Mussel scale - biological control

Mussel scale is a pest which is well controlled by natural enemies in many parts of the world. It came to New Zealand from Europe and is attacked by some of the exotic natural enemies which control its populations overseas. The most important of these is a tiny wasp of the family Aphelinidae, Aphytis mytilaspidis, assisted by a close relative Encarsia citrina. 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 the parasitoid wasps, as well as other natural enemies, for the control of our introduced scale pests in classical biological control programmes. These programmes have incidentally assisted control of mussel scale but accidental introductions of parasitoids of mussel scale take most of the credit for biological control of mussel scale in New Zealand. In addition to the two main species mentioned above, several encyrtid wasps have arrived here by accident. A predatory mite, Hemisarcoptes malus, has been shown to be a key natural enemy controlling mussel scale in North America and Europe. Attempts to introduce this mite to New Zealand in the 1960s have apparently failed, despite initial establishment on oystershell scale and black scale in Nelson. There remains scope for further introduction of natural enemies, including H. malus. These natural enemies cause high mortality of mussel scale in Canada, U.S.A. and Europe. The more important wasps which attack mussel 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.

Biological control of mussel scale in New Zealand appears to be reasonably effective because populations do not routinely reach high densities (unlike San Jose scale and oystershell scale). However, outbreaks do sometimes occur and improved biological control would assist control of mussel scale in organic orchards and reduce the 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.