The toxicity of 2 reduced-risk pesticides, 2% horticultural oil with 0.05% Silwet L-77 and Grandevo (Chromobacterium subtsugae Strain PRAA4-1T), to 2 predators: Metaseiulus occidentalis (Nesbitt) (Acari: Phytoseiidae) and Hemicheyletia wellsina DeLeon (Acari: Cheyletidae), and 3 orchid pests: the two-spotted spider mite Tetranychus urticae Koch (Prostigmata: Tetranychidae), the phalaenopsis mite, Tenuipalpus pacificus Baker (Acari: Tenuipalpidae), and the longtailed mealybug Pseudococcus longispinus (Targioni-Tozzetti) (Hemiptera: Pseudococcidae) were tested. Grandevo did not kill either predator, and it failed to suppress populations of T. urticae and P. longispinus, but it reduced T. pacificus by 84%. The Silwet and horticultural oil mixture caused mortality of at least 82% in all 5 species tested.
Orchid hobbyists are known to use toxic pesticides in an unsafe manner when trying to control pests (Cating et al. 2011). Three products, dimethoate, diazinon and chlorpyrifos, have previously been used but have been removed recently from sale for non-commercial use in the United States because of serious health and environmental problems (Johnson 2008). Orchids are susceptible to many types of arthropod pests, including scales, mealybugs, thrips, mites, aphids, and whiteflies (Bottom 2012). Of these, the most difficult to control are mites, which require a high rate of pesticide use on ornamental plants (Naher et al. 2005). Two types of mites, spider mites (Tetranychidae) and flat mites (Tenuipalpidae), are common orchid pests that can become problematic in greenhouses (Johnson 2008). The American Orchid Society recommends products for managing dense infestations of mites, most of which are toxic to both people and natural enemies (Johnson 2008). The goal of this study was to identify lower-risk products that are effective on key pests of orchids, while being less toxic to 2 natural enemies.
Grandevo is an organic insecticide meant to control a diverse range of pests while being soft on beneficial arthropods (Marrone Bio Innovations 2012). It is naturally derived from the bacterium Chromobacterium subtsugae Strain PRAA4-1T (Martin et al. 2007). Grandevo uses complex modes of action for control of pest insects and mites, including repellency, oral toxicity, reduced egg hatch, and reduced fecundity (Marrone Bio Innovations 2012). Martin et al. (2007) tested this product on the Colorado potato beetle (Leptinotarsa decemlineata (Say); Coleoptera: Chrysomelidae), Western and Southern corn rootworms (Diabrotica virgifera LeConte and D. undecimpunctata howardi Barber; Coleoptera: Chrysomelidae, respectively), small hive beetle (Aethina tumida (Murray); Coleoptera: Nitidulidae), diamondback moth (Plutella xylostella (L,); Lepidoptera: Plutellidae), gypsy moth (Lymantria dispar (L.); Lepidoptera: Erebidae), tobacco hornworm (Manduca sexta (L.); Lepidoptera: Sphingidae), sweet potato whitefly (Bemisia tabaci (Gennadius); Hemiptera: Aleyrodidae), southern green stink bug (Nezara viridula (Linnaeus); Hemiptera: Pentatomidae), and a mosquito (Culex pipiens L.; Diptera: Culicidae). It has been tested as well on the black pecan aphid (Melanocallis caryaefoliae (Davis); Hemiptera: Aphididae) and pecan weevil (Curculio caryae (Horn); Coleoptera: Curculionidae) (Shapiro-Ilan et al. 2013).
