Plants may provide potential alternatives to currently used pest-control agents because they constitute a rich source of bioactive chemicals. The present study aimed to evaluate the insecticidal and antifungal activities of Curcuma longa extracts. The isolation and identification of two compounds, ar-turmerone and curcumin were also described. Curcuma longa rhizomes dried powder was extracted with n-hexane, methylene chloride, methanol and water, successively. The resulting extracts were evaluated for their insecticidal activity against fourth instar larvae of Culex pipiens by dipping assay. Hexane, methylene chloride and methanol extracts exhibited remarkable insecticidal activity against the larvae. The LC50 values were 12.87, 17.81 and 23.63 mg/L, respectively. In contrary, the extracts exhibited weak toxic effect on the third instar larvae of S. littoralis. After 24 h of treatment, the tested extracts caused mortality less than 50% at the highest concentration (1000 mg/L). After 48 h, water, methylene chloride and hexane extracts showed moderate insecticidal activity with LC50 values of 495.9, 565.7 and 709.7 mg/L, respectively. Concerning the efficiency of the tested extracts against plant pathogenic fungi, Fusarium oxysporum, Pythium debryanum, Phytophthora infestans, Fusarium solani and Alternaria alternata, the extracts showed variable antifungal activity. Methanol extract had the highest antifungal activity among the tested extracts with EC50 values of 159.8, 242.7 and 322.2 mg/L on P. infestans, F. solani and A. alternata, respectively. Hexane and water extracts were effective only against P. infestans with EC50 values of 287.2 and 295.8 mg/L, respectively. Moreover, methylene chloride extract revealed antifungal activity against F. oxysporum (EC50 = 395.5 mg/L) but not active against other fungal strains. Two compounds ar-turmerone and curcumin have been isolated from C. longa rhizome powder after extraction with methylene chloride/methanol (1:1). The chemical structures of these compounds were identified by using spectral data obtained from UV, IR and 1HNMR analysis. The results of insecticidal activity showed that ar-turmerone had moderate toxicity against the fourth instar larvae of C. pipiens. The LC50 values were 158.5 and 117.6 mg/L after 24 and 48 h, respectively. In addition, ar-turmerone showed moderate antifungal activity against P. infestans (EC50 = 588.9 mg/L) and weak activity against F. solani (EC50 = 820.6 mg/L). On the other hand, curcumin showed antifungal activity against F. oxysporum and P. infestans with radial growth inhibition of 51.1 and 54.32 % at 250 mg/L, respectively.
Key words: Curcuma longa; ar-Turmerone ; Curcumin ; Culex pipiens; Spodoptera littoralis; Plant pathogenic fungi; Insecticidal activity; Antifungal activity.
Introduction
Higher plants are a rich source of natural compounds that can be used effectively in pest control. Insecticidal, herbicidal and fungicidal activities of many plants against several pests have been demonstrated (Isman, 2006; Dyan et al., 2009). Although botanicals are now a small part of the global pesticide market due to replacement by synthetics, the new environmental movement has provided a favorable environment for the renewal of botanical pesticides. Public concern over use of synthetic pesticides is growing. This has led to the great growth in organic agriculture in which botanicals play an important role in pest control. Controversy over genetically modified crops is another factor favoring the use of botanicals in pest control. Botanicals have certain advantages, such as rapid degradation, lack of persistence and bioaccumulation in the environment and low mammalian toxicity (Cantrell et al., 2012).
Curcuma longa, an eternal herb that affiliate the Zingiberaceae family, It develops to a height of 3 to 5 feet and cropped widely in Asia, India, China, and many countries with a tropical weather. It has oblong, pointed leaves and funnel-shaped yellow flowers. Rhizome is the medicinal part, which boiled, cleaned, and dried, producing a yellow powder. Dried Curcuma longa is the reason of the flavor turmeric, the part that gives curry powder its distinctive yellow color. Turmeric is used extensively in foods for its flavor and color, as well as having a long tradition of use in the Chinese and Ayurvedic systems of medicine; India has a rich history of using plants for medicinal purposes. Turmeric is a medicinal plant widely used in Ayurveda system of medicine as home remedy for several diseases. Turmeric (Curcuma longa) used widely as a spice, food preservative and colouring substance in India. Curcumin (diferuloyl methane), the main yellow bioactive component of turmeric showed a wide spectrum of biological effects. These comprise its antidiabatic, antibacterial, antifungal, antiprotozoal and antiviral activities. Curcumin is the major curcuminoid of the common Indian spice turmeric. The else two curcuminoids are desmethoxy curcumin and bis-desmethoxy curcumin. The curcuminoids belong to polyphenols, it is answerable for the yellow color of turmeric. Curcumin could be found in at least two tautomeric forms, keto and enol. The enol form is more stable whether in the solid or solution (Akram et al., 2010).
The Egyptian cotton leafworm Spodoptera littoralis (Boisduval) is one of the most series agricultural lepidopterous pests of cultivated crops foremost in tropical and subtropical countries (Bakr et al., 2013). Culex pipiens are vectors of West Nile virus and an important pest to humans, causing allergic responses that include local skin reaction and systemic reactions such as angioedema, and urticaria (Cheng et al., 2008). Plant pathogenic fungi cause significant pre-harvest and post-harvest loss in crop production. It is estimated that the world crop loss due to plant diseases may amount to 14%. Plant pathogenic fungi share 40-60% of total plant diseases loss (Mahy and van Regenmortel, 2009).
In our continuing efforts to find new natural products with potential for pest control, the insecticidal and antifungal activities of hexane, methylene chloride, methanol and water extracts of C. longa were examined against C. pipiens and S. littoralis, and five plant pathogenic fungi, Fusarium oxysporum, Pythium debryanum, Phytophthora infestans, Fusarium solani and Alternaria alternata. Moreover, the isolation, structure determination and bioactivity of two active compounds (ar-turmerone (1) and curcumin (2)) were conducted.