Résumé
The predacious phytoseiid mite Neoseiulus fallacis (Garman) is an important agent for the biological control of spider mites in deciduous fruit orchards in North America and Canada. It would be helpful to monitor the fate of released individuals to improve the results of introductions of the predators in biological control trials. We have used two types of genetic markers, pyrethroid resistance and allozymes, for indirect estimation of the survival of N. fallacis introduced in an apple orchard in Ontario, Canada. Mite samples were submitted to toxicological tests. The polymorphism of four enzymes was studied in individual females using an isoelectric focusing technique. A mite sample was taken from the field, mass-reared in the laboratory, and selected for permethrin resistance. This strain was released on several apple trees treated with permethrin, and mite samples were collected from the same trees 10 to 90 days later. The genetic composition and the insecticide resistance level of this sample were compared to those of two other samples from trees where mites had not previously been released, either in the same orchard or in a neighboring block. A control susceptible strain was compared using mites collected earlier from trees on the same site but outside the present experiment. The mites collected from the release trees and those from the strain used for the releases were found to be genetically closely related, as judged from a small genetic distance, and from similar levels of insecticide resistance in both samples. The control samples from the nonrelease trees were genetically distant from these and displayed low resistance levels. These results indicate that the released genotypes established and persisted in the release trees for the period of the experiment. The utility of the two approaches in assessing the fate of released natural enemies is discussed.