Silwet L-77 is an organosilicone surfactant that can be used as an adjuvant to enhance coverage of pesticides (Foy 1989). Silwet L-77 and horticultural oil have been shown to reduce populations of pest mites (Tenuipalpus pacificus Baker; Acari: Tenuipalpidae) and scales (Diaspis boisduvalii Signoret; Hemiptera: Diaspididae) on orchids in the greenhouse, although complete coverage of the plants is necessary and there is no residual toxicity (Cating et al. 2010). Silwet L-77 also was tested on the Asian citrus psyllid (Diaphorina citri Kuwayama; Hemiptera: Liviidae) and its parasitoid (Tamarixia radiata (Waterston); Hymenoptera: Eulophidae) (Cocco & Hoy 2008; Srinivasan et al. 2008), citrus leafminer (Phyllocnistis citrella Stainton; Lepidoptera: Gracillariidae) (Shapiro et al. 1998). It was tested by Tipping et al. (2003) on pests of table grapes (Vitis spp.; Vitales: Vitaceae) including the grape mealybug (Pseudococcus maritimus (Ehrhorn); Hemiptera: Pseudococcidae), omnivorous leafroller (Platynota stultana Walsingham; Lepidoptera: Tortricidae), Pacific spider mite (Tetranychus pacificus McGregor; Acari: Tetranychidae), cotton aphid (Aphis gossypii Glover; Hemiptera: Aphididae), and western flower thrips (Frankliniella occidentalis (Pergande); Thysanoptera: Thripidae).
Five species were tested with Silwet L-77 plus oil and Grandevo in the laboratory, including 2 predatory mites, Metaseiulus (=Typhlodromus or Galendromus) occidentalis (Nesbitt) (Acari: Phytoseiidae) and Hemicheyletia wellsina De Leon (Acari: Cheyletidae), 2 phytophagous mites: the two-spotted spider mite [Tetranychus urticae (Koch) (Acari: Tetranychidae)] and Tenuipalpus pacificus Baker (Acari: Tenuipalpidae), and the long-tailed mealybug, Pseudococcus longispinus (Targioni-Tozzetti) (Hemiptera: Pseudococcidae).
Materials and Methods
Grandevo (Marrone Bio Innovations, Davis, California) was applied at the suggested high label rate of 3.6 g/1000 mL (3 lbs/100 gallons). Silwet L-77 at 0.05% (Helena Chemical Company, Collierville, Tennessee) was applied in combination with 2% of Prescription Treatment® Ultra-PureTM horticultural oil (BASF Chemical Company, Triangle Park, North Carolina) diluted with tap water. Only 40 ml of the pesticides were made daily, and all pesticide concentrations were made fresh daily.
All colonies in this experiment were established at the University of Florida Department of Entomology and Nematology, except the M. occidentalis colony, which was developed in 1984 and is resistant to carbaryl, sulfur and organophosphates (Hoy 1984). It is kept on paraffin-coated paper squares resting on water-soaked cotton and fed all life stages of T. urticae. The H. wellsina colonies were established in November 2012 after being discovered in a greenhouse on orchids in August 2012, and were fed all life stages of T. urticae. Hemicheyletia wellsina were maintained on Phalaenopsis orchid leaf squares on water-soaked cotton. Metaseiulus occidentalis, T. urticae, and H. wellsina were maintained in the laboratory at 25 °C, 45–55% RH and a 16:8 L:D photoperiod. The T. urticae colony has been reared on pinto beans, Phaseoulus vulgaris L. (Fabales: Fabaceae), in a greenhouse since 1992. The Phalaenopsis mite and long-tailed mealybug colonies were established in March 2012 from infested Phalaenopsis plants in a greenhouse, and were held in a growth chamber at 27 °C, 65–70% RH and a 16:8 L:D photoperiod on Phalaenopsis leaves.
General Experimental Conditions
Assays were conducted in the laboratory between May and Aug 2013. Phalaenopsis orchids (Kerry’s Orchids, Apopka, Florida) were used as substrates for H. wellsina, T. pacificus and P. longispinus, and pinto beans (P. vulgaris) were used for M. occidentalis and T. urticae. Plants were grown in a greenhouse at the University of Florida, Gainesville. All species were placed on the abaxial side of the orchid or pinto bean leaf discs, which were kept on wet cotton to discourage run off. Sprays were applied using aerosol sprayers (Preval, Chicago Aerosol, Coal City, Illinois) under a fume hood. Leaf discs were sprayed to drip then allowed to dry for 1 h. Twenty adult T. urticae were added daily after treatment to leaf discs containing H. wellsina and M. occidentalis so that a lack of food would not affect survival. Each product was tested on at least 2 dates for each species. Each species was tested with the high label rate of Grandevo, 0.05% Silwet L-77 in combination with 2% horticultural oil, and a tap water control. Metaseiulus occidentalis and T. urticae were also tested with 0.05% Silwet and 1% horticultural oil.
