Argentine Ants on Santa Cruz Island, California: Conservation Issues and Management Options Island Invasives: Eradication and Management
Randall, J.M.; K.R. Faulkner, C. Boser, C. Cory, P. Power, L.A Vermeer, L. Lozier, and S.A. Morrison. Argentine ants on Santa Cruz Island, California: conservation issues and management options Island invasives: eradication and management
Argentine ants on Santa Cruz Island, California: conservation issues and management options
J. M. Randall1, K. R. Faulkner2, C. Boser1, C. Cory1, P. Power2, L. A. Vermeer1, L. Lozier1, and S. A. Morrison1 1The Nature Conservancy, 201 Mission Street, 4th Floor, San Francisco, California 94105, U.S.A.
INTRODUCTION There are approximately 20,000 species of ants on recognised as a cause for concern. Today the ants are likely Earth (Ward 2006) but only about 150 species, fewer than the most damaging invasive species remaining on the 1% of the total, are known to have established outside their island. To assess the extent of the threat it poses, presence/ native ranges after dispersal by humans (McGlynn 1999). absence surveys of Argentine ants were conducted around Most of the introduced species are restricted to human- the three previously recorded infestations and other high modified habitats, but a small subset has spread into natural human use sites on the island. We also convened an on-site environments where they have significant and sometimes meeting of 18 experts on Argentine ant ecology and control disproportionate negative effects on native biological and conservation land management from the U.S. and New diversity. Six species of ants are particularly widespread, Zealand to advise us on a course of action. Here we present abundant, and damaging worldwide (Holway et al. 2002), findings from those efforts. including the Argentine ant (Linepithema humile). Many studies have documented harmful effects of Argentine ants HISTORY OF ISLAND MANGEMENT AND on native ants, other invertebrates, some vertebrates, plants ARGENTINE ANT INVASION and plant communities, and in countries that include New Zealand (Ward and Harris 2005; Ward 2009), Australia The arthropod fauna of Santa Cruz Island is not fully (Walters and Mackay 2003; Rowles and O’Dowd 2007, known but includes at least 8 endemic insect taxa (Miller 2009), South Africa (Buys 1987; Christian 2001), the 1985; Schoenherr et al. 1999). More systematic collections Mediterranean region (Way et al. 1997; Quilichini and of ants have revealed 32 native species. None of these Debussche 2000), and in California (Ward 1987; Gambino are endemic although Messor chamberlini is relatively 1990; Human and Gordon 1997; Sockman 1997; Holway common on the island but quite restricted and rare on the 1998, 1999; Bolger et al. 2000; Fisher et al. 2002; Suarez California mainland (D. Holway, UC San Diego pers. and Case 2002; Suarez et al. 2000) where they are invasive comm.). Besides the Argentine ant, the only other non- in coastal areas and the Central Valley. As with other native ant species on the island is Cardiocondyla ectopia invaders, islands seem to have been particularly vulnerable (Wetterer et al. 2000); however, its known extent is small to the harmful effects of invasive ants (O’Dowd et al. 2003; and it is currently not a species of concern. Lach and Hooper-Bui 2010). For example, Argentine ants Santa Cruz Island has a long record of human occupation have been particularly damaging on the island of Maui and use, dating back at least 7000 years (Glasow 1980). By (Cole et al. 1992). the time the effort to set the island aside as a conservation Santa Cruz Island (250 km2) is approximately 40 km area began in the late 1970s, Santa Cruz harboured large south of Santa Barbara, California and is the largest of populations of several non-native animals and many species the eight California Channel Islands. A highly valued of invasive plants. Particularly damaging were thousands conservation area, it falls entirely within Channel Islands of feral sheep (Ovis aries) and pigs (Sus scrofa) as well as National Park although just 24% of it is owned and managed smaller numbers of cattle (Bos taurus), all of which had by the U.S. National Park Service (NPS). The remaining been intentionally introduced in the mid-1800s (Junak et 76% is owned and managed cooperatively by The Nature al. 1995). These ungulates severely damaged vegetation on Conservancy (TNC), an international conservation the island, stripping much of the island almost completely organisation. The island harbours at least 21 endemic taxa of plant cover, leading to severe erosion (Brumbaugh et of animals (insects, birds, mammals, and herptiles) and 8 al. 1982). endemic taxa of plants (Junak et al. 1995; Schoenherr et By 2006, NPS and TNC successfully eliminated cattle, al. 1999). It also supports many other species found on feral sheep and feral pigs, which allowed a spectacular some of the other Channel Islands but not on the mainland, recovery of native vegetation in many areas (Junak et al. as well as several plant communities now rare on the 1995; Morrison 2007). Feral honeybees (Apis mellifera) California mainland. were also eliminated from the island by the early 2000s, Argentine ants were first detected on Santa Cruz reducing threats to native bees as well as pollination Island in 1996 (Calderwood et al. 1999) and were quickly services provided to invasive plants favoured by honeybees (Wenner and Thorp 1994; Barthell et al. 2001).
