The Australian taro industry

Taro is produced commercially in New South Wales, Queensland and the Northern Territory. Traditionally it was grown as a domestic garden crop and used and traded by the growers through an informal system. Taro is grown all year round in the northern parts of Australia.

It is estimated that more than 1000 tonnes of taro is produced commercially in Australia each year, and more than 3000 tonnes of taro is imported (Daniells et al. 2004; Vinning 2003). Very little of the commercially grown and imported taro is processed. More than two million fresh taro corms are likely to be traded and consumed domestically each year.

Estimates of the size of the Australian market vary widely, but most support the idea that the size of the market is expanding. Production in Australia is expected to increase because of the influx of new growers, stable prices, demand exceeding supply, opportunities for import replacement and the development of new products such as taro chips (Horsburgh and Noller 2005).

Pest risk assessments for quarantine pests

1.6Quarantine pests for pest risk assessment

Pest risk assessments for these quarantine pests are presented in this section. The results of these pest risk assessments are summarised in Table 4.2, together with the overall restricted risk estimates. The rationale for each value in the pest risk assessments is described in the relevant sections below.

Very little is known of the developmental biology of Elytroteinus subtruncatus because it is difficult to rear under laboratory conditions (Mau and Martin Kessing 1992a). The adult lays a single egg in corms, tubers, fruits or soft stems of a range of plants. The larva bores through the tissues, completing its development inside the plant. Elytroteinus subtruncatus is associated with a diverse range of plant hosts, including taro corms, ginger (Zingiber officinale) rhizomes, avocado (Persea americana) seeds, daylily (Hemerocallis spp.) stems, kava (Piper methysticum) stems, cycad (Cycas spp.) trunks, ti (Cordyline fruticosa) cuttings, lemons (Citrus limon), dwarf mondo (Ophiopogon japonicus) roots, Marrattia fern trunks and dead sugarcane (Saccharum spp.) (Follett et al. 2007; Mau and Martin Kessing 1992a).

In the United States of America (USA), the Animal and Plant Health Inspection Service (APHIS) considers Elytroteinus subtruncatus to be a high-risk pest requiring mitigation for sweet potatoes exported from Hawaii to the mainland. This is the result of five weevil interceptions in 1995 and 1997 (nine sweet potato roots containing a total of eight larvae and two pupae) found in passenger baggage at Keahole International Airport, Hawaii (Follett et al. 2007). However, sweet potato is not reported in the literature as a host of Elytroteinus subtruncatus. The weevil has never been observed on sweet potato in the field, and was not found in a survey of sweet potatoes prepared for export in Hawaii (Follett et al. 2007). Taro exports from Hawaii to the mainland USA require visual inspection and cutting of corms to identify infestations by a number of arthropod pests. No ginger weevils have been observed in over 30 years of this inspection regime in taro corms grown in the production area in east Hawaii (Follett et al. 2007).

The likelihood that Elytroteinus subtruncatus will arrive in Australia with the importation of fresh taro corms from any country where this pest is present is: LOW.

• 
  The weevil larva burrows into the taro corm where it completes development. Feeding gives rise to wilting, loss of vigour, and in severe infestations the plants die (Mau and Martin Kessing 1992a). Affected corms will show signs of rot and are likely to be detected and rejected during normal harvesting and grading operations.
  
• 
  The main risk would be from corms in which eggs were laid late in the season, just before harvest, or after harvest during storage. This is possible, as Fiji ginger weevil is noted as a long-term storage pest in root crops such as yams (Dioscorea spp.) (Wilson 1987).
  
• 
  Taro corms are known to host Elytroteinus subtruncatus, although no weevils have been detected in over 30 years of taro exports from east Hawaii where the weevil is present (Follett et al. 2007).
  
• 
  Weevil larvae and pupae have been found on sweet potato from Hawaii imported into mainland USA, but has not been confirmed as a pest of sweet potato under natural field conditions (Shea 2004; Follett et al. 2007).
  
