Biological control of the bulb mite, Rhizoglyphus robini, by the predatorymite, 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 acule ifer, 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 r elease of predatory mites either slowed down the increase-as observed in th e field-or caused the bulb mites populations to decrease-as observed in the greenhouse. In all cases the population of predatory, mites increased as l ong 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 a nginal number released. Greenhouse experiments on intact lily bulbs in pots boxes and I m(2) plots with peat soil showed that when released in a ratio of I predator to 2 or 5 prey, the predatory mite, Hypoaspis aculeifer, sup pressed populations of bulb mites to less than 10 individual per bulb withi n 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 li hen released in ratio of I predator to I or 2 prey, the predatory mite, H. aculeifer, di d nor cause the population of bulb mires to decrease, but it did reduce the ir population growth. The initial predator-to-prey ratios required to achie ve suppression (ca 1:2) or elimination (3:1) in the sail environment are mu ch higher than those required for bulb mite elimination when lily bulb scal es were embedded in a medium of vermiculite (ca 1. 20). Among the possible causes are: (I) the initial losses of predators in the greenhouse and even more so in the field clue to mortality and/or emigration from the experimen tal plots; (2) the lower temperatures in the greenhouse and especially in t he field, which slow? clown the growth and predation processes and thereby delay prey extinction; and (3) the spatial complexity of the soil environme nt which creates refuges for the bulb mites.