Conservation biological control has been the least studied area of all biological control techniques and has been dominated by arthropod pest systems. This technique adds plant biodiversity to agricultural sys tems through the provision of shelter and nonprey food, especially in high value crops such as wine grapes. One of the most successful conservation biolo gical control techniques has been 'beetle banks' which were developed by the Game Conservancy Trust and the University of Southampton in the United Kingdom. They comprise overwintering refugia for predatory arthropods in arable land and have the added benefit of providing nesting sites for rare farmland birds and mammals. Although floral resources for pests' natural enemies were planted in the 1980s and 1990s, their selection and deployment were not informed by ecolo gical science. There later developed a growing awareness of the possible impact of the plants on the pest itself and on any parasitoids of natural enemy species. Bioassays to measure the effects of flowers on fecundity and longevity in these trophic levels have recently been carried out so that the deployment of selective floral diversity in agricultural systems is now possible. Recently, floral nectar has been analyzed for its sugar ratio, then the same plants were assessed with pests' natural enemies. Flower structure and size also have an impact on the system. Long, narrow flowers may provide nectar only to larger insects with long mouth parts, whereas large, shallow flowers provide nectar to all species. Further knowledge of such aspects of floral resources will allow plant selection to benefit the third trophic level more than the second and the fourth, which will allow agriculturalists to maximize the effects of the technology.
An area of recent development in conservation biolo gical control is herbivore induced plant volatiles (HIPVs). These (mainly terpenoids and indoles) are sys temically released by the plant during insect herbivore feeding and attract predatory and parasitic insects to the plant. In tomato, this response led to twice as many para sitoid larvae (Hyposoter exiguae) developing on tomato fruitworm (Spodoptera exigua) than in the control. There is a large range of volatiles released by different plant species; some attract a narrow range of natural enemies, others a broad range. If natural enemies can be attracted into a crop in this way ('attract') and their 'fitness' enhanced by the provision of appropriate selective nectar resources ('reward'), then the prospects for an 'attract and reward' pest management program will have been developed.
Attention is now greatly focused on not only the importance of the floral resource and the species on which it impacts, but also the importance of each species with regard to ecosystem function. Ecologists generally agree that higher biodiversity per se does not necessarily increase ecological function, so the role of particular species or groups of species is being investi gated. This will further refine research efforts to effectively maximize ecosystem services, including bio logical control.
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