Pages 108-113 In: Veitch, C. R.; Clout, M. N. and Towns, D. R. (eds.). 2011. Island invasives: eradication and management. IUCN,108 Gland, Switzerland. Randall et al.: Argentine ants on Santa Cruz I., California
Argentine ants were first detected on Santa Cruz known infestations (1997 and 1998 for the Valley Anchorage Island in 1996 (Calderwood et al. 1999) and were quickly and Blue sites; 2004 for the UC Field Station site). These recognised as a cause for concern. In 1997 and 1998, distances were based on Argentine ant invasion expansion delimitation surveys were conducted around areas where rates of 10 to 100 m/yr recorded elsewhere in California Argentine ants had been detected and in other locations (Holway 1998). Each bait station consisted of a 3 x 5 inch with a recent history of human use and movement of (7.6 x 12.7 cm) paper card with a few drops of organic goods (Calderwood et al. 1999). Infestations were maple syrup and several small pecan (Carya illinoinensis) found at two sites, one covering approximately 1.5 km2 nut pieces. Bait stations were revisited 30-120 minutes (hereafter referred to as the Valley Anchorage site) and after they were set out, and the presence and quantity (1, the other covering 0.04 km2 (hereafter, the Blue site). The 2-10, >10, >100) of Argentine ants on or immediately larger infestation was of a size consistent with arrival of adjacent to the stations was recorded (Coastal Restoration the ants 5 to 10 years before the surveys were conducted Consultants 2009). Other ants on or immediately adjacent (Calderwood et al. 1999). Both infestations appeared to to the stations were identified to genus where possible and radiate from sites of U.S. Navy installations dismantled in recorded (data not reported here). 1995, which led Calderwood et al. (1999) to speculate that On 13-15 November 2009 and 21-24 June 2010, more heavy equipment transported to the island for use in these detailed surveys were conducted in the Cañada del Medio installations may have carried Argentine ants. The surveys and Cañada del Puerto drainage and riparian areas, i.e. from also found that native ants were largely absent wherever approximately 100 m upstream of the Field Station, to the Argentine ants were established (Wetterer et al. 2000, drainage’s mouth at Prisoner’s Harbor, approximately 5.5 2001). Unfortunately, no control action was taken at that km downstream of the Field Station. This catchment was time, in part because problems caused by invasive feral judged most likely to harbour additional infestations since it pigs (Sus scrofa) were severe and given higher priority. is downstream of the Field Station infestation, and is used as Both sites thus remained infested (Fig. 1). the primary transportation corridor from the island’s north There are two building complexes in the central valley coast to its interior. The surveys used a sucrose-water mix of the island: a historic ranch compound that serves as or a sweet gel (the non-toxic attractant in Xstinguish Ant the island headquarters for The Nature Conservancy, and Bait produced by Bait Technology of New Zealand; www. a nearby University of California (UC) Field Station. flybusters.co.nz/Bait+Technology.html) placed in small Searches around both complexes failed to detect Argentine plastic vials as baits. Bait vials were placed at intervals of ants in 1997 and 1998, but a third infestation was found 15 m or less, and retrieved after 5 and 24 hours. Ants in the around the Field Station in 2004 (L. Laughrin, UC Natural vials were identified and recorded. During the November Reserve System pers. comm.; Fig. 1). At this time, however, 2009 and June 2010 surveys, entomologists familiar with a feral pig eradication programme and efforts to recover Argentine ants and native ants of southern California also the endangered island fox (Urocyon littoralis santacruzae) searched on foot through the vegetation and cobbles in the were in full swing, and took priority over addressing the watercourse and associated floodplain and recorded the Argentine ant invasion. With the recent completion of the locations of any Argentine ants found. feral pig eradication programme (Morrison 2007) and the A separate survey of twelve beaches commonly used island fox showing strong signs of recovery (Coonan and as landing areas by recreational boaters was conducted in Schwemm 2008), managers have the capacity to focus on June 2010. This survey also used the non-toxic attractant other management priorities. in Xstinguish Ant Bait placed in small plastic vials. The baited vials were placed >15 m inland of the high water METHODS mark, and approximately 15 m from any other monitoring Argentine ant surveys tube. Baits were left in place for 1 -3 nights then collected and the vials examined for the presence of Argentine ants. Surveys of the three known Argentine ant infestations, plus 15 other sites with high human use on the island, were Argentine ant management recommendations conducted between 20 May and 16 June 2009. Non-toxic We convened a three day meeting of an Expert Working baits were placed at a total of 468 bait stations at the 18 sites. Group in October 2009 on Santa Cruz Island, which Bait stations were placed around and up to 600 m beyond included 18 experts in the ecology and control of Argentine the boundaries identified in previous surveys for the three
Fig. 1 Argentine ant infestations detected on Santa Cruz Island, California as of June, 2010 and sites where no Argentine ants were detected during 2009 and 2010 surveys. Inset shows the location of the Santa Cruz Island in California.
