Habitat diversity: Implications for the conservation and use of predatory insects of Helicoverpa spp. in cotton systems in Australia

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Abstract
Adoption of within-field monocultures of annual crops in modern farming systems is known to discriminate against and reduce the activity of predatory insects. In Australia, cotton fields are strictly monoculture and lack ecological diversity which could be the major cause of pest problems in the cotton agroecosystem. The utility of crops such as sunflower (Helianthus annuus), safflower (Carthamus tinctorius), sorghum (Sorghum bicolor), lucerne (Medicago sativa) and tomato (Lycopersicon esculentum) as refugia for predatory insects of Helicoverpa spp., when they were planted as strips in cotton fields, was evaluated from 1993 to 1995. Densities of beneficial insects, mainly predatory beetles, bugs and lacewings, were higher in lucerne crops than any other crop tested. In an experiment where lucerne was planted in strips within commercial cotton, the number of predators was highest in the lucerne strip and declined with increasing distance from the lucerne strip to reach their lowestlevel300 m away in the cotton crop. At the end of the study, 7.1, 6.1, 5.8 and 1.5 times more predatory beetles, bugs, lacewings and spiders per metre row respectively were recorded on the lucerne strips than on cotton. When lucerne was interplanted with commercial cotton to compare densities of predators on cotton with and without lucerne strips, 2.1, 2.5 and 1.2 times more predators were recorded on cotton with lucerne strips than cotton without lucerne strips. This indicated that the lucerne strips can serve as a refugium for predators which can then be moved into cotton fields. This study supports the findings of previous researchers that increased habitat diversity in monocultural crops by strip-cropping can increase population density of indigenous predators.

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