Final Import Risk Analysis Report for Fresh Mango Fruit from India

4.2. Mango pulp weevil – Sternochetus frigidus [Coleoptera: Curculionidae]

The species examined in this pest risk analysis is:

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  Sternochetus frigidus – Mango pulp weevil.
  

4.2.1. Probability of entry

Probability of importation

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  Mango pulp weevil has been reported in north-east India on cultivated and wild mango (Srivastava 1997; De Jesus et al. 2002). Fruit infestation rates of 65–80% and, at times 100% have been recorded for Sternochetus species in north-east India (Dey and Pande 1987).
  
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  Adult mango pulp weevils feed on flowers, panicles and fruits. Eggs are laid singly on the developing mango fruit (Srivastava 1997). An incision is made in the fruit and eggs are covered with fruit exudate (De and Pande 1988). Infested fruit usually contain two to three eggs, although as many as seven eggs have been observed (Srivastava 1997).
  
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  The newly hatched larvae tunnel into the fruit, where they develop (De and Pande 1988; Altoveros et al. 2001), feeding in the pulp as the fruit matures (Waite 2002).
  
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  Approximately 30–50% of newly hatched larvae die through contact with gum laden tissues while they tunnel through the fruit pulp (CAB International 2007). Up to 20% of larvae die when the fruits are harvested, because they are unable to complete their development (CAB International 2007), but many larvae are unaffected.
  
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  Infested fruit are difficult to detect as there are few external symptoms of infestation prior to exit of adult weevils from the ripe fruit through holes in the peel (CAB International 2007). Although all fruit is visually inspected during sorting and packing, the cryptic development of this species within mango fruit means current sorting practices are unlikely to detect invested fruit.
  

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  As an internal pest feeding on mango fruit, mango pulp weevil is likely to survive during transport and storage.
  
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  Mango pulp weevil completes a significant proportion of its life-cycle inside mango fruit. Larvae feed in the pulp of the mango fruit where pupation also occurs (De and Pande 1988; Srivastava 1997; De Jesus et al. 2002). Adult weevils may remain inside ripened mango fruits for 37 days after pupation (Altoveros et al. 2001), and a large proportion of adults may hibernate (overwinter) inside mango fruits (De and Pande 1988).
  
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  The mango pulp weevil has been intercepted on mangoes entering the USA (USDA 2006), demonstrating that it can survive transport and storage and could be imported into Australia via the movement of fruit.
  

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  The incidence of Sternochetus species can be reduced by field hygiene practices such as the removal of fallen fruits and leaves (Dey and Pande 1987).
  
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  However, Sternochetus species have been detected in fruit in numerous countries, and intercepted by others, demonstrating that Sternochetus species can survive existing pest management procedures.
  

Probability of distribution

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  The mango pulp weevils associated with fruit are likely to be in the immature stage or adult life stage (De Jesus et al. 2002).
  
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  The mango pulp weevils that survive cool storage (13˚C) would be capable of laying eggs, but a suitable host would need to be located. From the release of imported fruit at the point of entry to Australia, through to the retailing of fruit, there would be limited opportunities where suitable hosts are likely to be in close proximity to the imported commodity.
  
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  Mango pulp weevils have a restricted host range (Mangifera indica, M. sylvatica and M. foetida) and develop slowly; the time from egg to adult takes 35–53 days, and only one generation is produced per year (Srivastava 1997).
  
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  In Australia, some of these hosts can be found in domestic gardens, as well as in urban environments as amenity plants. This would limit the opportunity for reproductively active weevils to locate a suitable host.
  
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  Adult weevils feed on mango flowers and mate and lay eggs on mango fruit when fruits are small to medium size (De Jesus et al. 2003). For counter-seasonal reasons, suitable sites for feeding may not be readily available when mango fruit is imported from India.
  
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  The mango pulp weevil might enter the environment through:
  

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  eggs maturing into larvae within stored fruit, fruit at the point of sale, or fruit that has been purchased. The immature stages of the pest feed and develop in the pulp into adult weevils (De Jesus et al. 2005). Adult weevils may remain inside ripened mango fruits for several days (Altoveros et al. 2001). Some of the adult weevils may leave the fruit but some may hibernate within it (De and Pande 1988). Newly emerged adults are able to move directly from the fruit into the environment.
  
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  wholesalers, retailers or consumers discarding fruit with spoiled flesh or visible larvae. Adult Sternochetus can emerge from the discarded fruit but rarely move far from the point of emergence (Shukla and Tandon 1985). Adults may transfer to a suitable host by crawling (preferred) (De Jesus et al. 2003), or flying (Srivastava 1997). However, flight is limited to a distance of 50–90 cm in a horizontal direction (Srivastava 1997).
  

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  The commodity is likely to be distributed to retail destinations throughout Australia for sale.
  
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  Eggs could develop into larvae, and larvae to adult weevils within fruit throughout the distribution chain. The ability to pupate or even hibernate in fruit gives the weevil the ability to survive distribution within Australia.
  
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  Wholesalers, retailers or consumers could discard spoiled fruit containing eggs or larvae or adults at multiple locations.
  

