¹ Agronomical Engineer, Universidade Federal do Espírito Santo (UFES), Brazil. Doctor Student in Plant Technology, Universidade Federal de Viçosa (UFV), Brazil. UFV, Brazil. Address: Departamento de Fitotecnia/Pós-graduação, Universidade Federal de Viçosa, 36.570-000, Viçosa, Minas Gerais, Brasil. e-mail: gdalmeida.ufv@hotmail.com.

² Agronomical Engineer, UFES, Brazil. M.Sc., Universidade Federal de Lavras (UFLA), Brazil. D.Sc., Universidade de São Paulo (USP), Brazil. Professor, UFES, Brazil. e-mail: dirceu@cca.ufes.br.

³ Forest Engineer, UFV, Brazil. M.Sc., USP, Brazil. Ph.D., University of British Columbia, Canada. Professor, UFV, Brazil. e-mail: zanuncio@ufv.br.

⁴ Agronomical Engineer and M.Sc. student, UFES, Brazil. e-mail: victorbvicentini@hotmail.com.

⁵ Agronomical Engineer, UFES, Brazil. M.Sc. and D.Sc. UFV, Brazil. Professor, Escola Agrotécnica Federal de Santa Teresa, Brazil. e-mail: aholtz@insecta.ufv.br.

⁶ Biologist, Universidade Federal de São Carlos, Brazil. M.Sc., Universidade Estadual Paulista, Brazil. D.Sc., USP, Brazil. Professor, UFV, Brazil. e-mail: jeserrao@ufv.br

SUMMARY

Thrips palmi Karny (Thysanoptera: Thripidae) is a phytophagous insect associated with the reduction of eggplant productivity. The aim of this study was to evaluate the effect of calcium silicate and/or an organic mineral fertilizer, together or separately, in increasing the resistance of eggplants to T. palmi. The treatments were calcium silicate, organic mineral fertilizer, calcium silicate associated with this fertilizer and the control. Mortality and number of lesions caused by nymphs of this insect on eggplant leaves were evaluated after 3, 6, 9 and 12 leaf applications of these products. The calcium silicate and the organic mineral fertilizer reduced both the population of T. palmi and the damage caused by its nymphs, suggesting a possible increase in eggplant resistance to this pest as a result of the treatments.

RESUMEN

Thrips palmi Karny (Thysanoptera: Thripidae) es un fitófago asociado a la reducción de productividad en el cultivo de berenjena. El objetivo del estudio fue evaluar el efecto del silicato de calcio y un fertilizante organomineral, individualmente y asociados, en el aumento de la resistencia de plantas de berenjena al T. palmi. Los tratamientos fueron silicato de calcio, fertilizante organomineral, silicato de calcio con fertilizante organomineral y testigo. La mortalidad y el número de lesiones de ninfas en las hojas de la planta de berenjena fueron evaluadas después de 3, 6, 9 y 12 aplicaciones foliares de los productos. El silicato de calcio y el fertilizante organomineral disminuyeron tanto la población de T. palmi como los daños producidos por las ninfas, mostrando un posible aumento de la resistencia de las plantas de berenjena a esa plaga.

RESUMO

Thrips palmi Karny (Thysanoptera: Thripidae) é um fitófago associado à redução da produtividade da berinjela. Dessa forma, objetivou-se avaliar o efeito do silicato de cálcio e de um fertilizante organomineral, isolados ou associados, no aumento da resistência de plantas de berinjela ao tripes T. palmi. Os tratamentos foram silicato de cálcio, fertilizante organomineral, silicato de cálcio associado com fertilizante organomineral e o controle. A mortalidade e o número de lesões de ninfas desse tripes nas folhas das plantas de berinjela foram avaliadas após três, seis, nove e doze aplicações foliares desses produtos. O silicato de cálcio e o fertilizante organomineral reduziram a população de T. palmi e os danos das ninfas, mostrando possível aumento da resistência das plantas de berinjela a essa praga.

