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

4.2.2.1.2 Citrophilus mealybug

The mealybug examined in this pest risk analysis is:

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  Pseudococcus calceolariae Maskell [Hemiptera: Pseudococcidae] – citrophilus mealybug.
  

Probability of importation

The likelihood that citrophilus mealybug will arrive in the PRA area with the importation of stone fruit from New Zealand: High.

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  Citrophilus mealybug has been recorded on nectarines and plums in New Zealand (Charles, 1993; McLaren et al., 1999).
  
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  Mealybugs feed on phloem sap from the stem and fruit. They are typically found in protected sites such as crevices on branches or in the stem end of the fruit (McLaren et al., 1999).
  
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  Honeydew, the waste product of the mealybug feeding process, is a perfect growth medium for sooty mould fungi (Hely et al., 1982). Fruit with sooty mould may be detected during pre-export inspections.
  
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  Post-harvest grading, washing and packing procedures may remove this pest from the fruit. However, mealybugs often favour cryptic habitats, such as the stem end of fruit, and may remain with the fruit.
  
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  Citrophilus mealybug can survive packinghouse procedures. AQIS inspectors have intercepted citrophilus mealybug on peaches from New Zealand. Numerous other interceptions of mealybugs, including Pseudococcus spp., are recorded (PDI, 2003).
  

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

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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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  Mealybugs are likely to survive cold storage and transportation i.e. Pseudococcus affinis can survive up to 42 days storage at 0°C (Hoy & Whiting, 1997).
  
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  Disposal of waste material could occur near hosts.
  
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  Citrophilus mealybug may enter the environment as adults discarded with fruit or as juveniles blown by wind or carried by other vectors.
  
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  Mealybugs are mobile at all life stages. Crawlers are mobile while adults are slow-moving (Smith et al., 1997).
  
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  The natural dispersal mechanism that allows the movement of mealybugs from discarded fruit waste to a suitable host is a significant limiting factor. Mealybugs have a limited ability to disperse independently from the stone fruit pathway.
  
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  Adult females are wingless and would need to be carried onto hosts by vectors such as other insects or people. Adult females can only crawl a few metres, restricting their ability to move from discarded fruit waste to a suitable host.
  
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  Because citrophilus mealybug is polyphagous and its life stages have limited mobility, it is possible it could be transferred to a susceptible host.
  

The likelihood that citrophilus mealybug 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: Moderate.

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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 citrophilus mealybug 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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  Citrophilus mealybug is native to eastern Australia and now also occurs in the USA, South America, New Zealand, South Africa and Europe (Smith et al., 1997).
  
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  Citrophilus mealybug is a highly polyphagous species that has been recorded on 40 plant families (Ben-Dov, 1994), including many commercial and nursery plants such as apple, pear, grape, stone fruit, potato, hibiscus and rose. These hosts are widespread in Western Australia.
  
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  The rate of development of citrophilus mealybug is temperature dependent. There is a minimum threshold temperature for each particular species of mealybug, below which development either ceases totally or is slowed significantly. There is also a maximum threshold temperature, beyond which development is slowed significantly or ceases all together. If temperatures remain elevated for prolonged periods, insect mortality increases rapidly with a consequent crash in population size.
  
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  Mild to warm conditions are most favourable with temperatures of about 25°C and a high relative humidity being optimum for mealybug development. In Australia, mealybug populations reach peaks in spring and autumn.
  
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  Mealybugs have high reproductive rates with multiple generations in a year (Smith et al., 1997). Mated females commonly move to a protected site to lay eggs over a period of up to 2 weeks. Females lay approximately 500 eggs within a cottony sac. Females cease feeding before egg laying and die at the end of egg laying. A population can be started from these eggs.
  
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  Existing control programs may be effective. Control strategies are already in place as Western Australia has several economically important mealybug species. These existing control strategies would minimise the impact of citrophilus mealybug within Western Australia. Biological control agents are available that provide control of citrophilus mealybug.
  

The likelihood that citrophilus mealybug 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: High.

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  Citrophilus mealybug has limited independent dispersal capabilities. The long distance dispersal of this pest requires the movement of nymphs and adults on infested host material, such as fruit and nursery stock.
  
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  There are no intrastate quarantine controls in place in Western Australia on the movement of nursery stock.
  
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  Commercial fruit crop hosts of citrophilus mealybug are grown in south-western Western Australia and there are natural barriers between some districts. It would be difficult for the mealybugs to disperse from one district to another by natural means.
  
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  Female mealybugs do not have wings and are therefore limited in their ability to disperse. However, the spread of this pest would be aided if other host plants occurred between the commercial fruit orchards in different districts of Western Australia.
  
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  Short distance dispersal of juveniles could occur through the movement of crawlers in wind currents or as contaminants on biological or mechanical vectors (Williams, 1996).
  
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  Adult males are winged, capable of short flights and are short lived. 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 of the citrophilus mealybug, such as Cryptolaemus montrouzieri and parasitoids Tetracnemus pretisous and Coccophagus gurneyi, are used to control this pest in Australia and other countries. However, only Cryptolaemus montrouzieri is known to be present in Western Australia.
  

The overall likelihood that the citrophilus mealybug 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: Moderate.

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  The probability of entry, establishment and 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 the citrophilus mealybug: Low.

Unrestricted risk estimate

The unrestricted risk estimate for citrophilus mealybug, determined by combining the overall ‘probability of entry, of establishment and of spread’ with the ‘consequences’ using the risk estimation matrix (Table 4): Low.