Impact on soil-dwelling arthropods in citrus orchards of spraying horticultural mineral oil, carbaryl or methidathion

Impacts of an nC24 horticultural mineral oil (HMO) and two synthetic insecticides (carbaryl and methidathion) on the abundance and species diversity of soil-dwelling arthropods were evaluated in two citrus orchards in the coastal region of New South Wales, Australia. In the first orchard, mature Valencia orange trees were sprayed in summer with one of HMO, carbaryl or methidathion delivered at low (2000 L/ha) or high (10 000 L/ha) volumes; the sprays were applied either once (February) or twice (December and February). HMO had no significant impact but the synthetic insecticides, irrespective of spray frequency or spray volume, significantly reduced the abundance and species diversity of the arthropods including springtails and generalist predators, such as spiders, staphylinid beetles and mesostigmatid mites. In the second orchard, blocks of Washington navel and Valencia orange trees were sprayed with either HMO or methidathion; sprays were applied twice, 2 weeks apart, in late summer (February), first at a rate of 6500 L/ha and second at 5500 L/ha. Results were similar to those in the first orchard, except that the effect of methidathion on carabid beetles and ants was not significant.