The influence of the predaceous miteTyphlodromus (A.) potentillae (Garman) on the development ofPanonychus ulmi (Koch) on apple grown under various nitrogen conditions

Abstract

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.

Hits and Misses. The Ecological Approach to Pest Control in Orchards

“…The experiment was continued for one season, in the beginning of which the trees were inoculated with a known number of Panonychus ulmi and, after some time, with a certain number of phytoseiid predators. In contrast to Post's results, the predators proved able to reduce the spider mites to a low level within 8 weeks at all N-levels (Van de Vrie & Boersma, 1970 In view of this history, it was a matter of course that a test of the relevance of food quality in the effectiveness of spider mite predators was incorporated in the lay-out of long-term experiments on IPM in orchards started in 1965. For these experiments, aimed at the development of a practicable IPM-programme, the National Council for Agricultural Research made available a 12-ha fruit farm, De Schuilenburg, and funds to run it for 20 years.…”

“…The experiment was continued for one season, in the beginning of which the trees were inoculated with a known number of Panonychus ulmi and, after some time, with a certain number of phytoseiid predators. In contrast to Post's results, the predators proved able to reduce the spider mites to a low level within 8 weeks at all N-levels (Van de Vrie & Boersma, 1970). The question of what would happen in the long run, in orchards and in practice, remained open.…”