19 : Insecticidal Properties of Some Medicinal Plants
Abstract
Medicinal plants contain a large amount of biologically active substances applied both in traditional medicine and as inexhaustible sources of substances for various industries. Crop protection is one of those areas where substances obtained from plants have found their applications, as well. Botanical insecticides (BI) play a significant role in plant protection in biological agriculture systems. Nevertheless, the current world-wide trend to increase the share of biologically cultivated crops forces us to seek new, environmentally safe substances to find their application in production of new safe BI. For this reason, those plant extracts must be tested, which can be expected to contain substances of environmental potential, and at the same time, which provide sufficient efficiency against crop diseases and pests. Some medicinal plants may have such a potential. Screening of insecticide efficiency of wild plants in the Euro-Asian region has been performed within the framework of the international project currently being carried out. 82 species of wild plants have been sampled at localities in the Czech Republic and in the Russian Federation until present. The plants have been extracted using methanol and the extracts have been assessed for their biological efficiency against larvae of Spodoptera littoralis and Culex quinquefasciata. This work summarizes current results of the biological tests. It shows that some plant species have a high potential for production of new plant insecticides, and that biological activity screening of the substances obtained from the plants is important for development of new, environmentally safe BI.
Key words : Insecticidal activity, Medicinal plants, Plant extracts, Botanical insecticides, Culex quinquefasciata, Spodoptera littoralis
1. Crop Research Institute, Drnovská 507, 161 06 Praha 6 Ruzyn, Czech Republic.
2. All-Russian Institute of Plant Protection, Podbelsky sh., 3, St.-Petersburg, Pushkin 196608, Russia.
* Corresponding author : E-mail: pavela@vurv.cz
Introduction
Plants have evolved for over 400 million years and to defend themselves from insect attack they have developed protection mechanisms such as repellents and even insecticidal effects. The use of plant extracts as traditional protectants of plant products is an old practice used all over the world. Our ancestors were quite successful in exploring and exploiting this natural treasure. The documented use of plant extracts and powdered plant parts as insecticides goes back at least as far as the Roman Empire. There are reports of the use of pyrethrum (Chrysanthemum cinerariaefolium, Asteraceae) already in 400 B.C. The first pure botanical insecticide, used as such, dates back to the XVIIth Century when it was shown that nicotine, obtained from tobacco leaves, would kill plum beetles. Around 1850 a new plant insecticide known as rotenone was introduced. Rotenone is a flavonoid derivative extracted from the roots of two different Derris spp. and Lonchocarpus spp., all Fabaceae (Ador, 1995). Pyrethrins, which are esters with insecticidal properties, are obtained from pyrethrum (C. cinerariaefolium and C. cocineum) flowers. The compounds obtained from this plant which have known insecticidal activity are six esters formed by the combination of the acids chrysanthemic and pyrethric and the alcohols piretrolone, cinerolone and jasmolone. These compounds act both on the central nervous system and in the peripheral nervous system causing repetitive discharges, followed by convulsions. Research has shown that these compounds block sodium ion influx resulting in the channels being affected by intermolecular forces which causes alterations in moving ion conductivity as a result of tchanges in the channels. There is no doubt that the most important characteristic of these compounds is their irritating effect or “knock down” which causes the insect to stop feeding as soon as it encounters a treated surface.
These traditions were largely neglected by farmers after the Second World War. When synthetic insecticides appeared in the 1940’s some people thought that botanical insecticides would disappear gradually. However, problems like environmental contamination, residues in food and feed, and pest resistence has led to a renewed interest in nature as a source of novel crop protection compounds. The fact that plants during evolution have developed an effective defense system against most insects makes plants the richest natural source for biocidal compounds as toxicity (Hiremath et al., 1997), growth retardation (Breuer & Schmidt, 1995; Pavela, 2004b), feeding inhibition (Pavela, 2005), oviposition deterrence (Hermawan et al., 1994), suppression of calling behaviour (Khan & Saxena, 1986) and reduction of fecundity and fertility (Pavela, 2007b). Such a wide variety of effects provide potential natural alternatives for the use of synthetic chemical insecticides. For example Azadirachtin is a tetraterpenoid characteristic of the Meliaceae family but particularly from the Neem tree (Azadirachta indica), indigenous to India. The compound is found in bark, leaves and fruits of the tree but seeds have the highest concentration. In the extract more compounds have been identified among which salanine, meliantrol and azadiractin are most prominent, the latter being in the highest concentration. Azadirachtin shows antifeedant activity, is a growth regulator, inhibits oviposition and is also a sterilizing compound. Today, commercial formulations of neem may be found with names like Neem Gold, Neemazal, Econeem, Neemark, Neemcure and Azatin among others, in many countries including the United States, India, Germany and several Latin American countries.
Pongamia is a monospecific genus i.e. Pongamia pinnata L. (syn. P. glabra Vent.; Derris indica Lamk.) which belongs to the Leguminosae family. P. pinnata (common names: puna oil tree, Pongamia, Kharanja, or Karanja oil) is a rich source of flavonoids, the B-ring linked either to a furan or a pyran ring, some of which possess biological activity. The secondary metabolites (flavonoids, chalcones, steroids and terpenoids) in pongam oil serve as defensive agents against insect pests. The use of pongam oils against greenhouse pests is possible: pongam oil-treated potato Lycopersicum esculentum) and chrysanthemum (Chrysanthemum indicum) plants showed a strong repellent effect on the adults of greenhouse whitefly and deterred oviposition (Pavela & Herda, 2007).