• 
  One weevil was intercepted in Sydney on unspecified goods from Fiji in 1963 (APPD 2009). Taro has been imported into Australia for many years from a number of Pacific countries where Elytroteinus subtruncatus is present without any further interceptions of the pest.
  

The likelihood that Elytroteinus subtruncatus will be distributed within Australia in a viable state to a susceptible part of a host, as a result of the processing, sale or disposal of fresh taro corms from any country where this pest is present, is: MODERATE.

• 
  The weevil larva will remain within the corm for some time, as pupation occurs at the feeding site inside the corm (Mau and Martin Kessing 1992a). Emergence of adult weevils may not occur until some time after arrival in Australia.
  

• 
  Corms will be distributed to many localities by wholesale and retail trade and by individual consumers.
  
• 
  Individual consumers could carry small quantities of taro corms to urban, rural and natural localities. Small amounts of corm waste could be discarded in these localities.
  
• 
  Some corms will be distributed to areas where taro or other host species grow.
  
• 
  Small amounts of corm waste will be discarded into domestic compost.
  

• 
  Infested corms that escaped detection during processing and importation are likely to be distributed in the wholesale and retail supply chain.
  
• 
  Elytroteinus spp. weevils are flightless (NZ MAF 2008) and have a limited ability to seek out new hosts once they leave the corm.
  

The likelihood that Elytroteinus subtruncatus will enter Australia and be distributed in a viable state to a susceptible host, as a result of trade in fresh taro corms from any country where this pest is present, is: LOW.

The likelihood that Elytroteinus subtruncatus will establish within Australia, based on a comparison of factors in the source and destination areas considered pertinent to its survival and reproduction, is: LOW.

• 
  On lemons, Miller (1923) reported that only a single egg was laid in each fruit, so the fruit only contained a single larva. It is not known if this behaviour also occurs on taro corms, but it is considered likely.
  
• 
  For this pest to establish, it would need to complete its lifecycle. This could occur if several infested corms remained together in the supply chain, or if a single corm carried several eggs (oviposited by multiple females, as each female probably lays only a single egg in the corm), the pupae hatched, the adults survived and mated, and several females found suitable plants for their eggs. The combined probability of all these events happening is considered to be low.
  

The likelihood that Elytroteinus subtruncatus will spread within Australia, based on a comparison of those factors in the source and destination areas considered pertinent to the expansion of the geographic distribution of the pest, is: MODERATE.

• 
  Climatic conditions in parts of northern Australia are similar to conditions in the Pacific countries where the weevil is found.
  
• 
  Host plants, including avocado, lemon, sugarcane and taro (Follett et al. 2007), are available in many parts of Australia, so some spread could be anticipated.
  
• 
  Elytroteinus subtruncatus has not spread widely in Hawaii since it was first reported in 1918, despite the presence of hosts such as avocado and taro. Its distribution is restricted to parts of the island of Oahu (Follett et al. 2007).
  
• 
  Elytroteinus spp. weevils are flightless (NZ MAF 2008), so natural spread would be slow. Longer distance spread would only occur via movement of infested plant material.
  

The likelihood that Elytroteinus subtruncatus will be imported as a result of trade in fresh taro corms from any country where this pest is present, be distributed in a viable state to a susceptible host, establish and spread within Australia, is: VERY LOW.

Assessment of the potential consequences (direct and indirect) of Elytroteinus subtruncatus for Australia is: VERY LOW.

The unrestricted risk for Elytroteinus subtruncatus is: NEGLIGIBLE.

Unrestricted risk is the result of combining the probability of entry, establishment and spread with the outcome of overall consequences. Probabilities and consequences are combined using the risk estimation matrix shown in Table 2.5.

The unrestricted risk estimate for the Elytroteinus subtruncatus of ‘negligible’ achieves Australia’s ALOP. Therefore, specific risk management measures are not required for this pest.