In Brazil, the eggplant (Solanum melongena L) cultivation area is relatively small, but the consumption of this vegetable is increasing, mainly as a result of its nutritive value and medicinal properties, as it helps reduce blood cholesterol levels (Silva et al., 1999; Antonini et al., 2002). This Solanaceae grows in tropical and subtropical zones and can be cultivated in areas with rather high temperatures, 18-25ºC (Kikuchi et al., 2008).

Thrips palmi can be controlled with chemical products (Leite et al., 2005), but this method may be inefficient, because the populations of this insect can be highly resistant to insecticides (Cannon et al., 2007a). Besides, the intensive use of chemical insecticides can cause pest resurgence, and most of such products have high levels of non-target biological action, persistence in the environment and impact on human health, besides reducing the abundance of natural enemies and increasing production costs (Leite et al., 2003; Leite et al., 2005; Atakan, 2006). The maintenance of natural enemies in an integrated management program (IPM) is a crucial strategy for the sustainability of the agroecosystem, because of the environmental, economical, social and ecological problems that arise with the use of insecticides (Lemos et al., 2005).

The increase in plant resistance after the application of silicon is an established strategy in pest management programs (Gomes et al., 2005). This material can be accumulated and polymerized in cell walls, thus providing a mechanical barrier against phytophagous pests (Aguirre et al., 2007). This also increases the synthesis of phenolic compounds, lignins and phytoalexins in the damaged areas (Ghanmi et al., 2004) and activates endogenous plant chemical defenses (Rains et al., 2006), although they are not essential (Fauteux et al., 2006). The use of silicon can be made compatible with other strategies of pest integrated management. For example, the application of sodium silicate reduced feeding preference of Schizaphis graminum (Rondani) (Hemiptera: Aphididae) on wheat plants (Triticum aestivum) without affecting the populations of natural enemies (Morães et al., 2004). The application of silicate materials could increase the resistance of eggplants to T. palmi, as they did reduce the feeding of Eldana saccharina Walker (Lepidoptera: Pyralidae) when deposited on stems of sugar cane plants (Kvedaras and Keeping, 2007). Calcium silicate is the largest source of silicate products in the world (Snyder et al., 2005). The organic mineral fertilizer (silicate sources, essential nutrients, organic acids and proteins), in association with a silicate source, reduced the severity of Asperisporium caricae (Speg) Maubl. on papaya trees, possibly due to silicon deposition on the epicuticular layer, which impedes the penetration of the pathogen in the leaves (Pratissoli et al., 2007) and, when administered alone it reduced the oviposition of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) on bean plants (Phaseolus vulgaris L), in choice and no-choice tests (Almeida et al., 2008).

The aim of this study was to evaluate the effect of calcium silicate and an organic mineral fertilizer, together or separately, on the resistance increase of the eggplant to T. palmi.

Adults of T. palmi were collected from commercial eggplant plantations and taken to the laboratory of Entomology of the Center of Agrarian Sciences, Universidade Federal do Espírito Santo (CCA-UFES), in Alegre, Espírito Santo State, Brazil. The insects were reared/maintained in the laboratory on leaves of Canavalia ensiformis L., which had been cultivated in a greenhouse with an anti-aphid screen to avoid contamination by other arthropods species. T. palmi nymphs were placed on leaves of this plant in plastic containers (15´10´5cm). The adaxial face of the leaf was covered with filter paper and the lateral edges were also lined with paper towel to avoid water accumulation. The petioles of the C. ensiformis leaves were wrapped with a moistened cotton ball to reduce water loss. Leaves were replaced when they lost their turgidity and nutritional quality. Pollen grains of C. ensiformis were put onto the leaves to increase their nutritional quality for T. palmi.

Calcium silicate (17.45% Si₂O) and the organic mineral fertilizer containing (ppm): 35.00 Si₂O, 100.00 N₂, 25 P₂O₅, 50.00 K₂O, 13.75 S, 0.069 Zn, 0.475 B, 11.25 Fe quelated with HEDTA, 0.375 Mn, 2.75 Cl, 0.75 Cu, 0.075 Mo, 0.875 Co and 87.5 organic C were applied, together or separately, on the leaves using a manual mini-sprayer, after transplanting young eggplants to plastic bags (16´34cm). The calcium silicate was sprayed at a rate of 15g·l^-1 water; and the organic mineral fertilizer at 2ml·l^-1 water. When the two products were applied in association, 4g of calcium silicate and 2ml of the organic mineral fertilizer per liter of water plus the adhesive Hi Ten® (200g·l^-1 of polioxytilen alquil phenol eter) were used. The plants in the control treatment were sprayed with water plus the adhesive.

