Some effects of pre-release host-plant on the biological control of Panonychus ulmi by the predatory mite Amblyseius fallacis

Amblyseius fallacis Garman has been selected for pyrethroid resistance and mass reared for experimental release as a biological control agent for tetr anychid mites on a number of crops in Canada. Several releases of this pred ator onto apple and peach trees have failed to result in the establishment of A. fallacis, or in the biological control of Panonychus ulmi Koch. Here, we test the hypothesis that the change of host-plant at the time of releas e is a critical factor in the establishment of A. fallacis for biological c ontrol of P. ulmi. Functional and numerical response studies were undertake n on two populations of A. fallacis: a wild strain collected from the canop y foliage of an apple orchard near Vineland, Ontario; and a second strain r eared on bean plants in a commercial insectary with Tetranychus urticae as prey. Each population consumed significantly more P. ulmi and produced sign ificantly more eggs when on leaf disks from the plant species they were rea red on, than on leaf disks from the novel host plant. A further experiment was conducted to determine if establishment and biological control of mass- reared A. fallacis could be affected by rearing a population for a short te rm on apple leaves prior to release on apple trees. Three release treatment s were made into potted apple trees in a glasshouse, using predators commer cially mass-reared on bean and T. urticae: A. fallacis released directly; A . fallacis reared in the laboratory for four weeks on bean and T. urticae; A. fallacis reared on apple leaves and T. urticae for four weeks. They were compared with a control treatment lacking predator release. Contrary to re sults of the functional and numerical response studies, no difference was o bserved between release treatments. All release treatments adding A. fallac is resulted in a similar, if limited, degree of biological control of P. ul mi. These results indicated that there may be short-term effects of host pl ant on the establishment of A. fallacis and biological control of P. ulmi, which in our study were observed as an initial reduction of the predatory r esponse. However, in a rest, the predators appeared to overcome these short -term effects and successfully established on the new host-plant to control P. ulmi.