Efecto insecticida y repelente de extractos de Pluchea sericea (Nutt.) sobre adultos de Bemisia tabaci (Genn.)

Carlos Enrique Ail-Catzim¹*; Alejandro Manelik García-López¹; Rosalba Troncoso-Rojas²; Rosario Esmeralda González-Rodríguez¹; Yuliana Sánchez-Segura¹

¹ Instituto de Ciencias Agrícolas, Universidad Autónoma de Baja California. Carretera a Delta s/n. Ejido Nuevo León, Mexicali, Baja California, C. P. 21705, MÉXICO. Correo-e: carlos.ail@uabc.edu.mx, tel.: (686) 523 00 88/79 (*Autor para correspondencia).

² Centro de Investigación en Alimentación y Desarrollo, A.C. Carretera a La Victoria km 0.6. Hermosillo, Sonora, C.P. 83304, MÉXICO.

Abstract

The use of repeated insecticide applications in agricultural crops to control Bemisia tabaci (Genn.) has resulted in resistance problems and environmental pollution. Inputs from plant species are an alternative to reduce this problem. Plants are a source of bioactive chemicals that can have insecticidal, repellent, attractant, anti-feeding or growth regulator effects. The aim of this study was to determine the insecticidal and repellent activity of ethanolic, acetonic and aqueous extracts of both leaves and stems of Pluchea sericea (Nutt.) on Bemisia tabaci (Genn.). The LC₅₀ of each of the extracts on adult whiteflies was estimated using bioassays with the residual film technique. Also, the repellent effect from the LC₅₀ of each extract was evaluated, and the repellency index (RI) was estimated. If RI = 1 the effect is neutral, if RI <1 the effect is repellent and if RI > 1 the effect is attractant. The leaf-ethanol, leaf-water and leaf-acetone extracts showed the lowest LC₅₀ values of 700, 1,190 and 1,250 ppm respectively, indicating high efficiency on B. tabaci, followed by the stem-water extract and lastly the stem-ethanol and stem-acetone extracts showed the least effectiveness. All extracts had repellent action, regardless of the solvent and plant organ at 24 and 48 h. However, water-based extracts may have greater potential for whitefly control in integrated pest management systems because they presented insecticidal and repellent effects, and the solvent is harmless and inexpensive.

Keywords: Cachanilla, whitefly, lethal concentration, repellency index, botanical extracts.

Resumen

Para el control de Bemisia tabaci (Genn.) en los cultivos agrícolas se usan repetidas aplicaciones de insecticidas, lo que ha provocado problemas de resistencia y contaminación ambiental. Los insumos provenientes de especies vegetales son una alternativa para reducir esta problemática. Las plantas son una fuente de químicos bioactivos, que pueden tener efecto insecticida, repelente, atrayente, antialimenticio o regulador de crecimiento. El objetivo fue determinar la actividad insecticida y repelente de extractos etanólicos, acetónicos y acuosos, tanto de hojas como de tallos de Pluchea sericea (Nutt.) sobre Bemisia tabaci (Genn.). Se estimó la CL₅₀ de cada uno de los extractos, mediante bioensayos con la técnica de película residual, sobre adultos de mosquita blanca. Además, se evaluó el efecto repelente a partir de la CL₅₀ de cada extracto, se estimó el índice de repelencia (IR), si IR = 1 el efecto es neutro, IR < 1 el efecto es repelente e IR > 1, el efecto es atrayente. Los extractos de hoja-etanol, hoja-agua y hoja-acetona presentaron valores menores de CL₅₀ 700, 1,190 y 1,250 ppm respectivamente, lo que indica alta eficiencia sobre B. tabaci, seguidos del extracto tallo-agua, por último los extractos tallo-etanol y tallo-acetona presentaron menor efectividad. Todos los extractos tuvieron acción repelente, independientemente del disolvente y órgano de la planta a 24 y 48 h. Sin embargo, los extractos a base de agua podrían tener mayor potencial para el control de la mosquita blanca en sistemas de manejo integrado de plagas, debido a que presentaron efecto insecticida y repelente, además el disolvente es inocuo y de bajo costo.

Palabras clave: Cachanilla, mosquita blanca, concentración letal, índice de repelencia, extractos botánicos.

INTRODUCTION

The whitefly (Bemisia tabaci Genn.) is one of the most important pests worldwide in both field and greenhouse agricultural production (Oliveira, Henneberry, & Anderson, 2001). The direct damage caused by this insect is due to its feeding and to physiological disorders in the plant, while indirect damage is due to fungal growth on the honeydew produced by the pest and to its ability to transmit viruses (Perring, 2001).

In 1992 the whitefly invaded the Mexicali Valley in Baja California and the San Luis Río Colorado region ofSonora (González et al., 1992), affecting cotton (Gossypium hirsutum L.), melon (Cucumis melo L.), watermelon (Citrillus lanatus Thunb.) and sesame (Sesamum indicum L.) crops. In cotton, the honeydew contaminates the fiber, making it sticky, which hampers the spinning process and diminishes its quality, resulting in economic penalties to cotton producers when the limits of honeydew content in the fiber are surpassed (Hendrix, Steele, & Perkins, 1996).

