Final Import Risk Analysis Report for Fresh Mango Fruit from India

4.3. Mango seed weevil – Sternochetus mangiferae [Coleoptera: Curculionidae]

• 
  Sternochetus mangiferae – Mango seed weevil ^WA .
  

Adult weevils are dark brown to black with grey markings and are 6–9 mm long. Weevils spend the winter living under loose bark around the base of mango trees or in the forks of branches (Pinese and Holmes 2005). They may also live in leaf litter around the tree, and approximately 25% of the adults overwinter in seed. Adult weevils can live for two years, so even with a crop failure in one season some weevils can survive into the following year (Pinese and Holmes 2005). Mango seed weevil has one generation per year (Pena et al. 1998; Verghese et al. 2005a) completing the egg to adult cycle in 1–2 months (Shukla and Tandon 1985; Srivastava 1997).

4.3.1. Probability of entry

Probability of importation

• 
  The mango seed weevil is considered to be a serious pest throughout its range in India (Srivastava 1997), including Andhra Pradesh, Orissa, Tamil Nadu, Karnataka, Kerala, Maharashtra and Gujarat (Shukla and Tandon 1985). Infestation of fruit by this weevil is known to reach up to 100% in some mango varieties (Vereesh 1989).
  
• 
  Eggs are laid in a depression on the surface of fruits and covered with a fruit exudate (Follet and Gabbard 2000). Initially the oviposition site is seen as a brownish spot on the rind but this heals and the spot fades as the fruit ripens (Smith 1996; Pinese and Holmes 2005).
  
• 
  Larvae tunnel into immature fruit before the seed endocarp thickens (Follett and Gabbard 2000). As the fruit matures the seed coat becomes hard; hampering the weevil’s ability to tunnel into the seed and leading to higher morality of first instar larvae (Shukla and Tandon 1985; Hansen et al. 1989). Larvae feed within the seed and pupate in the seed cavity (Follett 2002). Complete larval development usually occurs within the maturing seed, but also occasionally within the flesh (Hansen et al. 1989). Infested seeds usually contain one or two weevils, but seeds containing six or more have been recorded (Balock and Kozuma 1964; Srivastava 1997).
  
• 
  Infested fruit is difficult to detect as there are no external symptoms of infestation (Cunningham 1989; Srivastava 1997). Cutting fruit open for weevil presence is an effective method of detection (Shukla and Tandon 1985). Fruit inspection procedures are concerned primarily with quality standards of fruit with regard to blemishes, bruising or damage to the skin and are not specifically directed at the detection of internal pests that may be feeding under the skin of the fruit.
  
• 
  Mango seed weevil larvae can survive in picked fruit and are likely to be present in fruit that is packed for export.
  

• 
  As an internal pest feeding on mango fruit, mango seed weevil could survive during transport and storage.
  
• 
  Weevils tunnel inside the seed of mango fruits and destroy the cotyledons (Cunningham 1989). Pupation occurs inside the fruit and adults usually emerge within two months of fruit fall, but it may be longer (Cunningham 1989).
  
• 
  Adults are able to survive for considerable periods of time in the absence of food sources (Srivastava 1997). In the related species S. gravis, female weevils are able to survive for 82 days without food, and 135 days with food and water (De and Pande 1988). In plantations, weevils may hide in bark, crevices and other niches during hibernation (Balock and Kozuma 1964; Shukla and Tandon 1985; Srivastava 1997; Verghese et al. 2005a).
  
• 
  Mango seed weevil may survive for more than two years when provided with food (mango fruit) and water (Balock and Kozuma 1964; Srivastava 1997; Follett 2002). Mango seed weevil has been recorded to survive for two seasons with a hibernation period between these seasons (Balock and Kozuma 1964).
  
• 
  The mango seed weevil has been intercepted on mango entering the USA (USDA 2006), indicating that it would survive transport and storage and be imported into Western Australia via the movement of fruit.
  

• 
  The incidence of Sternochetus species can be reduced by field hygiene, for example removing fallen fruits and leaves (Dey and Pande 1987; Smith 1996).
  
• 
  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

• 
  The mango seed weevils associated with fruit are likely to be in the immature stage or adult life stage (De and Pande 1988; Smith 1996).
  
• 
  The mango seed weevils that survive cold storage would be capable of laying eggs, but a suitable host would need to be located to establish a founding population. 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.
  
