San Jose scale - biological control

There are numerous examples of scale insect outbreaks due to the destruction of their natural enemies by pesticides or interference by dust. San Jose scale is an exotic pest which is attacked by a complex of mainly exotic natural enemies. The more important of these are tiny wasps of the family Aphelinidae: Encarsia citrina, Encarsia perniciosi, Aphytis diaspidis, and Aphytis mytilaspidis. These arrived in New Zealand primarily by accident, probably with scale-infested plant material. Limited attempts have been made to introduce and establish further strains of some of these wasps, as well as other natural enemies, in classical biological control programmes. The presence of these natural enemies contributes to control of San Jose scale but does not prevent this pest from causing severe damage to unsprayed apple trees and reaching high densities on a range of other host plants. There remains considerable scope for further introduction of natural enemies, particularly parasitoid wasps. The use of these wasps has been successful in reducing populations of San Jose scale and related scale insects on a variety of crops in other parts of the world. Attempts have also been made overseas to rear and mass release E. perniciosi for the control of San Jose scale in orchards. The more important wasps which attack San Jose scale are attracted by the sex pheromone of the females and their presence/activity in orchards can be monitored with pheromone traps.

San Jose scale is a quarantine pest with extremely low tolerance on export fruit. Biological control with natural enemies is unlikely to reduce scale insects to such extremely low levels, and other supplementary controls (e.g. oil sprays) may always be needed. However, improved biological control could make an enormous contribution to control of San Jose scale by reducing the large reservoirs of the pest on its numerous host plants outside orchards. These include many shelter tree species from which scale crawlers disperse into the orchard in summer.

Fungal diseases of scale insects may be another useful source of biological control agents and up to 50% fungal mortality has been reported in populations of San Jose scale.