Appendix C3
Appendix C Biosecurity risk assessment – Cottony Urbicola Scale (Pulvinaria urbicola) and Yellow Crazy Ant (Anoplolepis gracilipes)
Status
Cottony Urbicola Scale (Pulvinaria urbicola, CUS) is native to the West Indies and was first described in Jamaica (Smith, Papacek et al. 2004). CUS has established throughout the Indian and Pacific Oceans, northern continental Australia, Central and North America and Israel (Smith, Papacek et al. 2004). CUS is established on Christmas Island and was first detected in June 2011 (Neumann, Green et al. 2011). In Australia, it’s naturalised on mainland Queensland and its islands, including in the Capricornia Cays, where is has had catastrophic impacts (Greenslade 2008, Queensland State Government 2010, Biosecurity Queensland 2012). CUS are not present on North Keeling Island (Pulu Keeling National Park, PKNP).
Originating from West Africa (Slip and Comport 2008) or the West Indies (Neumann, Green et al. 2011), yellow crazy ants (Anoplolepis gracilipes, YCA) are one of the most abundant and damaging ant species in the world and have invaded most continents with tropical and subtropical climates (Figure 1). YCAs have naturalised on the continents of Africa and Asia, the Caribbean islands, and the islands of the Indian and Pacific Oceans (O’Dowd, Green et al. 2003, Slip and Comport 2008). They are widespread in pan-tropical areas, where they are considered environmental pests (Slip and Comport 2008).
Figure 1 Global distribution of Yellow Crazy Ants (Global Biodiversity Information Facility n.d.)
The YCA is a national conservation priority and has been identified as a species that is likely to threaten the biodiversity of Australia and its territories (Commonwealth of Australia 2006). It is also listed as a ‘key threatening process’ on Christmas Island and in New South Wales (Biosecurity Queensland 2012).
On Christmas Island, various species of scale insect and YCA benefit from a mutualistic relationship (Abbott 2006). YCA were accidentally introduced to Christmas Island sometime between 1915 and 1934 (Donisthorpe 1935). They were first detected at high densities in 1989 and reached widespread supercolony densities in the 1990’s (O’Dowd, Green et al. 2003, Abbott 2006). By 2001, YCAs had covered a quarter of the islands forests (O’Dowd, Green et al. 2003). The interactions between scale insects and YCAs have disrupted the islands trophic web causing an “invasional meltdown” (O’Dowd, Green et al. 2003, Abbott 2005, Abbott 2006).
YCAs occupy every island in CKIs (Slip and Comport 2008) and were first recorded on North Keeling Island in 1941 (Gibson-Hill 1948, Gibson-Hill 1950). They are widespread although sparse in the southern atoll (Slip and Comport 2008) and have not reached supercolony densities on PKNP (Detto 2012). It’s likely that ongoing monitoring will be required to ensure that CUS are not introduced to the park so that YCAs do not reach densities that will cause environmental damage of the parks values.
Biology & ecology
The following knowledge of CUS was collected in their naturalised distribution where they have caused environmental damage. It is particularly relevant in terms of assessing risk of introduction to PKNP and its potential impact on island biodiversity. The biology and ecology of CUS follows:
· hatchlings (crawlers) have 3 pairs of legs, paired eye spots and antennae. Crawlers are mobile and travel towards the light on shoots and tree tops
· once in the air currents they can disperse by wind hundreds of metres to fresh hosts. Long distance dispersal may occur between islands by birds (up to a few days survival time)
· males are winged and are short-lived mating before death
· adult females produce ~8000 eggs held in a white ovisack ~8 millimetres (mm) length
· live in terrestrial habitats and are host species generalists attacking dicots, monocots and fern species
· host species include: Pisonia grandis (pisonia), maiden hair fern, capsicum, tomato, dahlia, dianthus, lantana and blackberry nightshade, guava, frangipani, taro, pineapple, milk thistle, cobblers peg and cape gooseberry
· feed by inserting a tube into the host plants sap and high densities on host leaves are up to 100/leaf
· lifecycle is two months producing 2-4 generations per annually (observed between 6–7 weeks in Christmas Island)
· CUS excretes honeydew that is tended by ants
· in most places where it occurs, CUS are attacked by natural enemies (both predators and parasitoids) and is not generally regarded as a pest (Smith and Papacek 2001, Smith, Papacek et al. 2004, United States Government 2005, Neumann, Green et al. 2011).