Due to the importance of B. tabaci, cotton farmers are forced to make repeated insecticide applications for control purposes, depending solely on this method, which causes development of resistance by pests, accumulation and persistence of the compounds in the environment, and lethal effect on beneficial organisms (Dent, 2000). Therefore, control alternatives that reduce the use of chemicals are needed. The use of natural products for agricultural pest control is becoming increasingly accepted because of the need to employ effective compounds that do not harm the environment (Ateyyat, Al-Mazra'awi, Abu-Rjai, & Shatnawi, 2009; Isman, 2006). Therefore, it is necessary to evaluate products that come from plant species, since plants are a rich source of bioactive chemicals (Wink, Schmeller, & Latz-Bruning, 1998) that may have insecticidal, repellent, attractant, anti-feeding and growth regulator effects on insects (Champagne, Koul, Isman, Scudder, & Towers, 1992). In addition to being selective to pest insects, they commonly have little adverse effect on non-target organisms (parasitoids and predators) and the environment (Arnason, Philogene, & Morand, 1989).

The plant Pluchea sericea (Nutt.), commonly known as "Cachanilla," is a wild evergreen shrub that grows in sandy or saline soils in the deserts of Mexico, such as the Baja California, Sonora and Chihuahua deserts (Villaseñor & Villareal, 2006). Previous studies on aerial parts of this plant revealed the presence of flavonoids, triterpenes and sesquiterpenes (Romo, Reyes, Delgado, & Schlemper, 1982), which are important secondary metabolites in pest control (Sepúlveda, Porta, & Rocha, 2003). However, few studies in Mexico confirm the effect of these plant compounds on B. tabaci; therefore, it is important to study whether P. sericea extracts can be a useful tool for B. tabaci management in integrated pest management systems. Due to the above, the aim of this study was to evaluate the insecticidal and repellent effect of ethanolic, acetone and aqueous extracts of both leaf and stem of P. sericea on B. tabaci adults.

MATERIALS AND METHODS

Experiment location

This study was conducted from October 2013 to May 2014 in the Institute of Agricultural Sciences' Entomology Laboratory at the Universidad Autónoma de Baja California, located in the Nuevo León ejido in the Mexicali Valley, situated at 32° 24' 34'' N latitude and 115° 11' 31'' W longitude. The climate of this region is characterized as arid and dry (García, 1985).

Biological material

To perform bioassays, adult whiteflies were collected in the institute's experimental fields, with which a colony of B. tabaci was established on bean (Phaseolus vulgaris L.) plants in controlled temperature (25 ± 2 °C) and photoperiod (12:12 h light dark) conditions. Sufficient biological material was obtained in this way to carry out assessments by means of bioassays. Whitefly specimens were identified using the guide published by Hodges and Evans (2005).

Preparation of extracts

For the preparation of extracts, the plant P. sericea was collected in the Nuevo León ejido in the Mexicali Valley. The collected material was dried, the leaves were separated from the stems, and the cachanilla leaf and stem samples were crushed with a power mill to obtain particles of 1-3 mm. In 3-L plastic vials, 100 g of ground sample were placed and 1 liter of solvent was added; each extract was placed on an orbital shaker (Orbit 1900, Labnet) for 24 h at 0.306 x g. The solvents ethanol (99.9 %), acetone (99.7 %) and distilled water were used to obtain six extracts, three each of stem and leaf. The initial concentration of each extract was 10,000 ppm.

Concentration-mortality experiment

We used six concentrations of each extract of P. sericea (250 to 10,000 ppm) to estimate the median lethal concentration (LC₅₀) on B. tabaci, using the residual film technique, in glass vials (Xu, Shelton, & Chen 2001). The technique consisted of treating 200-mL vials with 1 mL of solution of the leaf and stem extracts at their different concentrations. After adding the solution, each vial was rotated in every direction until the solvent evaporated; subsequently, 20 adult whiteflies were placed into each vial using a brush. Three replicates for each concentration, plus the control (only distilled water, ethanol or acetone depending on the treament), were included. A replicate consisted of a vial containing 20 adult whiteflies (experimental unit). Treatment evaluation was conducted 24 h after the introduction of specimens.

To determine the repellency effect, the LC₅₀ repellency of each of the P. sericea extracts, obtained from the above experiment on B. tabaci adults, was selected. The bioassay procedure consisted of using a hand sprayer to spray eight bean (P. vulgaris) plants with the lethal concentration of each of the extracts. In addition, eight plants were included as the control, in which only water was applied, for each extract. After 2 h, when the excess water and solvent had evaporated, the treated and control plants were placed in cages of 1 x 1 x 1 m covered with fine mesh; subsequently 100 adult whiteflies were introduced (Baldin et al., 2014). Three replicates for each concentration were included, with a replicate consisting of a cage containing eight treated and eight untreated plants. At 24 h after treatment, the number of whiteflies perched on each plant was counted. At the end of this evaluation, another 100 adult whiteflies were introduced into each of the cages and were counted again at 24 h.