109 Island invasives: eradication and management ants and conservation land management from the United reduced abundances of flies, springtails, beetles, cynipid States and New Zealand. The goals of the meeting were to: wasps, ticks, mites and spiders in northern and southern 1) describe known and potential impacts of Argentine ants California (Human and Gordon 1997; Bolger et al. 2000) on the island; 2) characterise management options; and, 3) and of arthropods from eight orders on the Hawaiian island make recommendations for management and monitoring. of Maui, including endemic spiders, moths, beetles, bees and flies (Coleet al. 1992). Argentine ants have been observed RESULTS AND DISCUSSION attacking yellow jacket (Dolichovespula arenaria; Vespula germanica; V. pensylvanica; and V. vulgaris) colonies in Surveys in 2009 and 2010 northern California (Gambino 1990) and were shown to The survey conducted in May and June 2009 found compete successfully for nectar sought by honeybees continuing infestations at the three known sites. At Valley (Apis mellifera) in South Africa (Buys 1987). Similarly, Anchorage and Field Station, Argentine ants had expanded on Maui they entered the nests of a native solitary bee, in and near riparian corridors by rates of 30-60 meters Hylaeus volcanica, flushed out the adults and apparently per year since 1997. There was little or no evidence of preyed on larvae, none of which were found in areas expansion at the Blue site nor of expansion into dry uplands infested by the ants (Cole et al. 1992). On the other hand, around any of the sites. No Argentine ants were found at Holway (1998) found that while Argentine ants displaced any of the other 15 sites with heavy human use (Coastal all native ant species that feed above-ground except for the Restoration Consultants 2009). cold-tolerant, winter-active species Prenolepis imparis, The November 2009 and June 2010 surveys detected they did not appear to affect the diversity or abundance three small infestations in the ephemeral stream drainage of non-ant arthropods at his riparian woodland study site of Cañada del Puerto between the main ranch compound in inland northern California. Some studies (Cole et al. and Prisoners’ Harbor (Fig. 1). One covered perhaps 75 1992; Human and Gordon 1997) also found that a variety m2 in a wetland area north of the TNC ranch (Bridge site). of other non-native isopods and insects were actually more Another measuring approximately 600 m2 was found along abundant in sites with established Argentine ant colonies, the banks of an ephemeral waterway about a third of the possibly examples of what has been termed “invasional distance between the ranch and Prisoners’ Harbor (Cañada meltdown” by Simberloff and Von Holle (1999). del Puerto site); and the third measuring approximately 300 Argentine ants also have direct and indirect negative m2 was found further north along the ephemeral waterway effects on native vertebrates, including birds, mammals close to Prisoners’ Harbor (north Cañada del Puerto site). and reptiles as well as on plant-animal interactions such A total of six infestations are now known on the island. as pollination and seed dispersal that ultimately affect No Argentine ants were detected in the June 2010 plant regeneration and community composition. Sockman survey of the 12 beaches used as landing areas used by (1997) found that Argentine ants were responsible for recreational boaters. California gnatcatcher (Polioptila californica) nest failures. In coastal southern California, Argentine ants appear to Recommendations from the Expert Working Group negatively impact the coastal horned lizard (Phrynosoma The Expert Working Group reached consensus coronatum) by displacing the native ant species the lizard on three major points (numbered below). Each of the prefers to eat and which support higher lizard growth rates 18 group members was also asked to provide their (Suarez et al. 2000; Fisher et al. 2002; Suarez and Case personal conclusions and recommendations, which gave 2002). Similarly, surveys at the Point Loma Ecological additional guidance on the three consensus points. We Reserve in San Diego found that Argentine ant density then summarised this input in order to outline the effort was negatively correlated with capture rates of lizards and necessary to delimit, detect, prevent spread and re-invasion salmanders (Atkinson et al. 2003). of existing infestations of Argentine ants, and to contain In South Africa and southeastern Australia, invasive and ideally eradicate these populations. Details of the Argentine ants displaced native harvester ants and altered Expert Working Group’s findings and recommendations, seed dispersal patterns in ways that may influence the species and our plans and actions to date to implement them are composition of native vegetation. For example, native as follows. large-bodied harvester ants, which preferentially gather seeds of large-seeded shrubs in the family Proteaceae, were 1: Argentine ants are likely to spread and cause absent from areas invaded by Argentine ants in the Cape significant damage to the island’s biological diversity, Province of South Africa (Christian 2001). Other species particularly to native ants and other arthropods. of native ants that preferred seeds of smaller seeded