Metaseiulus occidentalis Adult Females
Five 20-mm diam pinto bean leaf discs that had been naturally infested with abundant T. urticae prey were made, and 5 newly mated M. occidentalis females were added to each leaf disc. The leaf discs were sprayed with each pesticide and allowed to dry. Leaf discs were then held in the laboratory at 25 ± 1 °C with 45–55% RH. Leaf discs were scored after 24, 48, and 72 h to record mortality of M. occidentalis. This was repeated for a total of n = 50 mites for each treatment.
Another experiment was conducted with a total n = 25 mites using 1% horticultural oil with 0.05% Silwet L-77 after preliminary results indicated that not all M. occidentalis were killed with 2% oil and 0.05% Silwet L-77.
Hemicheyletia wellsina Adult Females
Five 20-mm diam Phalaenopsis orchid leaf discs with all stages of T. urticae prey were made and 5 newly emerged and recently mated H. wellsina females were added to each leaf disc. The leaf discs were sprayed with each treatment and allowed to dry. Once removed from the fume hood they were held at 25 ± 1 °C, 45–55% RH and a 16:8 L:D photoperiod. Leaf discs were scored after 24, 48, and 72 h to record mortality of H. wellsina. This was repeated for a total of n = 50 for each treatment.
Tenuipalpus pacificus Adult Females
Leaf discs were made from Phalaenopsis leaves for T. pacificus because it is one of the few host plants for this mite. Preliminary studies showed that T. pacificus preferred to settle on the edge of the leaf disc, often resulting in accidental death from run off. To reduce the likelihood that this would affect the results, melted paraffin wax was painted around the edge of the disc to form a barrier and this kept 94% of the Phalaenopsis mites on the leaf. Five 20-mm leaf discs were placed on wet cotton and 5 adult female T. pacificus were added to each. The leaf discs were sprayed with each pesticide and allowed to dry. Because this species requires a higher temperature and RH, the leaf discs were held in a growth chamber at 27°C, 65–70% RH and a 16:8 L:D photoperiod. Total mortality was recorded after 24, 48, and 72 h. A total of n = 50 for each treatment was evaluated.
Pseudococcus longispinus Adults
Because P. longispinus is larger than the mites tested, Phalaenopsis leaf squares measuring approximately 60 × 60 mm were made for each treatment. To each leaf square, 15 P. longispinus adults (mixture of males and females) were added, and sprayed with either a water control, Grandevo, or 2% horticultural oil with 0.05% Silwet. The long-tailed mealybugs were held in a growth chamber at 27 °C and 65–70% RH. Mortality was recorded after 24, 48, and 72 h. This was repeated for a total of n = 60 mealybugs for each treatment.
Tetranychus urticae Adults and Larvae
Experiment One. Five pinto bean leaf discs were used for each of the 3 treatments, and 5 newly emerged and mated T. urticae females were placed on each disc and sprayed. After drying, the leaf discs were kept at 25 ± 1 °C, 45–55% RH and a 16:8 L:D photoperiod, and scored after 24, 48, and 72 h. This was repeated for a total of n = 50 mites for each treatment. An additional spray was completed using 0.05% Silwet with 1% horticultural oil, for a total of n = 25 mites.
Experiment Two. An additional experiment was conducted using a total of n = 25 T. urticae larvae in which the high label rate of Grandevo was applied using distilled water to dilute the product to rule out the possibility of water quality being a factor in the effectiveness of Grandevo.
Experiment Three. Five recently hatched T. urticae larvae were added to leaf discs and sprayed with the high label rate of Grandevo. Mortality was assessed after 24, 48, and 72 h, and surviving mites were allowed to develop to adulthood on the residues. Once these mites became adults, new leaf discs were made and 5 recently emerged and mated T. urticae females that had been treated with Grandevo as larvae were added to each. An additional treatment was established to assess recently emerged and mated spider mites that had not been previously exposed to Grandevo, as well as a tap water control using unexposed mites. The spider mite adults that had been previously sprayed with Grandevo as larvae, as well as 1 treatment that had not been exposed previously, were sprayed with the high label rate of Grandevo. After 24, 48, and 72 h mortality and the total cumulative number of eggs laid was recorded at each time interval.
