Applied plant science
History of agriculture
To Understand why we need a more sustainable approach in our agricultural practices, we need to look at how agriculture evolved to be the way we know it today.
As nomadic populations of hunter-gatherers settled to form communities that are more permanent (20.000-10.000 years BC) the need for producing more food within a particular area arose (although the two phenomena likely developed in parallel rather than one being the cause of the other). The first forms of agriculture were most probably practiced in areas around dwellings and on alluvial deposits resulting from receding flood waters, that is, on ground already fertilized and requiring hardly any clearing (Mazoyer and Roudart, 2006). From that moment on and for thousands of years, agricultural practices relied heavily on crop rotation or mixed crop planting, which not only optimized soil fertility, but also natural pest control (such as predation, parasitism, and competition) (Martin and Sauerborn, 2013). Therefore, the concept of ‘natural pesticides’ arose early in the development of agriculture. As proof of that, there are written records by Greek, Roman and Chinese scholars ranging from the 4th century B.C. until about the 3rd century A.D. (Dayan et al., 2009).
The European agricultural revolution that followed in the 17th - 19th century (the so-called first agricultural revolution of modern times) was characterized by systems of cereal and feed grain cultivation without fallowing (Mazoyer and Roudart, 2006) and it was accompanied by more extensive and international trade that resulted in the discovery of botanical insecticides from pyrethrum (Tanacetum cinerariifolium) flower heads and Derris root which contain pyrethrins and rotenone, respectively (Dayan et al., 2009). The advent of extensive monoculture and intensive agricultural practices of the 20th century, characterized by motorization, mechanization and the use of mineral fertilizers, was accompanied by increases in yields (Glaeser, 2010). New cultivars were selected based on their higher yields, but many of these lines appeared to have lower resistance to pests, as there is often a trade-off between yield and defence (Evenson and Gollin, 2003; Herms and Mattson, 1992; Kooke and Keurentjes, 2012). This resulted in greater pest pressures, which has mostly been addressed by the use of synthetic pesticides.
Synthetic pesticides have been developed throughout the 20th century. Major classes of synthetic pesticides are: organochlorides, organophosphates, carbamates, pyrethroids, neonicotinoids and ryanoids (Fig. 1.3).
Use of synthetic insecticides led to numerous problems unforeseen at the time of their introduction: acute and chronic poisoning of applicators, farmworkers, and even consumers; destruction of fish, birds, and other wildlife; disruption of natural biological control and pollination; extensive groundwater contamination, potentially threatening human and environmental health; and the evolution of resistance to pesticides in pest populations. Governments responded to these problems with the introduction of new pesticide registration procedures, such as the Food Quality Protection Act in the United States. These new regulations have reduced the number of synthetic pesticides available in agriculture. Therefore, the current paradigm of relying almost exclusively on chemicals for pest control may need to be reconsidered (Dayan et al., 2009). In addition, the development of pesticide resistance in insects, due to persistent use, has rendered this need for alternatives – which include plant derived insecticides – ever more urgent (Isman, 2006).
Integrated Pest Management (IPM) is an ecosystem-approach to crop production and protection that combines different management strategies and practices to grow healthy crops and minimize the use of pesticides (Kogan, 1998). It is defined as the selection, integration and implementation of pest control based on predicted economic, ecological and sociological consequences, and as such it makes maximum use of naturally occurring control agents, including weather, disease organisms, predators and parasites (Bottrell, 1979). IPM aims to suppress pest populations below the economic injury level (EIL) (Higley and Pedigo, 1993). As defined by the Food and Agriculture Organisation (FAO) of the UN, IPM is "the careful consideration of all available pest control techniques and subsequent integration of appropriate measures that discourage the development of pest populations and keep pesticides and other interventions to levels that are economically justified and reduce or minimize risks to human health and the environment. IPM emphasizes the growth of a healthy crop with the least possible disruption to agro-ecosystems and encourages natural pest control mechanisms" (FAO, 2013). IPM includes managing insects, plant pathogens and weeds.
The most effective IPM programs are based on an ecological understanding of the pests’ interactions with the crops, other organisms, and the environment (Dufour, 2001). The principles of IPM include: a) setting of action thresholds (acceptable pest levels), with emphasis on control rather than eradication; b) monitoring and identification of the pest by regular observation and record keeping; c) prevention through the selection of best varieties for local growing conditions, as well as soil ecosystem management; d) control strategies adoption from less risky to most risky (mechanical control, followed by biological control, followed in turn by responsible use of synthetic pesticides if the less risky approaches failed to achieve the desired result) (U.S. Environmental Protection Agency, 2012).
Beneficial insects can be used in pest control to complement other strategies. They can be predators or parasites (Fig. 1.4). The three strategies adopted in biological control using beneficial insects include importation, augmentation and conservation. Importation involves the introduction of a pest’s natural enemies to a new area where they do not occur naturally; this is often necessary when a pest had been accidentally introduced into a new geographic area without its associated natural enemies. Augmentation involves the supplemental release of natural enemies, boosting the naturally occurring population; it can be done by inoculative or inundative release, depending on the numbers of insects released at a given time. Conservation of existing natural enemies is usually a simple and cost-effective strategy to implement enemies already adapted to the habitat of the target pest; this can be achieved by modifying the cropping systems to favour the natural enemies, like for instance providing a suitable habitat for other life stages than the one that targets the pest (Rechcigl and Rechcigl, 1999).
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