• 
  Mango seed weevils have a restricted host range (Mangifera indica and its varieties (Srivastava 1997) and develop slowly; the time from egg to adult takes 35–54 days (Shukla and Tandon 1985) and only one generation is produced per year (Pinese and Holmes 2005).
  
• 
  In Australia, these hosts can be found in domestic gardens, as well as in urban environments as amenity plants. Adults move into the outer canopy of the tree during flowering to feed on new growth and mate prior to egg laying (Smith 1996). For counter-seasonal reasons, suitable sites for feeding may not be readily available when mango fruit is imported from India. This would limit the opportunity for reproductively active weevils to locate a suitable host.
  
• 
  The mango seed weevil might enter the environment through two scenarios.
  

• 
  Eggs, larvae, pupae or adults may be present in fruit at the point of sale, or fruit that has been purchased. Larvae may then develop into adults, which are able to move directly from the fruit into the environment.
  
• 
  Consumers may discard seed containing adults. Often only one adult will mature in each seed (Balock and Kozuma 1964; Follett 2002) but as many as six individuals may be present in some varieties (Srivastava 1997). Adults can emerge from the discarded seed and transfer to suitable hosts by crawling or flying (Shukla and Tandon 1985).
  

• 
  The adults are known to be poor fliers (Pinese and Holmes 2005), limiting the ability of mango seed weevil to move onto a suitable host from discarded fruit.
  

• 
  The commodity is likely to be distributed to multiple destinations throughout Western Australia for retail sale.
  
• 
  Eggs may develop into larvae and larvae to adults within fruit throughout the distribution chain. Consumers could discard seeds containing eggs or adults at multiple locations.
  

• 
  Waste material would be generated in the course of consumption. Infested fruits show no signs of infestation when ripe despite the immature life stages feeding and pupating inside the mango (Smith 1996; Srivastava 1997). When infested seed is disposed of, larvae are still likely to complete development.
  
• 
  Adults are able to survive for considerable periods of time in the absence of food sources (Srivastava 1997). In the related species S. frigidus, female weevils are able to survive for 82–135 days without food (De and Pande 1988).
  

Probability of entry (importation x distribution)

4.3.2. Probability of establishment

• 
  Mango seed weevil is capable of surviving and reproducing on Mangifera indica and its varieties (Srivastava 1997). Laboratory tests have demonstrated that the weevil either fails to lay eggs or cannot complete its development on other hosts (Balock and Kozuma 1964).
  
• 
  Mangifera species are grown widely in tropical and subtropical areas of Western Australia as ornamental, shade and fruit trees.
  
• 
  Wild and backyard mango trees are considered to be a continuous source of mango seed weevil in infected areas in Hawaii (Hansen et al. 1989).
  

• 
  Mango seed weevil, native to the Indo-Myanmar region (Follet 2002), now has a distribution extending to southern and eastern Africa, most Asian countries, parts of the South Pacific and Hawaii (Cunningham 1989, Smith 1996). Western Australia has similar environments that would be suitable for the establishment of this pest. This is supported by the fact that mango seed weevil is already established in New South Wales, the Northern Territory and Queensland (Smith 1996)
  
• 
  Sternochetus species could presumably establish in any climate suitable for mango production (USDA 2006). Limiting factors include its monophagy and sensitivity to temperature and humidity: the optimum temperature for the development of mango seed weevil is 24°C (De and Pande 1988).
  

• 
  Because the weevil is an internal pest of the fruit, current insecticide spray regimes for other pests may not have any impact on the establishment of mango seed weevil in Western Australia.
  
• 
  No parasites or other biocontrol agents have been identified for the species (Follett and Gabbard 2000; Verghese et al. 2005b).
  

• 
  Mango seed weevils reproduce sexually (Smith 1996). During flowering the adult weevils leave their shelter areas under loose tree bark or litter under the trees, and move into the outer canopy of the tree to feed on new growth and to mate prior to egg laying (Pinese and Holmes 2005).
  
• 
  Eggs are laid on small to medium sized fruit, but when populations are high, full size fruit can also be utilized (Smith 1996). Adult females may lay up to 15 eggs per day and can deposit almost 300 eggs over a three month period (Smith 1996). Reproduction is triggered by mango trees coming into flower (Peña et al. 1998).
  
