natural-products-in-plant-pest-management

Natural Products from Plants 73agent is already present at a level that does not provide adequate, continuouscontrol of the target, this strategy is known as augmentation biocontrol(Goeden and Andrés, 1999). In general, inundative biocontrol relies on thereleased organisms themselves to control the target without any expectationof continued control by future generations of the agent (Elzen and King,1999). In an inundative strategy the agent may need to be released or appliedseveral times during a single crop cycle in the event of re-growth orre- emergence of the target weed. Thus, inundative strategies typically applyto relatively high-input systems.Classical biocontrol, sometimes referred to as inoculative biocontrol,involves the practice of identifying co-evolved natural enemies from thenative range of a target weed species and releasing them into the invadedrange to reduce the presence of the weed to acceptable levels (McFadyen,1998).In classical biocontrol, the agents are expected to reproduce and proliferateon the target weed and disseminate throughout its invaded range, reachingan ecological equilibrium with the target weed and providing continuous,perpetual control. Successful control depends almost wholly on damagecaused by the descendants of the released individuals rather than by thereleased individuals themselves (Elzen and King, 1999). Classical biocontrolis generally practiced in low-input systems. Biologically based weed controlcan also take the form of weed-resistant properties in crop plants, akin tohost-plant resistance versus insect and pathogen pests.Allelopathy, the production by a plant of secondary metabolites thatinhibit growth of nearby plants, is a phenomenon that has been studied forits potential utility in weed control for many years (Rector, 2008). This impliesthe use of natural compounds from plants, referred to as allelochemicals, innatural product development programmes. The term allelopathy is derivedfrom two Greek words ‘allelos’ and ‘patos’ meaning ‘to suffer on each other’(Delabays and Mermillod, 2002). According to the authors, allelopathy isdefined as a chemical process whereby certain plants release natural compoundsinto the environment that can either stimulate or inhibit the growthand development of surrounding plants. In nature the latter is more likely, asa form of competition for growing space, where allelochemicals are releasedfrom the roots of one plant and absorbed by the roots of surrounding plantspreventing them from growing in the same area. There are several ways inwhich an allelopathic plant can release its protective chemicals. These include:(i) volatilization, where a chemical is released in the form of a gas throughthe leaves and, on absorption, sensitive surrounding plants are stunted ordie; (ii) leaching of allelochemicals from decomposing abscised leaves preventingsurrounding plants from establishing in the same area; and (iii) exudationof allelochemicals into the soil through the roots, preventingsurrounding plants from prospering (Delabays and Mermillod, 2002).Despite its long history, little progress has been made in incorporatingallelopathy into mainstream weed management programmes due to a failureto provide adequate weed control while maintaining other agronomic qualitiesof the crop (Belz, 2007). There is a strong possibility that formulation hasplayed a key role in the failure of bio-herbicides in the market and their