Tetranychus marianae McGregor (Acari: Tetranychidae) is a pest of several perennial crops and is widespread in the Pacific Basin, including the Mariana Islands where it was first reported. The mite is also present in the West Indies, Bahamas, southern USA, Nicaragua, Argentina, Brazil and Southeast Asia. Eggplant growers apply carbaryl 10 to 15 times against this pest during each cropping period. Some growers no longer cultivate eggplant and tomato because of uncontrollable mite infestations. Previous indoor studies have shown petroleum spray oil (PSO) to be effective against T. marianae. We therefore examined the comparative effectiveness of PSO (Sun-spray 6E®) at the rate of 5mL/liter, and carbaryl at the rate of 1.5 mL/liter, against T. marianae on eggplant at 2 locations in Guam. The mean percentages of mite infested leaves and the population densities of T. marianae at both the locations were significantly higher in control plots than in treatment plots. PSO treatments with 6 and 15 sprays more effectively reduced the number of T. marianae-infested leaves and populations of T. marianae than carbaryl treatments. Marketable yields of eggplant from PSO treated plots were significantly higher than from the other plots. We recommend 6 applications of PSO at 15, 25, 35, 45, 55 and 65 days of after transplanting for managing T. marianae on eggplant.
The red spider mite, Tetranychus marianae McGregor (Acari: Tetranychidae), is a severe pest on vegetables and ornamental plants in the Mariana Islands (Reddy et al. 2011) where it feeds and reproduces year round (Moraes et al. 1987; Reddy et al. 2013). The climate of the Mariana Islands is hot and humid year round, resulting in high mite populations and severe damage to eggplant (Solanum melongena L.; Solanales: Solanaceae) and tomato (Solanum lycopersicum L.; Solanales: Solanaceae) (Reddy & Tangtrakulwanich 2013). Even low numbers of mites cause visible leaf damage on vegetables and ornamentals (Reddy & Bautista 2012). Initial damage causes slight spotting on the leaves (Denmark 1970). As feeding continues, leaves become chlorotic and covered in webbing (Oatman et al. 1967). Extensive webbing may cover leaves, twigs, and fruit, with leaves eventually dropping from the plant (Noronha 2006). Leaf loss may significantly reduce yields of eggplant and tomato (Reddy et al. 2013; Reddy & Tangtrakulwanich 2013).
Some eggplant growers in the Mariana Islands apply between 10 and 15 chemical insecticide applications per cropping period, often resulting in the development of resistance in mite populations. Some eggplant growers apply the readily available carbaryl indiscriminately. Many farmers in the Western Pacific have ceased eggplant and tomato cultivation because of their inability to control mites. Extensive survey work on mites has been conducted on several crop and ornamental plants on Guam at the insistence of the Guam Department of Agriculture and with funding from multiple agencies. Subsequent surveys have confirmed the severe damage caused by T. marianae to vegetable crops (Reddy et al. 2011). The present study extends this work by developing an integrated approach to T. marianae control.
Reddy et al. (2013) reported that eggplant plots sprayed with petroleum spray oil (PSO) at a threshold of 2 or 4 mites/leaf in the dry season and at 2 –8 mites/leaf during the wet season experienced significantly less leaf damage. Similarly on tomato, based on T. marianae infested leaves, the population density of and degree of plant damage caused by T. marianae on tomato was significantly reduced when experimental plots were sprayed at a threshold of 8 –12 mites/leaf in the dry season and at 8 –14 mites/leaf during the wet season (Reddy & Tangtrakulwanich 2013).
Reddy & Bautista (2012) reported that combining the predatory mite Neoseiulus californicus (Evans) (Acari: Phytoseiidae) with petroleum spray oils (Volck® oil spray) produced significant control of T. marianae while not affecting the survival of N. californicus. Further, they reported that integrating petroleum spray oil with releases of N. californicus resulted in better control of T. marianae than did utilizing N. californicus alone. The optimum release rate of N. californicus against T. marianae populations was 200 N. californicus per plant. Subsequently, N. californicus was successfully released and established on Guam (Reddy & Bautista 2012). However, N. californicus can be seen only in some parts of Guam and several more years may be required for the populations to build-up sufficiently to adequatelycontrol T. marianae.