• 
  After 5–7 days, the first instar larvae hatch from the egg and burrow through the mango flesh to the soft, developing seed. The seed is often completely destroyed by the feeding activity of two or more larvae. After the fruit matures and falls to the ground (or is harvested), the adult weevils chew a hole through the seed covering to emerge. This can occur between 22 and 76 days after fruit drop, but averages 45 days (Smith 1996).
  
• 
  After emergence, adults enter a diapause, which varies in duration with the geographic range. For example, in southern India, all adults emerging during June enter a diapause from July until late February of the following year (Shukla and Tandon 1985). The onset and termination of diapause appear to be associated with long-day and short-day photoperiod, respectively (Balock and Kozuma 1964).
  
• 
  The time from egg to adult is 35–54 days (Shukla and Tandon 1985) and only one generation is produced per year (Smith 1996; Peña et al. 1998; Follett 2002; Verghese et al. 2005a). This limits the ability of the weevils to establish populations if conditions are suitable only in small ‘windows of opportunity’.
  
• 
  Mango seed weevils overwinter as adults in the fruit, under loose bark around the base of mango trees or in the forks of branches or in litter under mango trees (Shukla and Tandon 1985; Pinese and Holmes 2005).
  
• 
  Mango seed weevils are long lived and may survive for more than two years when provided with food (mango fruit) and water (Balock and Kozuma 1964; Srivastava 1997; Follett 2002). Mango seed weevils have been recorded to survive for two seasons with a hibernation period between these seasons (Balock and Kozuma 1964).
  

4.3.3. Probability of spread

• 
  Mango seed weevil has been reported from tropical and subtropical environments in various countries, including eastern Australia (New South Wales, Northern Territory and Queensland). There are similarities in the natural and built environments of these states with those in Western Australia. This suggests that mango seed weevil would be able to spread within Western Australia.
  
• 
  Mango seed weevils survive as adults hibernating in the fruit, under loose bark around the base of mango trees or in the forks of branches or in litter under mango trees (Shukla and Tandon 1985; Pinese and Holmes 2005). Once they emerge from the mango seed, mango seed weevils usually stay in close proximity to their host tree until the following fruiting season.
  
• 
  Mango seed weevil displays clumping distributions with large infestations appearing year after year at the same location and low infestations at other locations. In some areas in Australia, mango seed weevil infests more than 80% of fruit (Cunningham 1991).
  
• 
  Current mango seed weevil management programs in Australia (vigilance, hygiene, chemical control) may have some impact on reducing the spread rate of mango seed weevil (Smith 1996).
  

• 
  The presence of natural barriers such as deserts or mountain ranges may prevent long-range natural spread of mango seed weevil. Although the adult weevils are capable of flight, they are not strong fliers (Smith 1996). It is unknown how important flight is in the dispersal of the species.
  
• 
  The long distances between some of the main West Australian commercial mango production areas may make it difficult for this pest to disperse unaided from one production area to another. Adult weevils usually remain in the vicinity of the original infestation until the following fruiting season (Cunningham 1991).
  
• 
  Hosts of mango seed weevil are widely distributed in Western Australia. The long distances between commercial host crops in Western Australia may prevent long-range natural spread of mango seed weevil.
  

• 
  The major means of dispersal to uninfected areas is through transport of adults and immature stages in infested fruits (Pinese and Holmes 2005).
  
• 
  There are restrictions in place in Australia on the movement of fruit to prevent the spread of mango seed weevil into Western Australia. Domestic quarantine regulations require that mango fruit entering Western Australia is sourced from properties free from Sternochetus mangiferae and requires a sample of the fruit to be dissected and inspected for the presence of the weevil.
  
• 
  Mango seed weevil has demonstrated a capacity to spread, from its original range in the India-Myanmar region to many mango growing countries (Smith 1996).
  
• 
  Existing interstate quarantine control on the movement of nursery stock and other plant material could inhibit spread into Western Australia, but would be of limited use within the state where control measures may not be applied.
  

• 
  Predators of mango seed weevil are known, but all fail to control populations of the pest in the field (Verghese et al. 2005b). No parasites or other biocontrol agents have been identified for the species (Follett and Gabbard 2000; Verghese et al. 2005b).
  

4.3.4. Overall probability of entry, establishment and spread

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

4.3.5. Consequences

4.3.6. Unrestricted risk estimate