOR CONTROL OF FLAT MITE (TENUIPALPUS PACIFICUS) INFESTATIONS IN DENDROBIUM OR GRAMMATOPHYLLUM ORCHIDS.

It was discovered after experiment la that the addition of oil to Silwet L-77 not only increased mortality of Boisduval scale, but also removed them from the plant. This property could be beneficial where tolerance for arthropod infestations is essentially zero and plants are sold and appreciated based on aesthetic value (Bethke & Cloyd 2009). However, as suggested by Tipping et al. (2003) and Liu & Stansly (2000) it is essential that 100% coverage is achieved to obtain adequate control.

Tipping et al. (2003) demonstrated that Silwet L-77 alone increased mortality of Western flower thrips, Pacific spider mite, and cotton aphid, (a93.8%), but 100% mortality was not achieved, possibly due to lack of complete coverage. Liu & Stansley (2003) suggest that complete coverage is necessary for good control due to contact activity. For orchids, this includes the abaxial and adaxial leaf surfaces, stems, pseudobulbs, inflorescences, under sheaths (Cattleya orchids), and possibly the roots. This requirement may preclude the use of oil + Silwet L-77 by large commercial orchid growers where pesticides are generally delivered to large groups of plants by irrigation or by large overhead booms, which may make it difficult obtain complete coverage. In addition, because there is no residual activity, re-applications may be necessary to obtain acceptable control.

Several caveats must be considered before using Silwet L-77 + petroleum oil to treat arthropod infestations in orchids. Although no evidence of phytotoxicity was observed in any of the orchids used in these tests, it is possible that other genera, intergeneric hybrids, or certain cultivars may be susceptible to phytotoxicity. It is recommended that growers check a representative of the plants to be treated for signs of phytotoxicity before extending treatment to large groups of plants. In addition, environmental conditions may play a role in phytotoxicity, so label recommendations for the oil should be followed. Humidity also may be important for increased efficacy and should be taken into consideration. Liu & Stansly (2003) suggest that environmental conditions (drying) may affect efficacy, and Imai et al. (1995) demonstrated that the toxicity of Silwet L-77 to the green peach aphid Myzus persicae (Sulzer) increased from 23.8% ± 7.2 at 30% RH to 99.3% ± 0.46 at 90% RH.

Gottwald et al. (1997) suggested that an adjuvant, such as Silwet L-77, may increase the incidence of bacterial diseases. The decrease in water tension could allow entry sites for plant pathogenic bacteria into leaf stomata (Melotto et al. 2008), which may allow bacteria to gain entry and cause infection (Gottwald et al. 1997; Zidack et al. 1992). However, no bacterial diseases were observed on any of the orchids tested throughout this study. This could be due to the absence of bacteria or lack of a conducive environment, so caution should be exercised if it is known that pathogens are present or there is a history of bacterial infections.

There are several advantages to using oil + Silwet L-77 to control orchid arthropod pests. No arthropod is known to have developed resistance to petroleum oil (M. A. Hoy, Dept. Entomology, University of Florida, personal communication). Therefore, Silwet L-77 could be used as an adjuvant with petroleum oils to increase their effectiveness with the development of resistance a minimal concern (Butler et al. 1993). Also, the use of a petroleum oil with the addition of Silwet L-77 is safer and may be less expensive than some pesticides (Liu and Stansly 2000) and may assist in the conservation of natural enemies, one of the goals of an integrated pest management program. Wood & Tedders (1997) noted that Silwet L-77 had little or no effect on adults or larvae of lady beetles and green lacewings, suggesting that Silwet L-77 could be an important tool for an integrated pest management program.

We conclude that Silwet L-77 increases the efficacy of oil against Boisduval scale and the flat mite on orchids. Because complete coverage is required, its use may be most relevant to hobbyists or small commercial growers who want to use a chemical control with reduced toxic effects on non-target organisms, the environment, and human health.