Oystershell scale - Natural enemies and diseases

A complex of natural enemies attack oystershell scale in New Zealand. However, these beneficial species are not sufficient to prevent populations of this scale species from increasing to high levels and causing damage to unsprayed apple and pear trees. The contributions of the different predators and parasites in reducing oystershell scale numbers are poorly understood. The highest recorded scale losses are from tiny wasps of the family Aphelinidae (species of Aphytis and Encarsia). Parasitism was found to average 40% from July to December in a Canterbury study, and reached a peak of 85-100% in October and November. The most important species was Encarsia citrina. Lower levels of parasitism by this species have been recorded in Otago where the dominant parasitoid was Aphytis mytilaspidis. Predation by a camerobiid mite, Neophyllobius sp., was considered common in the Canterbury research. A wide range of natural enemies are reported to attack oystershell scale overseas, where populations are also known to be infected by several fungal pathogens, notably in the genus Nectria.

The female sex pheromone of San Jose scale not only attracts males of that species for mating but also several species of the parasitoid wasps which attack both San Jose scale and oystershell scale. It is probable that the pheromone of oystershell scale (which has yet to be identified) is also used by its parasitoid wasps to locate oystershell scale populations

The known and possible natural enemies in New Zealand are:

Aphytis mytilaspidis (Le Baron) is widespread and frequent in armoured scale populations of various species throughout New Zealand and is the dominant parasitoid attacking oystershell scale in Otago. It is also the most abundant parasitoid attracted to pheromone traps of San Jose scale in Otago. Mussel scale (Lepidosaphes ulmi), rather than oystershell scale, is a preferred scale host of this wasp although it is also listed as an important parasitoid of oystershell scale overseas. Adult Aphytis wasps also frequently feed on and kill scale insects as well as causing parasitism.

Encarsia citrina (Craw) was the dominant parasitoid of oystershell scale in a study in Canterbury from 1959-60, causing a maximum of 85-100% parasitism in October/November. Low levels of parasitism by this species were reported in Otago in the 1980s and monitoring with San Jose scale pheromone traps in Otago in the 1990s caught only very low numbers of this species.

Epitetracnemus zetterstedtii (Westwood) is a parasitoid wasp in the family Encyrtidae which has been recovered from Central Otago parasitising oystershell scale. It was probably introduced accidentally. Its importance in control of oystershell scale in New Zealand is unknown but it has been rarely reported.

Hemisarcoptes malus (Shimer) is a minute predaceous mite which feeds on scale insects, including San Jose scale, a close relative of oystershell scale. This was the first mite species ever used in a biological control programme, mainly against the mussel scale, Lepidosaphes ulmi L.. For New Zealand, it was imported to Nelson from Nova Scotia in 1967. Although it persisted at several release sites for a few years (on oystershell scale and black scale), its present status is uncertain and it is not known to have established in New Zealand. These mites are dispersed by hitching a lift (phoresy) on ladybird beetles of the genus Chilocorus, and the absence of these ladybirds in New Zealand may have contributed to the poor or failed establishment of H. malus.

Lacewings are reported to feed on scale insects of the genus Quadraspidiotus overseas. This may occur in New Zealand but has not been reported.

Ladybirds, such as the sevenspotted ladybird, Coccinella septempunctata, and the twospotted ladybird, Adalia bipunctata, are reported to be predators of armoured scale insects in the genus Quadraspidiotus overseas. However, these ladybirds appear to be more effective as predators of scale insects whose scale covering is softer than those of oystershell scale. On the other hand, ladybirds of the genus Rhyzobius are recognized as predators of the armoured scale insects. A species of Rhyzobius has been observed feeding on oystershell scale in Central Otago.

Predatory mites - Neophyllobius sp. is a mite of the family Camerobiidae which was found to be a common predator of oystershell scale populations in Canterbury in 1959-60. It was observed to feed on adult female scales in the laboratory, and crawlers of other scale species are known to be attacked by this predator. The same researcher also observed the whirligig mite, Anystis sp., feeding on adult female oystershell scales on apple bark. Predatory mites of the family Phytoseiidae ('typhs') are known to feed on the crawlers of scale insects. This includes Typhlodromus pyri, an important predator found in New Zealand orchards. These predators feed primarily on mites and are described more fully in the sections on European red mite and twospotted spider mite. Several other families of predatory mites are known to feed on scale insects and are found associated with scale insects in New Zealand. These include Cheyletidae, Pyemotidae, and Stigmaeidae mites. They have not been specifically observed to feed on oystershell scale. The stigmaeid, Eryngiopus nr. nelsonensis occurs commonly among oystershell scale colonies in Central Otago and is probably a useful predator.

Thrips of the family Phlaeothripidae are known predators of scale insects in many countries, and this includes oystershell scale. They move under the scale cover and feed on eggs or crawlers. A predatory species which occurs in New Zealand is Haplothrips kurdjumovi Karny, which is known here as a mite feeder but also attacks mussel scale. It is particularly common in Otago orchards. A close relative, Haplothrips subtilissimus (Haliday) attacks oystershell scale in Europe, and it is therefore possible that H. kurdjumovi feeds on this pest in New Zealand.

Zaomma lambinus (Walker) is a parasitoid wasp in the family Encyrtidae known to occur in New Zealand. This species is listed among the more important parasitoids of oystershell scale overseas but its importance in New Zealand is unknown.