The toxicities of Grandevo and 0.05% Silwet with 2% horticultural oil to M. occidentalis adult females were tested by direct application. Grandevo had no negative effect on M. occidentalis, resulting in a 100,100, and 96% survival rates of the adult females after 24, 48, and 72 h, respectively, (P = 0.885) (Fig. 1) compared with 100% survival of the water control.
The 0.05% Silwet and 2% horticultural oil resulted in 18% survival compared with the water control (P < 0.0001) (Fig. 2). Survival from 0.05% Silwet and 2% horticultural oil was consistent after 24, 48, and 72 h. After reducing the horticultural oil to a 1% concentration, 64% of M. occidentalis adult females survived after 24, 48, and 72 h, which was significantly different from the water control (P = 0.0002).
Grandevo did not affect the survival of H. wellsina adult females, resulting in a 92, 90, 90% survival rates (P = 0.0901) after 24, 48, and 72 h (Fig. 1) compared with the water control. The direct spray of 0.05% Silwet and 2% horticultural oil resulted in a 10% survival rate that was consistent after 24, 48, and 72 h (Fig. 2), and was significantly different from the water control (P < 0.0001).
There was a significant decrease in survival of T. pacificus adult females after 72 h compared with the water control when exposed to Grandevo (P < 0.0001). Only 30, 24, and 16% of 50 T. pacificus survived after 24, 48, and 72 h, respectively (Fig. 1). The 0.05% Silwet and 2% horticultural oil caused 0% survival of T. pacificus after 72 h (P < 0.0001) (Fig. 2).
Pseudococcus longispinus exhibited a 92, 87, and 85% survival rate after 24, 48, and 72 h when exposed to Grandevo (Fig. 1), which was not significantly different from the 98% survival rate of the water control (P = 0.204). However, when P. longispinus was sprayed with 0.05% Silwet and 2% horticultural oil, survival fell to 18% after 24 h and remained at that after 72 h (Fig. 2), which was significantly different from the water control (P < 0.0001). Silwet and oil appeared to cause a loss of the white, waxy coating of the mealybugs.
Experiment 1. Tetranychus urticae exhibited no significant difference in mortality compared to the water control, with a 94, 94, and 90% survival rate of Grandevo diluted with tap water after 24, 48, and 72 h (P = 0.163). When sprayed with 0.05% Silwet and 2% horticultural oil, 0% survival occurred by 24 h (P < 0.0001) (Fig. 2). The 0.05% Silwet plus 1% horticultural oil resulted in a 28% survival rate of T. urticae, consistent after 24, 48, and 72 h (P < 0.0001).
Experiment 2. When treated with Grandevo diluted with distilled water, 96, 92, and 88% survived (P = 0.178) after 24, 48, and 72 h, respectively (Fig. 1). There was no significant differ ence between the results of Grandevo tested with tap or distilled water (P = 0.995).
Experiment 3. The fecundity of T. urticae after being exposed once as adults to Grandevo was reduced approximately 26.5% compared to the water control, but was not significantly different after 72 h (P = 0.1965) (Fig. 3). There was also no significant difference between the T. urticae that were exposed once as adults and those exposed as both adults and larvae after 72 h (P = 0.1451). Those sprayed as larvae with Grandevo and again as adults had a 35% reduction in egg production after 72 h compared to the water control, and according to the Tukey-Kramer test (α < 0.05), the number of eggs produced in the multiple-exposure treatment was significantly different from the number of eggs in the water control for each 24-h period (P = 0.0133).