The application of calcium silicate and organic mineral fertilizer was done every five days. Ten nymphs of T. palmi were inoculated per replication on the abaxial face of the eggplant leaves, two days after the third, sixth, ninth and eleventh application of these products. These plants were then conditioned in cages, protected from external factors such as wind, rain and other organisms. Nymphs of T. palmi were introduced onto the eggplants leaves in circular arenas (0.7854cm²) consisting of a rubber ring 1cm in diameter fixed on the leaves of the plants with wood glue and pressed by two disks of fine polystyrene fixed by two wood picks (15´1cm), one at the inferior and the other on the superior part of the eggplant leaf. These wood picks were pressed by an elastic band in their extremities to avoid the escape of the insects.

The experiment was carried out in an entirely randomized 4´4 factorial design with four treatments and four numbers of calcium and/or organic mineral applications. Five replicates were used, each with an eggplant. The averages were compared using the Tukey test at 5% probability.

The mortality of T. palmi nymphs did not show any significant interrelation between the number of applications and the treatments (F= 0.88, df= 9, 64), but the number of applications (F= 13.43, df= 3, 64) and the treatments (F= 137.83, df= 3, 64) showed a significant increase in insect mortality. The numbers of lesions caused by T. palmi did decrease significantly as a result of the application of the treatments (F= 2.20, df= 9, 64) and both factors alone, number of applications (F= 104.22, df= 3, 64) and treatments (F= 83.36, df= 9, 64), reduced significantly the number of lesions.

Thrips palmi nymphs showed higher mortality under the treatments with calcium silicate and the organic mineral fertilizer, together or in isolation, than with the control (Figure 1a). There was no significant increase in mortality after the sixth application of the products (Figure 1b).

Figure 1. Mortality (±SD) of Thrips palmi (Thysanoptera: Thripidae) nymphs on eggplants leaves per treatment (a) and for each number of calcium and/ or organic mineral fertilizer applications (b). Treatments: calcium silicate (T1), organic mineral fertilizer (T2), calcium silicate plus organic mineral fertilizer (T3), and control (T4). Temperature: 25 ±5°C, photoperiod: 12 hours, relative humidity: 70 ±10%. Means followed by the same letters are not different by the test of Tukey (P<0.05).

The number of lesions was significantly higher on eggplant leaves in the control treatment than on plants that had received either calcium silicate and/or the organic mineral fertilizer in all evaluations, but the organic mineral fertilizer isolated was the least efficient one in reducing the number of lesions, differently from the treatments with calcium silicate alone or not. However, the damage caused by T. palmi nymphs showed a marked reduction after six applications of these products, with lower efficiency in the treatment with the organic mineral fertilizer alone. The treatment with three applications of calcium silicate was more efficient than those with mineral fertilizer alone, and the association of these two substances had no effect in decreasing the number of lesions in the plants by the nymphs (Figure 2).

Figure 2. Number of lesions produced by Thrips palmi (Thysanoptera: Thripidae) nymphs on eggplant leaves after three, six, nine and twelve applications of calcium silicate, organic mineral fertilizer or calcium silicate associated with the organic mineral fertilizer and in the control. Temperature: 25 ±5°C, photoperiod: 12h, relative humidity: 70 ±10%. Capital letters indicated differences between applications and small letters between treatments in a same application by the test of Tukey (P<0.05).

