A reconsideration of the role of predacious mites in the control of the european red spider mite in orchards (*)

A reconsideration of the role of predacious rnites 111 the control of the european red spider mite in orchards (*)The ability of Typhlodromus-mites, occurring in orchards ( of which T. pyri Scheuten is the best known) to regulate the numbers of the European Red Spider Mite (Panonychus ulmi Koch) still forms a matter of controversy in acarological literature.The reactions of these predators seem to be, indeed, far from evident.PosT (1962) was confronted with this problem in her work in experimental orchards over many years.She observed that in the unsprayed plots of the orchards, in which soil cultivation, fertilization, and pruning were applied, predation of mites and insects were insufficient to reduce the increased development of P. ulmi.On the other hand, an increase in the population density of the European Red Spider Mite also was observed after the application of chemical control only in the neglected orchard, but, as she stated, it could be seen clearly that the effect of chemical control on the excessive increase of P. ulmi was less important than that of cultivation, fertilization and pruning.The work of PosT made clear, therefore, that the difference in reactions found in << neglected )) and << well kept )) orchards only partly can be ascribed to the use of sprays.We recognize here an intriguing problem in the field of population dynamics.The rate of reproduction of P. ulmi is higher on the leaves from pruned, fertilized and cultivated trees (PosT Le.) which leads to an increase in numbers, but the predators evidently appear to lack their regulatory capacity under certain conditions.It is easily understood that these complex interactions gave rise to different opinions among the research workers.CHANT (1959) (*) Prof. J. D. LATTIN was so kind as to check the English text of this paper.