The Seasonal Abundance and Impact of Predatory Arthropods on 'Helicoverpa' Species in Australian Cotton Fields

Abstract

The problems of assessing the predatory impact of one species of arthropod upon another have been addressed for over 50 years, and still remain largely unsolved. The literature is reviewed and principles derived from it are applied to the case of generalist arthropod predators on 'Helicoverpa' spp. (Lepidoptera: Noctuidae) in Australian cotton. Theoretical aspects discussed include the validity of functional and numerical responses as indicators of field predation rates and some current ideas on the survival behaviour of predatory insects, along with their implications for the desirable properties of biological control agents. The production of cotton in Australia relies heavily on the use of broad spectrum insecticides to control two key pests, 'Helicoverpa punctigera' (Wallengren) and 'Helicoverpa armigera' (Hubner). The potential loss of two of these insecticide groups, the synthetic pyrethroids and endosulfan, because of resistance in 'H armigera' and environmental concerns respectively, has rekindled the search for ways of utilising alternative sources of pest mortality. An appealing possibility is that endemic predators and parasites can maintain pest populations below economic thresholds. However the extensive use of insecticides throughout the history of Australian cotton production means that the potential of endemic natural enemies to control 'Helicoverpa' spp. is not normally realised. Trials using realistic field sizes were being conducted by Auscott Pty. Ltd. (a large corporate farming group at 'Midkin', near Moree, N.S.W.) to broadly assess the prospects of growing cotton without using endosulfan and synthetic pyrethroids. This presented an opportunity to examine the predatory impact of arthropod populations which could reasonably be expected to increase under these 'softer' insecticide strategies. Pioneering efforts in 1993 by Coulton Farming (based at Goondiwindi and North Star, N.S.W.) to grow organic cotton provided further opportunities to examine predator populations under reduced insecticides practices in cotton fields isolated from regional insecticide drift. The experimental sections of this thesis report the abundance of all the arthropods, especially predators, collected in suction samples from the different treatments used at these farms. The treatments included: no insecticides, organically certified treatments, perceived softer insecticide options (essentially avoiding endosulfan, synthetic pyrethroids and organophosphates) and conventional broad spectrum insecticides.