One goal of integrated pest management (IPM) is to preserve natural enemies while still reducing pest populations. The results obtained in this study indicate that, although Grandevo is not harmful to either of the natural enemies tested, Grandevo did not suppress 2 of the 3 plant pests tested (Fig. 1). Only the Phalaenopsis mite exhibited a significant mortality after exposure to Grandevo, and mortality increased over the 72-h period. According to Marrone Bio Innovations (2012), other studies have found that Grandevo does, in fact, cause statistically significant reductions of T. urticae populations on strawberry, but the results of this study do not show a significant difference using Grandevo diluted with either tap or distilled water. The reasons for the difference in our results is unknown, but could be due to the population of T. urticae tested or the effects of the different leaf types used. Also, it is unclear how Grandevo was applied to T. urticae on strawberries. In a study by Martin et al. (2007), Grandevo was found to be lethal to the Southern green stink bug, sweet potato whitefly, diamond-back moth larvae, small hive beetle larvae, Western and Southern corn rootworm, and Colorado potato beetle larvae, but not to the mosquito Culex pipens, tobacco hornworm larvae, gypsy moth larvae, or Colorado potato beetle adults. Grandevo has also been shown to cause significant mortality to the black pecan aphid and pecan weevil when compared to a control (Shapiro-Ilan et al. 2013).
Exposure to 0.05% Silwet and 2% horticultural oil caused a significant reduction in survival of all pest and natural enemy species tested (P < 0.0001) (Fig. 2). All mortality occurred within the first 24 h. Previous studies using Silwet L-77 have also shown a high mortality of pests. Mortality was greater than 93% for the grape mealybug, omnivorous leafroller, Pacific spider mite, cotton aphid, and western flower thrips, though the Silwet was applied at higher concentrations (0.1, 0.25, 0.5, and 1.0%) (Tipping et al. 2003). Silwet L-77 with horticultural oil has also been shown to cause mortality of over 85% of Boisduval scales and over 98% of phalaenopsis mites (Cating et al. 2010).
Because these results were obtained under laboratory conditions, field experiments would be the next step to test these products. Results of toxicity studies under laboratory conditions may not accurately depict what might happen when products are used in the field or in a greenhouse. Coverage of the plant with the pesticide could be less thorough in the field, and natural conditions that could protect the predator or pest (e.g. webbing that some species produce) could reduce toxicity, and abiotic factors could reduce pesticide quality (e.g. rain or UV light).
In order for a pesticide to be beneficial to a pest-management program, it would need to cause a reduction in the targeted plant pest, while causing less mortality to any natural enemies present. The surviving predators could feed on any remaining pests, keeping the pest population density low. Because Grandevo caused significant mortality to the Phalaenopsis mite without affecting predator mortality, it could potentially be a candidate for management of the Phalaenopsis mite. If sprayed repeatedly at a sufficient interval, it is possible that Grandevo could also reduce fecundity of pests like T. urticae while not affecting the natural enemies tested here.
The 0.05% Silwet with 2% horticultural oil treatment resulted in 100% mortality of T. urticae and T. pacificus, and 82% mortality of P. longispinus, while leaving 18% of M. occidentalis females and 10% of H. wellsina females surviving (Fig. 2). The mortality of T. urticae and T. pacificus was significantly greater than the mortality of the predator M. occidentalis when exposed to 0.05% Silwet with 2% horticultural oil. When the concentration of horticultural oil was reduced to 1% with 0.05% Silwet, the survival of M. occidentalis increased to 64%, while survival of T. urticae only increased to 28% (Fig. 2). Silwet L-77 and horticultural oil thus could be an effective pest management tool, but the need for complete coverage and its short residual activity may restrict its use to plant hobbyists looking for a less-toxic way to control pests. Depending on the pest problem and natural enemies present, both of these reducedrisk pesticides could be beneficial for orchid hobbyists looking to manage pest populations in greenhouses or landscapes.
We thank Marrone Bio Innovations, Helena Chemical Company, and BASF Chemical Company for samples of the pesticides tested and Kerry's Orchids for donation of plants. We also thank Dr. Cal Welbourn of the Division of Plant Industry, Gainesville, Florida and Dr. Andre Bochkov of the Department of Parasitology, Zoological Institute of the Russian Academy of Sciences for species identification. This project was supported in part by the Davies, Fischer and Eckes Endowment in biological control to Marjorie A. Hoy.