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Biological Control of the Bulb Mite, Rhizoglyphus robini , by the Predatory Mite, Hypoaspis aculeifer , on Lilies: Predator-Prey Dynamics in the Soil, under Greenhouse and Field Conditions

We tested the capacity of the soil-dwelling predatory mite, Hypoapsis aculeifer , to control mites attacking lily bulbs. Experiments in the greenhouse and in the field showed that in the absence of predatory mites populations of the bulb mite, Rhizoglyphus robini , on lily bulbs increased, whereas the release of predatory mites either slowed down the increase - as observed in the field - or caused the bulb mites populations to decrease - as observed in the greenhouse. In all cases the population of predatory mites increased as long as bulb mite densities were not too low. However, within the first week after predator release there was usually a sharp decline to 10-40% of the original number released. Greenhouse experiments on intact lily bulbs in pots, boxes and 1 m 2 plots with peat soil showed that when released in a ratio of 1 predator to 2 or 5 prey, the predatory mite, Hypoaspis aculeifer , suppressed populations of bulb mites to less than 10 individual per bulb within 6 weeks. Elimination of bulb mites was observed only when the predator-to-prey ratio at release was equal to 3:1. Field experiments in 2 m 2 plots with intact bulbs in rather compact sandy soil showed that when released in ratio of 1 predator to 1 or 2 prey, the predatory mite, H. aculeifer , did not cause the population of bulb mites to decrease, but it did reduce their population growth. The initial predator-to-prey ratios required to achieve suppression (ca 1:2) or elimination (3:1) in the soil environment are much higher than those required for bulb mite elimination when lily bulb scales were embedded in a medium of vermiculite (ca 1:20). Among the possible causes are: (1) the initial losses of predators in the greenhouse and even more so in the field due to mortality and/or emigration from the experimental plots; (2) the lower temperatures in the greenhouse and especially in the field, which slow down the growth and predation processes and thereby delay prey extinction; and (3) the spatial complexity of the soil environment which creates refuges for the bulb mites.