Results analysis

The results of the concentration-mortality experiment were analyzed by Probit Regression (Finney, 1971), using the PROBIT procedure (SAS, 2001), to obtain the LC₅₀ values and their fiducial limits (95 %). It was considered that the LC₅₀ of the extracts are not statistically different when the fiducial limits (95 %) overlap (Robertson, Russell, Preisler, & Savin, 2007).

With the results of the repellency experiment, the RI was calculated using the formula RI = 2G / (G+P) (Lin, Kogan, & Fischer, 1990), where G = number of insects perched in the treatment, and P = number of insects perched in the control. To classify the effect of the extracts, the criterion taken was that if RI = 1 the effect is neutral, if RI < 1 the effect is repellent and if RI > 1 the effect is attractant.

RESULTS AND DISCUSSION

Table 1 shows the values obtained from the LC₅₀ in cachanilla leaf extracts, which were 770 ppm for ethanol, 1,250 ppm for acetone and 1,190 ppm for distilled water, whereas the LC₅₀ of the stem extracts were 2,617 ppm for ethanol, 3,912 ppm for acetone and 2,622 ppm for distilled water.

It can also be seen that the effectiveness of the three leaf-based cachanilla extracts was statistically similar (Table 1), regardless of the type of solvent, as there is overlap between their fiducial limits (Robertson et al., 2007). A similar result was obtained in stem-based extracts. However, the leaf-ethanol and leaf-acetone extracts can be considered the best, followed by the aqueous extracts based on both plant organs. Finally, the stem-ethanol and stem-acetone extracts showed the least effectiveness, according to the criterion of Robertson et al. (2007).

Table 2 shows the average number of insects perched on plants treated with cachanilla extracts, after 24 and 48 h of exposure. It can be seen that with all the extracts there was a lower number of insects compared to the control, regardless of the exposure period.

Table 3 shows the RI at 24 h for each of the cachanilla extracts evaluated on adult whiteflies. It can be seen that the RI values in all extracts were less than unity; therefore, based on the established criterion, they presented repellent effect, regardless of the type of organ that gave rise to the extract and the type of solvent. The same effect occurred after 48 h of exposure (Table 3). All P. sericea extracts showed repellent effect on adult whiteflies, results similar to those found by Al-mazra'awi and Ateyyat (2009) who report repellency for aqueous extracts of Urtica pilulifera and Thymus capitatus on B. tabaci. Similarly, Pimpinella anisum, Galium longifolium, Retama reatam and Ballota ondulata presented repellent activity against adult whiteflies in studies by Ateyyat et al. (2009).

The repellent activity of botanical insecticides can be attributed to a complex mixture of bioactive compounds (Akhtar & Isman, 2012). Previous studies on aerial parts of cachanilla revealed the presence of flavonoids, triterpenes and sesquiterpenes (Romo et al., 1982), compounds that may be related to the activity of the different cachanilla extracts on B. tabaci.

Whitefly control depends on the use of synthetic insecticides, but due to the levels of resistance that have been developed by this pest, new control methods, including bio-insecticides, biological control and insecticides of botanical origin, have been sought. Insecticides of botanical origin are more accepted than chemical insecticides, because they are less harmful to humans and other non-target organisms. There are reports of various plant extracts that have the potential for controlling whitefly (Al-mazra'awi & Ateyyat, 2009; Bezerra, Alves, Vendramim, & Dos Santos, 2012; Cruz et al., 2013), but there are no reports of the insecticidal and repellent effect of plant extracts based on P. sericea; the results of this study represent the first evidence of the potential of P. sericea to control B. tabaci. However, there have been several studies with other species of Pluchea on various organisms, where different biological, anticancer, immunosuppressive, antimicrobial, cytotoxic, anti-inflammatory, and antioxidant properties, among others, are reported (Hussain et al., 2013). In another study, Grace (2002) reports larvicidal effect of essential oil of Pluchea dioscoridis on Culex pipiens. Likewise, Fahmy, Al-Sawaf, Turki, & Ali (2012) report allelopathic effect of aqueous extracts of P. dioscoridis on Corchorus olitorius, Lepidium sativum and Cynodon dactylon.

Considering the toxic and repellent effects of P. sericea against B. tabaci, it is possible that the extracts of this plant can be used in integrated pest management control systems. However, water-based extracts may have greater potential because they are harmless to plants and non-target organisms. In addition, the cachanilla is widely distributed in the Mexicali Valley and extraction methods are simple, so that the extracts would be easy to prepare and with relative ease could be used in farming systems.

CONCLUSIONS

Based on the results obtained in this study, water-based extracts of P. sericea have great potential for managing B. tabaci. However, further studies are needed to confirm their potential, determine their effect on the pest's nymphs and identify the bioactive compounds in P. sericea.

The authors thank the PROFESSIONAL TEACHING DEVELOPMENT PROGRAM for the funds granted for this research.

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