Pest Risk Analysis for Stone Fruit from New Zealand into Western Australia

4.2.2.1.3 Oystershell scale

The scale examined in this pest risk analysis is:

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  Diaspidiotus ostreaeformis (Curtis) Borchsenius [Hemiptera: Diaspididae] – oystershell scale
  

Probability of importation

The likelihood that oystershell scale (OSS) will arrive in Western Australia with the importation of stone fruit from New Zealand: Low.

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  OSS is reported on stone fruit in the southern regions of New Zealand (McLaren et al., 1999).
  
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  OSS mostly infects the bark on the stems and branches of the host trees. Sometimes it can be found on fruit, where it causes red spots (CABI, 2004).
  
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  OSS is typically found in protected sites such as crevices, which provide refuge for the scale insects from predators and pesticides (Ker & Walker, 1990).
  
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  Eggs are laid on the stems or branches and after hatching, crawlers may settle on the bark or fruit (McLaren et al., 1999).
  
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  OSS produces one generation per year and during harvesting time, all stages of the scale are present. Crawlers are the only mobile stage that could contaminate clean fruit by moving from infested fruit.
  
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  Post-harvest grading, washing and packing procedures are likely to reduce the number of OSS on the fruit. OSS is usually found on the fruit surface near the stem-end and may not be detected during pre-export inspection.
  

The likelihood that OSS will be distributed to the endangered area as a result of the processing, sale or disposal of stone fruit from New Zealand: Low.

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  Adults or immature forms may remain on the surface of the fruit during distribution via wholesale or retail trade.
  
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  Distribution of the commodity in Western Australia would be for retail sale, as the intended use of the commodity is human consumption. Waste material would be generated.
  
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  Adults or immature forms are likely to survive storage and transport and thus be associated with waste material.
  
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  Disposal of waste material could occur near hosts.
  
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  The only stages likely to move off of the importation pathway are adult males or crawlers. Crawlers, after hatching from their eggs, move for a short time and affix to the host (Ker & Walker, 1990). Adult males are winged, fragile and short-lived and only persist for a few days.
  
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  Most crawlers are reported to move only a short distance before inserting their mouthparts into host material.
  
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  Adult females are immobile and would need to be carried onto hosts by vectors such as other insects or people. However, forced removal of female scales that are attached to the plant by their mouthparts is likely to injure or kill the scale.
  
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  The natural dispersal mechanism that allows the movement of scale species from discarded fruit waste to a suitable host is a significant limiting factor. Scales have a limited ability to disperse independently from the fruit pathway.
  

The likelihood that OSS will enter Western Australia as a result of trade in stone fruit from New Zealand and be distributed in a viable state to the endangered area: Very low.

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  The overall probability of entry is determined by combining the probabilities of importation and distribution using the matrix of ‘rules’ for combining descriptive likelihoods (Table 2).
  

The likelihood that OSS will establish based on a comparative assessment of factors in the source and destination areas considered pertinent to the ability of the pest to survive and propagate: High.

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  OSS has a wide host range, mainly on deciduous trees. Host plants have been reported from 41 genera in 18 families. These hosts are widespread in Western Australia.
  
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  OSS is widely distributed in Palaearctic and Nearctic regions and has been introduced into Australia, Argentina, Canada and New Zealand (Brookes & Hudson, 1969; Kozár, 1990). Modelling studies in Western Australia suggest that there are regions within Western Australia suitable for the establishment of this pest.
  
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  Although the precise climate tolerance of scales is unknown, they are considered to be tropical or subtropical pests, and are therefore less likely to establish in either cool or hot and dry climates.
  
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  Females release sex pheromones during the day when males are active attracting the winged males for mating. Females have a high fecundity and can lay 100 to 200 eggs. A population can be started from these eggs.
  
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  Populations of OSS are kept under control in its native range by the presence of a large number of parasitoids. Most of these parasitoids are not present in areas where OSS has been introduced resulting in inadequate natural regulation and subsequent outbreaks.
  
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  Existing control programs may be effective for some hosts (e.g. broad spectrum pesticide applications) but not necessarily all hosts.
  

The likelihood that OSS will spread based on a comparative assessment of those factors in the source and destination areas considered pertinent to the expansion of the geographical distribution of the pest: Moderate.

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  OSS is reported from Victoria, South Australia and Tasmania but is absent from Western Australia. There are similar environments in Western Australia that would be suitable for its spread.
  
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  Commercial fruit crop hosts of OSS are grown in southwestern Western Australia but there are natural barriers between some districts that could limit the natural movement from one district to another.
  
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  OSS has limited independent dispersal capabilities. Long distance dispersal is through wind dispersal (Ben-Dov, 1994) or infested host material (Beardsley & Gonzalez, 1975). Interstate quarantine controls are in place on the movement of nursery stock. However, these controls would have no effect on the spread of OSS within Western Australia.
  
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  Adult males are winged and are capable of short flights. Male dispersal by crawling or flight is strongly affected by the location of females and their production of sex pheromones.
  
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  Natural enemies that attack OSS in New Zealand include parasitic wasps, several predatory mites including Hemisarcoptes malus and a ladybird of the genus Rhyzobius (CABI, 2004). Several species of Rhyzobius occur in Western Australia.
  

The overall likelihood that OSS will enter Western Australia as a result of trade in stone fruit from New Zealand, be distributed in a viable state to suitable hosts, establish in that area and subsequently spread within Western Australia: Very low.

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  The probability of entry, establishment or spread is determined by combining the probabilities of entry, of establishment and of spread using the matrix of ‘rules’ for combining descriptive likelihoods (Table 2).
  

Consequences (direct and indirect) of OSS: Low