|
Mussel scale - Natural enemies and diseases |
|---|
A complex of natural enemies attacks mussel scale in New Zealand. Their effectiveness in limiting mussel scale populations has not been studied in detail. Throughout its range, mussel scale is often present at low levels on its various host plants and very high populations are encountered only occasionally, particularly on neglected apple trees. This may result from a combination of variable control by natural enemies and competition with the more aggressive scale insects, such as San Jose scale and oystershell scale. One parasitoid wasp species, Aphytis mytilaspidis, which is known to be critical in control of mussel scale overseas, is present in New Zealand. This is both a parasite and predator of mussel scale and is commonly encountered in New Zealand populations. This is probably the most important natural enemy here. Attempts were made to introduce to New Zealand the mite Hemisarcoptes malus, the most important predator of mussel scale in Canada, U.S.A. and Europe. This appears to have been unsuccessful. A wide range of other natural enemies are reported to attack mussel scale overseas, where populations are also known to be infected by several fungal pathogens, notably Verticillium lecanii and Aspergillus depauperatus.
The known and possible natural enemies in New Zealand are:
Aphytis diaspidis (Howard) is recorded as a parasitoid of mussel scale in Europe. This species is known to attack San Jose scale in New Zealand but has not been reported from mussel scale.
Aphytis mytilaspidis (Le Baron) is widespread and frequent in armoured scale populations of various species throughout New Zealand and is the dominant parasitoid attacking mussel scale. Mussel scale is a preferred scale host of this wasp, although it is also listed as an important parasitoid of oystershell scale overseas. Canadian research has shown that A. mytilaspidis is one of two key factors which regulate mussel scale populations at low levels (the other is the predatory mite Hemisarcoptes malus). Adult A. mytilaspidis wasps regularly feed on and kill all stages of mussel scale, except the mobile crawlers, as well as causing parasitism of second stage scales, young and mature adults, and egg-laying females. Where the wasp has parasitised a female that has already laid eggs, the developing wasp larva will eat many of the eggs as well as feeding on the mother scale.
Encarsia citrina (Craw) is known to parasitise mussel scale in New Zealand, as it does commonly overseas. The levels of mortality caused by this parasite in New Zealand are unknown but it is not regarded as a key natural enemy of the species in Europe or North America.
Epitetracnemus zetterstedtii (Westwood) is a parasitoid wasp in the family Encyrtidae which is known to parasitise mussel scale in New Zealand. It was probably introduced accidentally. Its importance in control of mussel scale in New Zealand is unknown but it has been rarely reported.
Eusemion cornigerum (Walker) is listed as another encyrtid natural enemy of mussel scale in Europe. It was probably introduced to New Zealand accidentally and has been collected at altitudes from 600 to 1700 m. It has not been recorded from mussel scale in New Zealand.
Habrolepis dalmannii (Westwood) is considered an important parasitoid (Encyrtidae) of mussel scale in some parts of the world. It was introduced to New Zealand for control of the golden oak scale, Asterolecanium variolosum (Ratzeburg) and this was a spectacular success. It has not been reported from mussel scale in New Zealand.
Hemisarcoptes malus (Shimer) is a minute predaceous mite which feeds on mussel scale, as well as its close relatives San Jose scale and oystershell scale. This was the first mite species ever used in a biological control programme, when it was successfully introduced from New Brunswick to western Canada in 1916-17 to control the mussel scale. 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. H. malus in all stages of development feeds on eggs, nymphs and adults of mussel scale. It appears to prefer to feed on the eggs, which it does through the winter. The eggs become brown and shrivelled and break into pieces. Up to 8 mites may be found under a single female scale cover. The mite itself lays eggs under the scale cover and these are tiny, white and oval, about half the size of the mussel scale eggs. H. malus can tolerate temperatures as low as -34oC. These hemisarcoptid 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 overseas. This may occur on mussel scale 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 (such as mussel scale) overseas. However, ladybirds appear to be more effective as predators of scale insects whose scale covering is softer than those of mussel scale and there is no information available on ladybird predation of mussel scale in New Zealand.
Predatory mites - The whirligig mite, Anystis baccarum, is known to be a common predator of mussel scale overseas and is frequently encountered in New Zealand orchards. Predatory mites of the family Phytoseiidae ('typhs') are known to feed on the crawlers of scale insects. This family includes Typhlodromus pyri, an important predator which is sometimes abundant in New Zealand apple orchards. Typhs 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 mussel scale. A mite has recently been found under mussel scales in winter in Otago and is associated with scale eggs which are shrivelled, suggesting predation.
Thrips of the family Phlaeothripidae are known predators of scale insects in many countries, and this includes mussel 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, and this species is known to attack mussel scale. It is particularly common in Otago orchards.
Zaomma lambinus (Walker) is a parasitoid wasp in the family Encyrtidae which is known to parasitise mussel scale in New Zealand. Its importance in control is unknown but it has been rarely reported. It may not be beneficial because this species parasitises the parasites (a hyperparasite) of several scale species overseas.