The cotton bollworm or American bollworm, Helicoverpa armigera Hubner (Lep: Noctuidae), is one of the most destructive and cosmopolitan pests of a variety of field crops in Africa, Australia, Europe and Asia including Iran (Farid 1986, Guo 1997). It is a highly polyphagous pest, feeding on more than 180 cultivated crop species (within 46 families) and causing heavy crop losses every year to agricultural, horticultural and ornamental crops (Pogue 2004). According to investigations, the worldwide annual costs for controlling this pest as well as its associated yield losses have been estimated as US$ 5 billion (Lammers and MacLeod 2007). The pest status of H. armigera has been argued to arise mainly from its high fecundity, high mobility, polyphagy, facultative diapause, migratory potential (Fitt 1989) as well as the propensity to rapidly develop resistance against pesticides (Forrester et al. 1993, McCaffery 1998). The female moths lay about 1000-1500 eggs on different parts of plants and the larvae after hatching feed on almost all aerial parts such as leaves, stem, flower buds, fruits and flowers, with more than 90% of all damage is caused by the third instar onwards (Fye and McAda 1972, Wang and Li 1984). In Iran, the annual yield damage caused by H. armigera has been estimated around 20-25%, which may increase up to 75% during outbreaks (Mojeni et al. 2005).
Around 30% of all pesticides worldwide are used for controlling this pest, directly which reveals the control of this pest has relied approximately on application of synthetic insecticides. During recent decades, other control methods such as biological and cultural control, Bt-transgenic cultivars, use of resistant plant cultivars and sexual pheromone traps are also in progress (Czepak et al. 2013). In some countries, such as India and Pakistan, more than 50% of the total synthetic pesticides are used only for controlling H. armigera (Puri 1995, Abbas et al. 2015). During the last two decades, the cotton bollworm has been reported to develop resistance to the CRY endotoxins of Bacillus thuringiensis as well as to a wide range of pesticides including chlorinated hydrocarbons, carbamates, synthetic pyrethroides and organophosphates (Forrester et al. 1993, Akhurst et al. 2003, Gao et al. 2009, Yang et al. 2013). Additionally, the improper use of pesticides, at high doses and in inappropriate times, has had many irrecoverable effects on human health, environment and non-target organisms especially natural enemies and pollinator agents (Pimentel 2005, Baskar and Ignacimuthu 2012, Menezes et al. 2013). Therefore, it is important to search and develop alternative strategies for controlling this pest with more specific effects on the target pest, but less detrimental impacts on the environment and non-target organisms in the shade of integrated pest management (IPM) concept.
Plants have developed a wide range of strategies in response to invasion by herbivores. It has been argued that 1 to 10% of the total protein storages in plant seeds are involved in resistance against pests and pathogens (Shewry 2003, Konarev et al. 2002, Korsinczky et al. 2004). A relatively large portion of plant defenses against herbivores are mediated by enzymes that impair digestive processes in insect gut. For example, the small protein molecules, known as digestive enzyme inhibitors, are capable of interfering with the normal growth and development of insects through inhibition of the activity of digestive enzymes such as hydrolases, ''-amylases and proteinases (Lipke et al. 1954, Volpicella et al. 2003). As an alternative to chemical strategies, insect digestive enzymes may provide an appropriate target for pest control if we can improve the natural defense of plants by the use of transgenic technology (Franco et al. 2002, Macedo and Freire 2011). To achieve this goal, it is essential to select for appropriate inhibitors with satisfactory inhibition against the target pest, besides the least detrimental effects on non-target organisms as well as the lowest rate of resistance development in target pest against the expressed inhibitor. Already, protease inhibitors have been extracted from a wide range of plant taxa such as Solanaceae, Graminaceae, and Leguminosae, and their inhibitory properties against digestive enzymes of various herbivore insects have been evaluated under both in vivo and in vitro conditions (Connors et al. 2002).
Datura is a small genus, with a dozen of species, within the family Solanaceae, all species have been well known for their poisonous properties and are widely used in traditional medicine. Different parts of Datura plants, particularly seeds and flowers, contain fatal levels of tropane alkaloids such as scopolamine, hyoscyamine, and atropine which have very harmful effects such as delirium, loss of body control, cramps and eventual death when taken in large quantities causing (Setshogo 2015). A large volume of studies has evaluated the insecticidal activity of extracts, taken from different parts of Datura species, against different insect pests and negative effects of feeding on these compounds, including larval mortality, prolonged development, and decreased fecundity, have been reported (Rahuman et al. 2008, Abbasipour et al. 2011, Elango et al. 2011, Panneerselvam et al. 2013). Additionally, recent studies have revealed that seeds of some Datura species are rich in enzyme inhibitors, which may represent them a potential agent for pest control through genetic engineering (Parde 2009, Mehrabadi et al. 2011, Esmaeily and Bandani 2016). In this study, we isolated and purified the trypsin inhibitor of extract taken from D. metel. The inhibitory effects of these inhibitors were then evaluated against the extracted digestive enzymes of the cotton bollworm larvae and some biological properties of th
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