o.~)2 M. VAN DE VR[E ~% A. BOERS~' VIAwere present, but they were unable to prevent increase in P. ubmi density, although a numerical response to the prey density was indicated.]:) UTMAN (1964), however, demonstrated for peach orchards in Ontario that the food quality was not the sole factor for a lower density of P. ulmi in unsprayed orchards. PUTMAN & HER~'E (1966) are of the opinion that, if the predators are left unaffected by cultural measures, they are able to keep P. ulmi at extremely low levels.As P. ulmi is a potential pest in apple orchards in the Netherlands, studies were conducted on the influence of predaceous mites on the population development of this species (wt.~ DE VRIE, 1965; VAN DE VRIE & KROPCZYNSK,% 1965). They showed that under glasshouse conditions Typhlodromus potentillae was able to reduce the density of P. ulmi considerably. These experiments were conducted in a greenhouse and the host plant conditions were uniform. Therefore it seemed worthwhile to conduct similar experiments under semi-field conditions, at the same time introducing a variation in the fertilization of the fruit trees. The aim was to see whether predaceous mites under optimal conditions for the reproduction of their prey, could exert a reducing influence on the population development of the prey.
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