Cover crops and related methods for enhancing agricultural biodiversity and conservation biocontrol: successful case studies.

Abstract

In modern agricultural systems mechanical cultivation and chemical pesticides are used for crop production, restricting diversity and promoting landscapes dominated by large monocultures. Through the use of equipment such as harrows and mowers, large portions of the biomass are often removed and/or tilled annually, thereby forcing the growth process to start over. Herbicides are used to manage weeds, and fertilisers are used to foster rapid, lush growth of the crop. The prevailing insect pest control strategy in these agricultural systems is application of toxic agrochemicals. Such prophylaxis 'insurance' approaches can lead to biological control failure or a least a reduction in effectiveness as a result of the direct and indirect effects of pesticides, tillage, cultivation, lack of nectar and pollen sources, scarcity of hosts and lack of shelter and hibernation, mating and oviposition sites (Corbett and Rosenheim, 1996; Landis et al., 2000 ; Heimpel and Jervis, 2005 ). In the absence of vital resources, colonisation by predatory species is often much lower than that by herbivores (Altieri and Whitcomb, 1979; Thies and Tscharntke, 1999), resulting in the failure of predators and parasitoids to control pests as they begin colonising crops (Landis et al., 2000 ). Long-term solutions to escalating economic and environmental consequences of combating pests in agricultural crops can be achieved by restructuring and managing agroecosystems in ways that enhance agricultural diversity to increase biocontrol and other ecological services for pest management.
One of the most important aspects of enhancing biodiversity in agricultural systems involves the provision of resources for natural enemies of pest insects and insect pollinators. Interestingly, many of the habitats incorporated into agricultural systems for enhancing natural enemies are multifunctional, for they can provide other ecological benefits such as conserving wildlife, protecting water quality and reducing erosion and runoff (Leidner and Kidwell, 2000 ; Thomas et al., 2001; SWCS, 2006; Triplett and Dick, 2008; FAO, 2010). Understanding the ecology of insect pests and their natural enemies in agroecosystems is essential in creating and designing habitats for enhancing agricultural biodiversity for pest suppression. It is important to have a clear understanding of what resources are needed and how specific habitats can successfully provide these resources. Strategic placement, in time and space, of a multifunctional habitat in an agricultural system may also be essential for successfully increasing biocontrol and other ecological services for pest management. For example, the southern green stinkbug (Nezara viridula L.) is a generalist feeder that exhibits edge-mediated dispersal from peanut into cotton at the common boundary of the two crops in peanut-cotton farmscapes (Tillman et al., 2009). Addition of a habitat of sorghum along this boundary apparently enhances biocontrol of this pest by the adult fly Trichopoda pennipes F. (Tachinidae) (Tillman, 2006). Strategic establishment of a corridor composed of 65 flowering plant species enhanced predator colonisation and abundance on adjacent organic vineyards by providing timely circulation and dispersal of predators into the centre of the field (Nicholls et al., 2001).
In this chapter, we present three examples of establishing a habitat in an agricultural system at the right time and location for enhancement of agricultural biodiversity and conservation biocontrol. These include use of cover crops and conservation tillage in cotton fields, provision of insectary plants within lettuce fields and establishment of beetle banks within cereal fields. For each of these examples we cover the driving forces that led to the introduction of a biodiversity-based pest management system, discuss the development of the habitat and evaluate its effectiveness and uptake, while providing some information on the economics.