The higher mortality of T. palmi nymphs on eggplants treated with calcium silicate and/or organic mineral fertilizer can probably be attributed to the increase of the plants defense compounds, such as phenolics, lignin and phytoalexins. The oviposition, survival and longevity of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) were also lower on cucumber plants treated with calcium silicate and acibenzolar-S-methyl (BTH) (Correa et al., 2005). Silicon and BTH applications were used to activate enzymes such as chitinase, peroxidase, 1.3-glucanase, polyphenoloxidase, phenylalanine ammonia-lyase proteinase and lipoxygenases in cotton, tomato and wheat plants (Inbar et al., 2001). These enzymes are related to an increase of quinone and reactive O₂ species, which have antibiotic properties favoring tissue lignification and, thus, decreasing the nutritional quality and digestibility of the cells (Aguirre et al., 2007). This effect causes lignification of the cellular tissues, which reduces their nutritional quality and tissue digestibility (Batista et al., 2005), affecting the survival of T. palmi nymphs.

The high mortality of T. palmi nymphs in the treatments with calcium silicate and the organic mineral fertilizer is possibly related to an increase in the hardness of cell tissues of the eggplant, causing feeding difficulties. The nymphs may not be able to scrape the leaves to obtain enough nutrients, due to the direct physical impediment caused by the silicon accumulation in the cellular walls of plants (Heine et al., 2007). A similar effect reduced the longevity of Schizaphis graminum (Rond.) (Hemiptera: Aphididae) by the physical effect of silica on wheat plants (Goussain et al., 2005), and also reduced the feeding of Eldana saccharina Walker (Lepidoptera: Pyralidae) caterpillars on sugar cane plants with high silica levels (Kvedaras et al., 2007a). The deposition of silica in the cell walls of plant tissues reduces their digestibility and can cause death by starvation of phytophagous insects (Massey et al., 2006).

The marked reduction in the number of lesions caused by T. palmi nymphs on plants treated with calcium silicate and the organic mineral fertilizer may have been due to physical impediment for the accumulation of silica on the epicuticular layers of the eggplant cells (Epstein, 1999), resulting in more rigid tissues. The increased tissue hardness can wear out the mandibles of T. palmi nymphs, because their mandibular apparatus is of the scratching-sucking type. For this reason they can feed only after scratching the tissue surface, thus reducing the damage caused by the insect on eggplants treated with calcium silicate and the organic mineral fertilizer. Corn (Zea mays L.) plants treated with silicon resulted in higher mortalities of Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) caterpillars due to the wear and accentuated tear of their mandibles during all instars (Goussain et al., 2002). Calcium silicate also increased the cuticle thickness under the surface of the leaves of coffee (Coffea arabica) plants, mainly due to a more developed epicuticular wax layer; this reduced the penetration of the fungi Cercospora coffeicola and diminished the severity of the disease it causes on the plant (Pozza et al., 2004). The increased rigidity of cellular tissues caused by the application of calcium silicate also reduced the damage caused by E. saccharina on sugar cane, possibly due to the abrasion of the mandibles of this insect (Kvedaras et al., 2007b). The application of silicon reduced the damage caused by sap-sucking Xylella fastidosa subsp. pauca on Nicotiana tabacum plants, mainly due to the accumulation and polymerization of the silica in the xylem cells of this plant (Martinati et al., 2007).

The increase of mechanical and chemical barriers of plants can hinder feeding by insects, reduce plant damage and increase insect mortality (Ma and Yamaji, 2006). The use of calcium silicate and the organic mineral fertilizer could be an alternative in integrated management programs of T. palmi on eggplants. The high insect mortality and the reduction in the number of lesions caused by nymphs of T. palmi with six or more applications of calcium silicate and/or the organic mineral fertilizer, suggest that the induced effect depends on a sufficient period of time from application, which was of 30 days after the beginning of the treatments. A single application is not enough to induce the desired resistance level. It is likely that this process involves the accumulation and silica polymerization in cell tissues as well as an increase on the activity of enzymes involved in the synthesis of phenolics and phytoalexins that can increase plant resistance to phytophagy (Aguirre et al., 2007; Massey et al., 2007).

Six or more leaf applications of calcium silicate, and/or the organic mineral fertilizer, in a period of 30 days, can cause a higher mortality of T. palmi, thus reducing the damage by this pest, probably due to the silicate deposition on the cellular tissues of eggplants turning them more resistant.

The authors acknowledgements the funds received from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), the Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), and TECNOBIOLBRASIL S.A.

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