Research is again focusing on the plant kingdom for solutions since the interaction between plants and insects has led to the production of a myriad of secondary compounds that include properties such as toxicity, growth retardation, feeding inhibition, oviposition deterrence, suppression of calling behaviour and reduction of fecundity and fertility (Jang et al., 2002; Prakash & Rao, 1997; Pavela, 2006, 2007b). Such a wide variety of effects provide potential natural alternatives for the use of synthetic chemical insecticides. Certain plant families, particularly Meliaceae, Asteraceae, Rutaceae, Lamiaceae, Annonaceae and Canellaceae, are viewed as exceptionally promising sources of plant based insecticides (Pavela, 2007a; Prakash & Rao, 1997). Nevertheless, the current world-wide trend to increase the share of biologically cultivated crops forces us to seek new, environmentally safe substances to find their application in production of new safe botanical insecticides.
For this reason, those plant extracts must be tested, which can be expected to contain substances of environmental potential, and at the same time, which provide sufficient efficiency against crop diseases and pests. Some medicinal plants may have such a potential. Screening of insecticide efficiency of wild plants in the Euro-Asian region has been performed within the framework of the international project currently being carried out. 82 species of wild plants have been sampled at localities in the Czech Republic and in the Russian Federation until present. The plant extracts have been assessed for their biological efficiency against larvae of Spodoptera littoralis and Culex quinquefasciata. This work summarizes current results of the some biological tests.
Materials and Methods Plant extracts
Fresh plant material of each of the selected species (Table 1) was collected in years 2006 and 2007. Voucher specimens of all of the plant species studied were deposited in herbaria of the respective our institute. The plant material was shade-dried and powdered. The dry powder was extracted with methyl alcohol (500 ml of MeOH for 100g of plant powder) for 24 h. The crude extracts were filtrated and dried under vacuum in a rotary evaporator at 40 °C. The extract was stored below 4 °C until further assay.
Bioassays
The experiments were performed with 2nd instar larvae (4–6 mg) of Spodoptera littoralis, reared on an artificial diet (Premix, prepared by Stonefly Industries, Inc., Bryan, TX, USA) for more than 30 generations. The stock solutions of crude plant extracts were diluted with water for final concentration of 0.5 (w/ v); 75 ml of these dilutions were stirred into 25g of the diet mixture (Premix). The diet without an extract but with water added was used as a control. Portions of 100g diet containing 3.75 mg/g of extracts were thus obtained.
Experiment 1 – larval mortality: Second instar larvae were placed on a slice of the diet in Petri-dishes (diameter 15 cm). The diet was changed every 2 d, surviving larvae received the diet until the pupal stage, and larval mortality was recorded. Each experiment was performed on 25 larvae in 5 replications.
A laboratory colony of Culex quinquefasciatus was used for the larvicidal activity. The larvae were fed on dog biscuits and yeast powder in the 3:1 ratio. The larvae at early fourth instar stage were used in study. The larvicidal activity was analyzed as per the standard procedures recommended by World Health Organisation (1996). The plant extracts were dissolved in 2 ml of dimethyl sulfoxide and prepared into concentration like 500 ppm with distilled water. Twenty larvae were taken in a class beaker (250 ml) containing 199 ml of water and 1 ml of respective concentration of plant extract. Four replicates were maintained, and the dead larvae were counted after 24 h.
The assays were placed in a growth chamber (L16:D9, 26 °C). The control mortality was corrected by Abbott’s formula (Abbott, 1925).
Results and Discussion
Raw extracts of 82 plant species of the Euro-Asian region were tested in this study. Total mortality of S. littoralis and C. quinquefasciata larvae was observed as the main criterion to select plants suitable for development of plant insecticides. Only one batch of extracts was selected for classification tests, chosen based on economic consideration. The relatively high tested dose caused mortality versus control in almost all extracts in both insect species. Nevertheless, significant differences in efficiency of the extracts were found (Table 1). Extracts of 37 plant species caused mortality higher than 80% in C. quinquefasciata larvae, and 9 extracts showed the same mortality level in S. littoralis larvae. Toxicity higher than 80% for both tested insect species at the same time was found only in 6 extracts (Acer campestre, A. platanoides, A. pseudoplatanus, Leuzea carthamoides, Juglans nigra and Quercus robur).
Table 1. Insecticidal efficiency of plant extracts to larval stages of Culex quinquefasciata and Spodoptera littoralis and ethnobotanical use of plants
Although some effects of the plant extracts tested by us (e.g. Salvium sp., Leuzea carthamoides) have been known (Pavela, 2002, 2004a,b), insecticide efficiency has not been determined for some plant species yet (e.g. Acer sp., Seseli sp.). Screening is very important for choosing suitable plants with a high insecticide potential (Janks et al., 2002; Pavela, 2004b, 2005; Omena et al., 2007). Although in general, mortality is seen as the most important criterion in development of new insecticides, other characteristics need to be studied, as well; such as, for example, growth inhibition, antifeedancy, repelency and antioviposition (Pavela, 2004b) or the effect of extracts and their sublethal doses on subsequent generations (Pavela, 2007b). It is true that the effects of sublethal doses do not cause immediate mortality; however, they can significantly reduce fertility and fecundity as well as total vitality of adult forms and subsequent generations, whereby significant reduction of population density of the pest may occur.
In development of new botanical insecticides, environmental safety and health safety represent an important criterion, as well. Such a potential can be offered precisely by medicinal plants, used in folk medicine, and their effects on the human body are known (Table 1). Nevertheless, the effect on other than target organisms must be monitored.
In our work, we have chosen 6 plants with high potential for development of new botanical insecticides based on the mortality determined. These plants or their extracts, respectively, shall be subjected to other biological tests to provide closer specification of their biological activity.
Acknowledgements
This study was supported by grants of the Czech Republic Ministry of Education, Youth and Sports (1P05ME764). The author is thankful to the Prof. V.M. Kalinkin from VIZR, St. Petersburg, Russia for help at realization of expedition and collection of plants.
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