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  The intended use of the commodity is for human consumption but waste material will be generated. Infested fruits show no signs of infestation when ripe, despite the immature life stages feeding and pupating inside the mango (Altoveros et al. 2001). Larvae in infested mangoes could complete development in discarded waste.
  
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  Adults are able to survive for some time in the absence of food sources (De and Pande 1988). Female weevils, for example, are able to survive up to 82 days without food (De and Pande 1988).
  

Probability of entry (importation x distribution)

4.2.2 Probability of establishment

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  The mango pulp weevil is capable of surviving and reproducing on Mangifera indica, M. sylvatica (Srivastava 1997) and M. foetida (CAB International 2007).
  
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  Mangifera species are commonly grown in tropical and subtropical areas of Australia, including major cities (Brisbane, Darwin, Sydney and Perth), as ornamental, shade and fruit trees.
  
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  Transport of infested fruits containing adults or immature stages of S. frigidus is likely to be the major means of dispersal to uninfected areas, as has been found for the similar species S. mangiferae (Pinese and Holmes 2005).
  
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  Suitability of the environment
  
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  Mango pulp weevil has a distribution from India to Indonesia (De Jesus et al. 2002; CAB International 2007). There are parts of Australia that have similar climatic conditions to countries where it already thrives. Sternochetus species could establish in any climate suitable for mango production (USDA 2006). Optimum temperatures for the development of mango pulp weevil are from 21–27°C (Srivastava 1997).
  
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  Sternochetus mangiferae (mango seed weevil), a closely related species, is already established in tropical and subtropical parts of eastern Australia (Smith 1996).
  

Control of Sternochetus species with insecticides has been shown to be difficult and impractical (Dey and Pande 1987). The implementation of clean cultivation (removal of fallen fruits and leaves) and soil disturbance has been shown to reduce infestations from 87% to 68–78% (Dey and Pande 1987). These practices, if implemented, would reduce but not eliminate the probability of establishment.

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  Mango pulp weevils reproduce sexually (De and Pande 1988). At full-bloom stage the adult weevils crawl on flowers to feed. When fruits are small to medium in size they become the mating and oviposition sites (De Jesus et al. 2003).
  
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  Mating occurs 10–15 days following emergence from hibernation (Srivastava 1997). Females lay about 15 eggs in a day, and up to a maximum of 115 eggs in a three week period (De and Pande 1988).
  
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  The time from egg to adult is 35–53 days and only one generation is produced per year (Srivastava 1997). This limits the ability of the weevils to establish populations if conditions are suitable only in small ‘windows of opportunity’.
  
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  Mango pulp weevil can survive as adult weevils in the fruit (De and Pande 1988). Female weevils are able to survive up to 82 days without food, and longer when food is provided (De and Pande 1988). Up to 58% of adult weevils of this species may hibernate inside mango fruit (Srivastava 1997). The remaining weevils undergo hibernation beneath fallen fruits, in leaf litter, in crevices and in refuse material discarded in the orchard (Srivastava 1997).
  

4.2.3. Probability of spread

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  Many environments (natural and cultivated) within Australia mirror those in countries where the weevil already thrives.
  
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  Mango pulp weevils survive as adults hibernating inside mango fruit or beneath fallen fruits, in leaf litter, in crevices and in refuse material discarded in the orchard (Srivastava 1997; De and Pande 1988).
  
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  Current mango pulp weevil management programs (clean cultivation, removal of fallen fruits and leaves) may have some impact on reducing the spread rate of mango pulp weevil (Dey and Pande 1987).
  

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  The presence of natural barriers such as deserts or mountain ranges may prevent long-range natural spread of mango pulp weevil to widely dispersed mango production areas. Adult weevils are capable of flight, but are poor fliers only covering distances less than one metre in horizontal directions (De and Pande 1988). Therefore adult weevils will move only a limited distance by natural dispersal. Adult weevils usually hibernate in the vicinity of the original infestation until the following fruiting season (De and Pande 1988).
  

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  The major means of dispersal to uninfected areas is likely through transport of infested fruits containing adults or immature stages of the pest, as has been found for the similar species S. mangiferae (Pinese and Holmes 2005). Movement of infected fruit by human transport is generally a significant means of dispersal (Baker et al. 2000).
  
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  Mango pulp weevil has demonstrated a capacity to spread, from its original range in the India-Myanmar region where mango is native, to Indonesia and Papua New Guinea (De Jesus et al. 2002; CAB International 2007).
  
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  Existing interstate and intrastate quarantine control on the movement of nursery stock and other plant material could reduce the rate of spread.
  

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  Natural enemies of mango pulp weevil include the parasitic wasp Flavopimpla mangae and ngrangrang ants (Oecophylla species), which repel most insects in the vicinity of their nest and are able to keep a tree free from weevil infestation (CAB International 2007). Camponotus species of ants can also disturb adult weevils. Larvae, pupae and hibernating adults can be parasitised by Rhizoglyphus species (De and Pande, 1988). No information is available on potential natural enemies that may be present in Australia.
  

4.2.4. Overall probability of entry, establishment and spread

The overall probability that mango pulp weevil will be imported as a result of trade in mango fruit from India, be distributed in a viable state to a suitable host, establish and spread within Australia, is: LOW.

4.2.5. Consequences

4.2.6. Unrestricted risk estimate