NB: Successful biological control of CUS has been undertaken on North East Herald Islet on the Great Barrier Reef by several natural enemies (Smith, Papacek et al. 2004). Combined predation from wasp parasitoids (Coccophagus ceroplastae, Euryischomyia flavithorax) and a predatory ladybird (Cryptolaemus montrouzieri) provided the best control (Smith and Papacek 2001, Smith, Papacek et al. 2004).
The mutualism between CUS and YCAs plays an important role in the success of both invasive species (O’Dowd, Green et al. 2003). Their interactions cause a cascade of detrimental ecological impacts. YCA biology and ecology is outlined below:
· workers have uniform morphology: a long slender body, legs and neck, antennae in segments to 1.5 times the head length, gaster has an acidopore, erect hairs and is darker than head and thorax, head is oval with convex eyes and hairs
· workers live for 76–84 days and the queen can live up to several years
· interbreed whilst maintaining high heterozygosity. Offspring can be produced throughout the year but generally 2–3 months prior to the monsoon season
· when ‘budding’ can disperse up to 1100 m but rely on human dispersal to establish distant colonies, winged dispersal is rare
· predominant nesting habit is on the ground (fallen vegetation, ground cracks, tree bases) and potentially transient
· require “a carbohydrate source (fruit, plant exudates, honeydew) for energy and a protein based source (insects, small vertebrates, dead vertebrates or invertebrates) for reproduction” (Slip and Comport 2008)
· opportunistic and scavenge, often overcoming prey 100 times their own biomass using their formic acid defence. This blinds prey leading to death by starvation often within 24–48 hours
· display unicoloniality (many queens in a small area) in their introduced ranges forming supercolonies that can have up to 300 queens and cover large spatial scales. This facilitates a rapid increase in population numbers, increasing their invasive impact (Abbott 2005, Slip and Comport 2008, Biosecurity Queensland 2012).
Establishment risk
There are a range of factors relevant to the assessment of the risk of introduction and establishment of CUS on PKNP which are considered below.
Departure points
The following table outlines the major pathways that CUS may use to invade PKNP (Table1). It shows areas where the CUS is currently established, origins with potential for naturalised distribution, the potential pathways and the risk of introduction. It also shows areas where YCA are currently established. The range of CUS does not currently extend to PKNP; however it is foreseeable the island could form part of the future range of this species as conditions are high suitable. The following information has been used to identify where threat prevention work is needed.
Table 1 Cottony Urbicola Scale invasive pathways and likelihood of introduction
Origin / CUS Est.? / Pathway / Likelihood of introduction / YCA Est.?Air / Sea
Perth (Australian Mainland) / N / Y / N / Very unlikely / N
Broome (Australian Mainland) / N1 / Y / N / Very unlikely / Y
Darwin (Australian Mainland) / N1 / Y / N / Very unlikely / Y
Fremantle (Australian Mainland) / N / N / Y / Very unlikely / N
Christmas Island2 / Y / Y / Y / Possible / Y
Cocos Home Island / N1 / N / Y / Unlikely / Y
Cocos Horsburgh Island / N1 / N / Y / Unlikely / Y
Cocos West Island / N1 / N / Y / Unlikely / Y
Remaining islands within the southern atoll / N1 / N / Y / Unlikely / Y
Indonesia3 / N1 / N / Y / Unlikely / Y
Sri Lanka3 / Y / N / Y / Unlikely / Y
1 Potential for naturalised distribution.
2 Detected on Christmas Island in 2011.
3 Suspected Illegal Entry Vessels (SIEVs). Recorded in Sri Lanka in 2011 (Sirisena, Watson et al. 2013), data for Indonesia lacking.
Currently, there are no direct local conveyance pathways, either air or sea, between the CKI and PKNP as CUS is not naturalised at any of the local origins points. The current risk via local pathway has been identified as low but if introduced[1] to the southern atoll from Christmas Island it would be high.
As CUS is established on Christmas Island the risk of introduction via the domestic pathway is possible. There are two potential pathways for transport:
· pelagic seabirds travel great distances between breeding colonies and it is possible that they would travel the ~970 km distance between the two islands in less than 60 hours (Neumann, Green et al. 2011). The likelihood of controlling this natural dispersal pathway is impossible as threat prevention strategies will not reduce the introduction risk.
