Antifeedant, larvicidal and growth inhibitory bioactivities of novel polyketide metabolite isolated from Streptomyces sp. AP-123 against Helicoverpa armigera and Spodoptera litura

According to the report of FAO, US $120 billion losses worldwide were caused by 20–40% decrease in crop yield, due to the attack from pathogenic organisms and insect pests [1]. Helicoverpa armigera and Spodoptera litura are the major polyphagous pests attacking more than 150 different host species and affect the vegetable yield [2]. Therefore these pests are considered as the most economically important insect pests in many countries including India, Japan, China and Southeast Asia. Controlling these polyphagous pests becomes the challenging work in agriculture field. There are few chemical insecticides and pesticides are commercially available in the market. Usage of different varieties of chemical insecticides and pesticides to control insects has resulted in emergence of pesticide resistance in the pests [3]. To ensure the stable and high output of crops, huge amount of pesticides were applied to control the pests, and this not only caused serious environmental pollution but also induced in a wide range of pesticide resistance. Meanwhile by applying these chemical pesticides different varieties of pest predators were killed and the ecological balance was destroyed, thereby causing pest resurgence and a greater outbreak of secondary pests [4]. Due to this reason, many researchers have involved on alternative control methods. Botanical and microbial pesticides are having advantage over chemical pesticides by its highly effective, safe, and ecologically acceptable nature. Fortunately, bio-pesticides have been gaining increased attention and interest among those concerned with developing environment friendly and safe integrated crop management, with compatible approaches and tactics for pest management [5].

Natural products derived from plants and microorganisms have been used for insect control [6]. Azadirachtin, a natural compound isolated from neem Azadirachta indica, is considered superior over other compounds since it has wide range of biological activities. Azadirachtin has been studied by many researchers and used as positive control. Bacterial and viral-based insecticides controlled different pests. Most of the pesticides from microorganisms have been isolated from entomo-pathogens and the terrestrial environment [7]. Recent studies on marine microorganisms have focused mainly on the discovery of human drugs, whereas limited information about marine microorganisms possessing insecticidal activities has been reported. However marine environment, representing more than two thirds of our planet, is still under-explored and is considered to be a prolific resource for the isolation of less exploited microorganisms [8]. The ocean is a resource of huge drug, where more than 6000 kinds of novel chemical compounds have been isolated from marine living organisms, among which more than 1000 compounds exert biological activities, such as anti-tumour, anti-microbial and anti-virus, etc. [9].

Recently, Streptomyces sp. AP-123 producing polyketide metabolite (Figure 1) was reported by analyzing the presence of polyketide biosynthesis (PKS) biosynthetic cluster [10]. Streptomyces sp. AP-123, a Gram positive, filamentous, spore-forming antagonistic bacteria recovered from marine region at Andhra Pradesh, India. Polyketide metabolite isolated from Streptomyces sp. AP-123 acted as a growth inhibitor of Gram-positive, Gram-negative bacteria and filamentous fungi. No reports are available on the effect of polyketide metabolite against the polyphagous pest H. armigera and S. litura. The present study was aimed at assessing the antifeedant, larvicidal, pupicidal and growth inhibitory effect of polyketide metabolite isolated from Streptomyces sp. AP-123 against H. armigera and S. litura .

In the present study, polyketide metabolite exhibited maximum antifeedant activity of 78.51% and 70.75% at 1000 ppm concentration against H. armigera and S. litura. This result coincided with earlier results of Kannan who had isolated violacein from Chromobacterium violaceum claimed more than 80% antifeedancy at 1000 ppm against H.armigera [11]. Xiang et al. isolated novel macrocyclic lactone from Streptomyces microflavus neau3, showed high acaricidal activity against adult mites and nematocidal activity against Caenorhabditis elegans [12].

In the present study, significant larvicidal activity was observed at 1000 ppm concentration against H. armigera and S. litura, respectively. Becher et al. reported that 12-epi-Hapalindole J isonitrile isolated from soil bacterium showed 100% larvicidal activity against Chironomus riparius [13]. Three different strains of B. thuringiensis showed larvicidal activity ranging between 62% and 96% against Spodoptera frugiperda and 100% against Anticarsia gemmatalis [14]. In this study some adults emerged and were small in size with varied abnormalities. 20% of the world’s top-selling pharmaceuticals and their and derived compounds were polyketide in nature and thus represent one of the most important classes of natural products, with combined revenues of over $18 billion per year [15]. Microbial polyketides are synthesized by serialized reactions of a set of enzymes called PKS with extraordinary structural diversity and an irregular distribution between strains and species, and they have been considered to play vital roles antimicrobial agents for pathogenic bacteria, fungi and also used as in pest control agents to kill insects and pests [16]. Spinosyns recovered from microorganism showed potent insecticidal activities against many commercially significant species that cause extensive damage to crops and other plants. They also exhibit activity against important external parasites of livestock, companion animals and humans [17]. Several microbial polyketides, such as avermectins and milbemycins, have been reported as potent insecticides against various insects and parasites. Furthermore, they are believed to be the biggest selling and arguably most effective acaricides and anthelmintics currently available [18]. Of the 7000 known polyketide structures, more than 0.3% has been commercialized [19]. Given the importance and potential of these compounds, the discovery of microbial polyketides has drawn increasing attention.

In conclusion, polyketide metabolite showed good antifeedant, larvicidal, pupicidal and growth inhibitory activities against H. armigera and S. litura. The results indicated that polyketide metabolite would be a potential insecticide. This study is the first report on antifeedant, larvicidal, pupicidal and growth inhibitory activities against H. armigera and S. litura. This metabolite could be used for the development of new insecticidal formulation for the management of field pests.

The authors are grateful to global Research Centre for Biotechnology, Taramani, Chennai, India, Entomology Research Institute, Loyola College and CNU for carrying out this work. Authors are thankful to Addiriyah Chair for Environmental Studies, Department of Botany and Microbiology, College of Science, King Saud University, Riyadh-11451, Saudi Arabia for financial assistance.