Oystershell 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. Oystershell scale is an exotic pest which is attacked by a group of mainly exotic natural enemies. The more important of these are tiny wasps of the family Aphelinidae: Encarsia citrina, and Aphytis mytilaspidis. These arrived in New Zealand primarily by accident, probably with scale-infested plant material. A camerobiid mite, Neophyllobius sp. is reported to be the most important predator of oystershell scale in New Zealand. Limited attempts have been made to introduce and establish further strains of some of the parasitoid wasps, as well as other natural enemies, for the control of San Jose scale in classical biological control programmes. These programmes have incidentally assisted classical biological control of oystershell scale but this does not prevent it 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. These natural enemies are reported to cause high scale mortality in Europe such that oystershell scale can "seldom multiply excessively". The more important wasps which attack oystershell scale in New Zealand are attracted by the San Jose scale sex pheromone and their presence/activity in orchards can be monitored with these pheromone traps.

Oystershell scale is so similar to San Jose scale that it is treated as 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 oystershell 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.