San Jose scale - Natural enemies and diseases

A complex of natural enemies attack San Jose scale in New Zealand. However, they 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 San Jose scale numbers are not well understood. There is very little information on predation, and the highest recorded scale losses are from tiny wasps of the family Aphelinidae (species of Aphytis and Encarsia). Parasitism often exceeds 70% and may reach 100% in parts of the scale populations, but it varies considerably from tree to tree and over the growing season. Fungi have also been reported to kill up to 50% of San Jose scale insects on some occasions. Several fungal pathogens have been reported to reduce San Jose scale populations overseas, including Nectria flammea (=Sphaerostilbe coccophila) and Fusarium coccophilum, and other species of Nectria, Fusarium, Podonectria, Myriangium, and Peziotrichum.

The female sex pheromone of San Jose scale not only attracts males of the species for mating but also several species of parasitoid wasps. This assists the wasps in locating scale insect populations.

The known natural enemies in New Zealand are:

Aphytis mytilaspidis (Le Baron) is widespread and frequent in scale populations throughout New Zealand and has sometimes caused high mortality of San Jose scales. It is the most abundant parasitoid attracted to pheromone traps of San Jose scale in Otago and has also been trapped by this method in other regions. Mussel scale (Lepidosaphes ulmi), rather than San Jose scale, is a preferred scale host of this wasp. Adult Aphytis wasps also frequently feed on and kill scale insects as well as causing parasitism.

Aphytis diaspidis (Howard) is widespread but of low incidence and has been reported to parasitise only a small proportion of San Jose scale in Nelson. This species is attracted to San Jose scale pheromone traps. Adult Aphytis wasps also frequently feed on and kill scale insects as well as causing parasitism.

Encarsia citrina (Craw) has been the dominant parasitoid of San Jose scale in a number of studies and is common in Nelson, Hawkes Bay and northern North Island locations. It was the primary cause of 20-88% parasitism of San Jose scales at a Nelson apple orchard from 1968-70 and is commonly caught in San Jose scale pheromone traps in Hawkes Bay and Nelson.

Encarsia perniciosi (Tower) is a common parasitoid of San Jose scale in both South and North Island locations. In the Nelson study (see above), it increased in abundance from 1968 to 1970, suggesting that it may become the dominant species at that site. Almost the entire population was female. It was also the most common parasitoid attracted to San Jose scale pheromone traps in Nelson from 1978 to the early 1980s, in Hawkes Bay in the 1990's, and has been caught in these traps throughout New Zealand. This species is considered an important biological control agent of San Jose scale in many overseas countries.

Epitetracnemus zetterstedtii (Westwood) is a parasitoid wasp in the family Encyrtidae which is known to parasitise San Jose scale in New Zealand. It was probably introduced accidentally. Its importance in control of San Jose scale in New Zealand is unknown but it has been rarely reported.

Halmus chalybeus (Boisduval), the steelblue ladybird, has been recorded feeding on San Jose scale in New Zealand. Other ladybirds, such as the sevenspotted ladybird, Coccinella septempunctata, and the twospotted ladybird, Adalia bipunctata, are reported to be predators of San Jose scale overseas. Several other families of beetles are also reported to feed on San Jose scale overseas (e.g. soldier beetles, Cantharidae).

Hemisarcoptes malus (Shimer) is a minute predaceous mite which feeds on scale insects, including San Jose 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 San Jose scale overseas. This is not known to occur in New Zealand.

Predatory mites - several species of predatory mite 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 Anystidae, Cheyletidae, Pyemotidae, and Stigmaeidae mites. They have not been specifically observed to feed on San Jose scale.

Signiphora flavella Girault is a small wasp which is known to parasitise San Jose scale overseas. This species, and S. merceti (Malenotti), are important parasites of greedy scale, Hemiberlesia rapax, in New Zealand, and may also attack San Jose scale.

Thrips of the family Phlaeothripidae are known predators of San Jose scale in many countries (e.g. Leptothrips mali (Fitch) in California). They crawl 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 overseas. It may also attack San Jose scale but this is unknown at present.