1.8Taro beetles

Eucopidocaulus tridentipes; Papuana biroi; Papuana cheesmanae; Papuana huebneri; Papuana inermis; Papuana japenensis; Papuana laevipennis; Papuana semistriata; Papuana szentivanyi; Papuana trinodosa; Papuana uninodis

At least 19 species of taro beetle are known, 18 in the genus Papuana and one in the genus Eucopidocaulus. The 11 species of taro beetles assessed in this PRA have been grouped together because of their similar biology. All pest species of Papuana and Eucopidocaulus tridentipes behave in a similar way, and they feed on the same hosts (TaroPest 2008). There are, however, minor differences in geographic and climatic ranges between the species, which do not affect the risk assessment.

The large (25 mm long and 12 mm wide) adult beetles burrow into the soft trunks, plant bases and corms of a range of plants, including taro, making large holes or cavities up to 2 cm in diameter (McGlashan 2006). Adult female beetles feed aggressively for about a week before leaving the host plant to seek suitable sites for egg laying (TaroPest 2008). These sites include felled logs, grassland with a silty loam topsoil, river beds and banks with good alluvial soil deposits, logging areas, gardens under fallow, and roadsides where there are host plants present (TaroPest 2008). The eggs are laid 5–15 cm beneath the soil (Onwueme 1999). The larvae hatch from the eggs in 11–16 days, and feed on plant roots and dead organic matter at the base of host plants (Onwueme 1999). The larvae rarely feed on taro corms (Macfarlane 1987). Adult males are most likely to be found in association with the corms, as they tunnel into the corms and remain there, whereas the females seek out sites for laying eggs after mating (SPC 2003).

Taro beetles are native to the Indo-Pacific region, with 18 species found in Papua New Guinea (Macfarlane 1987), 12–18 in the Solomon Islands, five in Vanuatu, and one each in Fiji, Kiribati and New Caledonia (Onwueme 1999; SPC 2003). About 11 or 12 species are serious pests of taro in the South Pacific region. There are a number of records of Papuana species being reported in Australia between 1909 and 1981 (APPD 2009). The records range from tropical to temperate regions. Five beetles identified only as Papuana sp. were recorded from the Iron Range in Queensland in 1968 (APPD 2009). A population of Papuana woodlarkiana is considered to be present in the far north of Queensland (Brooks 1965; Cassis et al. 1992). They have also been recorded at Onslow and Binningup in Western Australia (APPD 2009).

The likelihood that taro beetles will arrive in Australia with the importation of fresh taro corms from any country where these pests are present is: LOW.

• 
  Adult taro beetles are shiny and black, measuring approximately 25 mm long and 12 mm wide. Newly emerged beetles are brown in colour (Carmichael et al. 2008). Adult beetles are likely to be detected if present in corms during harvest and grading operations.
  
• 
  Adult beetles burrow into the underground part of taro corms, forming large holes and tunnels up to 2 cm in diameter (McGlashan 2006). The tunnels and associated frass are obvious. Affected corms are likely to be detected and discarded either at harvest or at the packing house.
  
• 
  Affected corms are not suitable for human consumption.
  
• 
  The female beetles remain in the corms for only about a week and then spend most of their time laying eggs in humic litter (SPC 2003). Beetles remaining in the corms are most likely to be males (SPC 2003).
  
• 
  There is only one record of a taro beetle (an unidentified Papuana sp.) being found during Australian quarantine inspections of imported taro consignments in the available AQIS pest interception data since 1986.
  

The likelihood that taro beetles will be distributed within Australia in a viable state to a susceptible part of a host, as a result of the processing, sale or disposal of fresh taro corms from any country where these pests are present, is: LOW.

• 
  It is possible that adult beetles could be transported in unopened cartons of taro. However, only gravid females pose any risk, and these leave the corms after about a week to seek suitable substrates (humic soil) for oviposition. They are unlikely to remain associated with corms that have undergone harvest and pre-export sorting, grading and packing. Any beetles that remained in the corms after transport to Australia would be predominantly male.
  
• 
  Should gravid females be imported, they are likely to leave the corms at the earliest opportunity and fly short distances seeking damp humic soil (leaf litter, compost, sawdust piles) to oviposit.
  

The likelihood that taro beetles will enter Australia and be distributed in a viable state to a susceptible host, as a result of trade in fresh taro corms from any country where these pests are present is: VERY LOW.