· Neumann and Green et. al.(2011) reported that there is no direct human traffic between Christmas Island and PKNP. However, Parks staff do travel periodically between the two islands to assist with on-island conservation. CUS could be transported in live or moist vegetative material and fruit/vegetable grown or sold on Christmas Island. There is also a chance that CUS could travel to PKNP in equipment or clothing if they had not been adequately disinfected.
For these reasons the risk via domestic pathways has been identified as high.
The risk of introduction by SIEVs via the international sea pathways is less certain due to the lack of ability to control its arrival using threat prevention strategies. However, CUS could be transported in moist vegetative material or fruit/vegetables, thus the risk has been identified as medium.
Transportability
The population of CUS on Christmas Island is the most likely origin for its introduction to the CKIs and PKNP (Neumann, Green et al. 2011). The insect must be transported between 24 hours and 3 days of its detachment from a host species. If the ~3 day survival time is exceeded the insect could desiccate and perish. For this reason, human transport by visitors or Parks staff from Christmas Island appears possible. Care must be taken by visitors not to accidentally infest PKNP via soil, wood and vegetative material including vegetable scraps and fruit rinds (Smith and Papacek 2001).
Natural dispersal methods are the most likely transport vector. Christmas Island is the closest land mass to PKNP where CUS is present. Both islands contain large populations of seabirds and transport between the islands is the most probable future source of introduction (Smith and Papacek 2001, Neumann, Green et al. 2011). Red-footed Booby’s (Sula sula), Greater Frigatebird’s (Fregata minor) are common on both islands are the most likely seabirds to migrate between the islands. Therefore, CUS transport to PKNP is possible.
Climatic & habitat suitability
CUS are habitat generalists and PKNP habitat is considered highly suitable. The tropical, high-humidity climate and distinct wet season on PKNP closely match that of CUS in its naturalised range and is also highly suitable. If introduced, it’s likely that CUS and YCA numbers would increase and populations would spread island-wide. One predicts that the interactions between YCAs and CUS on PKNP would mimic those of the populations on Christmas Island where climate and habitat are similar.
Impact on biodiversity
The principal biodiversity impacts of scale insects[2] stem from their establishment on host trees and the resulting interaction between them, their host and species and the supporting ant species. There is a high likelihood of the following adverse biodiversity impacts occurring if CUS was introduced and YCAs reached supercolony densities on PKNP:
· CUS numbers are amplified by a facultative mutualistic association with YCA species: scale provide carbohydrate food source (honeydew) to ants à ants protect scale insect from predators à populations of both insects elevated
· high numbers of YCAs in multi-queen supercolonies (1000s ants/m2) cause rapid, catastrophic shifts in the forest ecosystems, affecting multiple trophic levels
· YCA extirpate land crabs (the dominant forest floor consumer) which causes enhanced seedling recruitment, species richness, and litter breakdown. Thereby changing the structure of forest vegetation
· YCA have also been determined as the cause for seabird nesting failures and chick deaths and directly disrupt bird frugivory influencing bird abundances and behaviours
· CUS infestation correlates with reductions in pisonia forest canopy cover. Scale density correlates with tree health. High densities = sooty mould growth, poor health, canopy dieback and tree death
· CUS outbreaks have been implicated in island-wide pisonia mortality. Pisonia are the dominant canopy species and are structurally and functionally important. They provide vital nesting habitat for a wide range of seabird species, including EPBC listed migratory birds and the largest population of red-footed booby birds in the Indian Ocean. Pisonia are protected in their own right under the EPBC Act and worldwide (O’Dowd, Green et al. 2003, United States Government 2005, Abbott 2006, Neville, O’Dowd et al. 2008, Davis, O'Dowd et al. 2009, Queensland State Government 2010, Neumann, Green et al. 2011)
· YCA in supercolony numbers could impact upon the endemic subspecies Gallirallus philippensis andrewsi (Cocos Buff-banded rail) that nest on PKNP.
The extent of future impacts/consequences on PKNP is uncertain but could be majoràcatastrophic. It is integral that biosecurity threat prevention protocols be introduced addressing the threat of CUS introduction to PKNP (Slip and Comport 2008). The presence of this honeydew producing insect would allow YCA numbers to increase. Although there is no current threat, if YCAs reached supercolony densities, the impacts to the natural values of the park would be catastrophic.
In the event that CUS established on PKNP, it is likely the benefits provided by YCA chemical eradication would be greater than the off-target impacts of the control program. However, it is important that the control methods do not compromise the islands ecosystems and impacts on the islands natural values are minimised (Slip and Comport 2008).