Because PSO effectively controlled T. marianae and was not detrimental to N. californicus, the present study compared the effectiveness of SO against T. marianae on eggplant on Guam compared to carbaryl sprays long used by Guam's farmers.
Materials and Methods
Seeds of the eggplant variety ‘Pingtung Long' (Known-You Seed, Co., Ltd., Kaohsiung, Taiwan) were sown in 40 × 30 cm trays and raised in a shade house (30–32 °C, 60–80% RH, 14 : 10 h L:D). The seedlings were grown in culture for 40 days. Experimental Locations
Duplicate experiments were conducted at the University of Guam Agricultural Experiment Stations at Yigo (N 13° 31.930′ E 144° 52.351′) in northern Guam and at the Inarajan Experiment Station (N 13° 61.963′ E 144° 45.353′) in southern Guam. Treatment plots measured 6 m × 6 m and were arranged in a randomized block design and separated from other plots by 1.0 m buffer zones to prevent spray drift. Forty day-old eggplant seedlings were transplanted with 75 cm between rows and 50 cm between plants within each row. Eight spray treatments were employed and replicated 3 times for a total of 30 individual plots. Each plot consisted of 10 rows of 15 eggplant seedlings, for a total of 150 plants per plot. Fertilizer was applied according to published recommendations (Schlub & Yudin 2002). The entire experimental design was repeated from May–Aug 2012 at Yigo and from Jun–Sep 2013 at Inarajan.
Seven chemical application treatments and a water spray and no spray control were applied to plots (Table 1). Concentrations of chemical applications were: Sun-spray 6E® (Sunoco, Inc. R&M, Philadelphia, Pennsylvania; active ingredients: refined petroleum distillate: 98.8 wt % + emulsifier: 1.2 wt %) at 5mL/liter and carbaryl (50% wettable powder, AllPro, St. Joseph, Missouri) at 1.5 mL/liter. Carbaryl spraying corresponded to the set time intervals normally practiced by Guam farmers.
The amount of solution sprayed per application was 80 L/ha for small plants (up to 45 DAT) and 185.0 L/ha for larger ones (45 DAT until harvest). All chemical applications were made with motorized backpack sprayers (Solo Brand; Forestry Suppliers, Jackson, Mississippi) equipped with an adjustable, flat spray, hollow cone, and a jet stream nozzle.
Number and timing of various petroleum oil and carbaryl treatments carried out on eggplant fields against the red spider mite at Yigo and Inarajan, Guam
Sampling Method for Tetranychus marianae Populations
To determine T. marianae population levels, 3 leaves from each of 10 plants were randomly selected in each plot beginning and ending when, especially with respect to days after transplanting. One leaf was selected each from the top, middle and bottom level of the plant (Reddy et al. 2013). The number of mites present on the underside of each leaf was counted using a magnifying lens. Counts were performed at weekly intervals. The number of leaves infested by T. marianae per plot was recorded out of 30 randomly selected leaves counted in each plot. At harvest, yield was recorded for each treatment plot for 12 weeks. Data were averaged and expressed as the number of mites per leaf, the percent of infested leaves and yield in tonnes per hectare.
Data for the numbers of mites per leaf and infested leaves on 10 plants per plot and overall yield levels in different treatments were analyzed using repeated measures ANOVA (P < 0.05) over multiple dates, and differences between treatments means were compared using the LSD test. All statistical analyses were carried out using SAS Version 9.3 (SAS Institute 2009). The 5% levels of significance were used for all analyses.
Damage and Yield Assessment in the Experimental Plots
Mean percentage of mite infested leaves and the population density of T. marianae at both locations were higher in control plots than in the treatment plots (F8, 34 = 11.21, P < 0.05) (Figs. 1 and 2). PSO treated plots with 6 and 15 sprays significantly reduced the number of T. marianaeinfested leaves (F9, 18 = 12.7, P < 0.05; Fig. 1) and the populations of T. marianae (F9, 28 = 26.2, P < 0.05; Fig. 2) over the carbaryl treated plots and both controls at both locations.
