The ecology of Tetranychid mites in Australian orchards.
Three important questions concerning the basic cause of the injuriousness of Tetranychid mites in Australian commercial orchards are whether insecticides cause the mite problem, whether they stimulate the mites or kill predators and, which natural enemies are important and in what way. The first two questions were answered with respect to Tetranychus urticae Koch and Bryobia rubrioculus (Scheuten) in 1968-9 by sampling a series of paired trees in apple orchards throughout south-eastern Australia. One tree of each pair was sprayed with 0.1% DDT and the other was not. DDT was shown to cause an increase in the abundance of both mites through its lethal effect on predators. The possibility that the DDT was acting by stimulating the mites rather than by removing the predators was excluded by the experimental design and by tests on the fecundity of T. urticae. Predation by Stethorus spp. was the most important factor maintaining T. urticae at low density on the unsprayed trees, whereas the predatory Mirid Campylomma livida Reut. was important with B. rubrioculus. Observations in Tasmania also indicated that Stethorus spp. were important in controlling Panonychus ulmi (Koch). The interaction between Stethorus and T. urticae was described by a simple mathematical model derived from the field data of 1968-69 and 1969-70. This model was tested in 1970-71 by removing Stethorus temporarily from a 5-acre orchard at Canberra and applying two malathion sprays. The subsequent rates of increase and peaks of abundance of both predator and prey in each of three different sections of the orchard were well predicted by the model. Also, the experiment demonstrated the numerical response of Stethorus to prey density, the downwind dispersal of adult beetles and the effectiveness of the beetles in controlling T. urticae at very low density if undisturbed by insecticides. The model served to illustrate the dynamic nature of the orchard ecosystem and showed how small disturbances in the level of either predator or prey lead to large increases in abundance later. The practical significance of the results is discussed in terms of minimising the impact of insecticides on predators by using either fewer, more selective chemicals or resistant predators.