Control plots suffered the greatest damage from T. marianae. Results from all other treatments were intermediate. The marketable eggplant yield from the plots sprayed with PSO 6 or 15 times were significantly greater at both locations than those in other treatments (F9, 32 = 7.64, P < 0.05) (Fig. 3). The treatment combination of PSO + carbaryl was moderately effective but significantly better than the control.
Eggplant growers in the Mariana Islands usually apply carbaryl as many as 10 to 15 times during each cropping period. This is not only extremely expensive but also poses ecological and toxicological hazards. Carbaryl is also known to cause abrupt outbreaks in mite populations (Mathew et al. 1995). The current carbaryl-dependent mite management program used by growers probably causes the extreme mite pest problems on eggplant (Goyal 1982; Reddy 2001). There are no other pesticide options for Guam farmers, and carbaryl is readily available over the counter throughout the island.
Although carbaryl was more effective than control treatments, it was apparent that the number of infested leaves and populations of T. marianae had increased with the increase in number of carbaryl applications (Figs. 1 and 2). Because carbaryl is a broad-spectrum insecticide, it kills beneficial natural enemies of mites as well as the mites themselves (Wilkinson et al. 1975; Karban & Zalom 1998). Carbaryl overuse may lead to outbreaks of aphids and spider mites (Shaw & Wallis 2008). Such carbaryl induced outbreaks have been reported in Panonychus citri (MacGregor) (Ho 1984), Tetranychus telarious (L.) (Patel et al. 1982) and T. cinnabarinus (Boisduval) (Abrol & Singh 2003). Conversely, Croft & Hoying (1975) reported carbaryl resistance in a population of the predatory mite Amblyseius fallacis (Garman) in Michigan. Using a carbaryl resistant natural enemy such as A. fallacis may assist in developing successful biological or integrated control of spider mites in other systems. Given that this observation was reported 38 years ago, more studies are needed to confirm it.
In the present study, the PSO was not shown any phytotoxic effect to eggplants. Our results agree with Mizell (1991) reported that no phytotoxicity of Sunspray Ultra-fine Spray oil was observed on ornamental plants. Also, Baxendale and Johnson (1988) stated that Sunspray oil has limited phytotoxicity on certain species of deciduous nut trees. However, there were some reports indicating that PSOs cause phytotoxicity. Cloyd et al. (2009) reported the oils such as eugenol, sodium lauryl sulfate, peppermint, and citronella oil and Sharpshooter (sodium lauryl sulfate and clove oil) were phytotoxic to the poinsettia, Euphorbia ulcherrima Willd. Ex Klotzsch, plants. Of course, this study was the one of the first to quantitatively demonstrate that commercially vailable plant-derived essential oil products vary in their effectiveness against certain arthropod pests stated on the label and are phytotoxic.
Petroleum derived spray oils such as Sunspray ultra-fine oil, have potential as tools in IPM to replace conventional pesticides. Oils effectively control pests, particularly the European red mite, Panonychus ulmi (Koch), in commercial orchards while conserving beneficial arthropods (Jandial 2009). Knapp et al. (2001) reported that PSO is effective against citrus rust mite Phyllocoptruta oleivora (Ashmead) at higher doses in Florida. Oil emulsions for the control of P. ulmi on apple were more effective than other chemicals tested (Downing 1967). Lawson & Weires (1991) reported that of all materials they tested, sunspray 6E and Volck Supreme oil caused the most P. ulmi mortality.
PSO was effective against T. marianae on eggplant in the present study. PSO may therefore be important in developing IPM programs for mites that rely less on synthetic chemical acaricides.
This project was supported by FY 2010 Pacific Islands Area Conservation Innovation Grants (PIA-CIG) Program, Grant Agreement No. 69-9251-10-880, The Natural Resources Conservation Service (NRCS)-United States Department of Agriculture. The USDA is an equal opportunity provider and employer. We thank Mr. David M. H. Mantanona, Mr. Robert C. Mendi and Mr. Ray Gumataotao for their help in the field.