shrub Argentine ants are known to have harmful impacts species in the same family persisted in invaded areas. As a on populations of native ants and other insects in coastal result, the larger seeds were not dispersed in invaded areas southern California and in other parts of the world. and suffered very high rates of predation by native rodents, Typically, invasions by Argentine ants are followed by losses while the smaller seeds were dispersed and escaped of medium- and large- bodied ants, plus reduced variation predation. In turn, regeneration rates of the large seeded in dietary specialisation, behavioural repertoires and nest species were an order of magnitude lower in invaded areas architecture, all of which are important to other plant and than in uninvaded areas, while regeneration rates for the animal species (Ward 1987; Holway 1998; Holway et al. smaller-seeded species were not significantly different 2002). Several of Santa Cruz Island’s 32 native ant species (Christian 2001). Near the coast southeast of Melbourne, would be threatened if Argentine ants are not managed, Australia, Argentine ants displaced the native keystone including the relatively rare Messor chamberlini and disperser Rhytidoponera victoriae. They also dispersed Pogonomyrmex californicus, a harvester ant that collects significantly fewer seeds of a nativeAcacia but significantly and stores seeds. The ability of Argentine ants to displace more seeds of the non-native invasive shrub Polygala and dominate other species is apparently greatly enhanced myrtifolia, another possible example of “invasional in California and some other regions they have invaded, meltdown” (Rowles and O’Dowd 2009). Argentine ants in part because they show little or no aggression between may likewise be capable of displacing at least some of nests and effectively form huge supercolonies with multiple the Santa Cruz Island’s native seed harvesting ants in the queens. This apparently allows Argentine ants to maintain genera Messor, Pheidole, and Pogonomyrmex, and driving extremely high population densities (Suarez et al. 1999). significant changes in seed dispersal (D. Holway, UC San The presence of Argentine ants has been associated with Diego pers. comm.).
110 Randall et al.: Argentine ants on Santa Cruz I., California
Argentine ants have great flexibility and capacity to Detect – Survey to detect any other Argentine ant exploit a wide variety of honeydew-producing aphids infestations and scale insects (Choe and Rust 2006). In return for Delimit – Use bait stations with non-toxic attractants honeydew, Argentine ants protect the honeydew insects to delimit all known infestations immediately prior to from predators which allows their densities to increase launching control efforts. (Barzman and Daane 2001; Grover et al. 2008). Their partnership with honeydew insects gives them access Control – Use toxic baits to reduce Argentine ant to carbohydrates which may facilitate their invasion of numbers and the area they infest with the goal of containing natural habitats (Rowles and Silverman 2009) and can help their spread or eliminating them from the island entirely. them to thrive when other foods are scarce making them Biosecurity – Implement protocols to prevent the more difficult to control, both in agricultural settings and spread of Argentine ants on the island and new invasions in conservation areas. to the island. The workshop concluded that Argentine ants on Santa Many Expert Working Group members underscored Cruz Island will likely have similar impacts to those the importance of strong institutional commitment to the reported elsewhere if the species is not managed and success of this approach, because effort and funding will allowed to spread. need to be sustained over the long-term if containment or eradication of Argentine ants is to be achieved. 2: Additional Argentine ant detection work should be carried out, particularly at sites with high levels of human Argentine ant Management Framework use and along the island’s major drainage. By July 2010, the following actions had been The Expert Working Group agreed that the sampling undertaken. methods used in the survey conducted in May and June 1. Surveys were carried out in November 2009 and June 2009 may have failed to detect Argentine ants in some 2010 to detect any additional Argentine ant infestations in locations, and that the Canada del Puerto in particular the island’s largest drainage, the Cañada del Medio/Cañada should be re-surveyed. Most participants recommended del Puerto. that additional detection work be carried out with sweet liquid or gel attractant. 2. Another survey of twelve beaches was carried out in June and July 2010. Recreational boaters are known 3: A management programme to prevent the spread and to land at these beaches and sometimes bring picnic and additional introductions of Argentine ants and to suppress camping gear or other equipment which may harbour or eradicate existing populations should be launched. Argentine ants. Because of the scattered nature of known infestations 3. A delimitation survey to determine the spatial extent on Santa Cruz Island, the Expert Working Group concluded of all six known infestations was scheduled for September that Argentine ants could be eradicated, but only if targeted 2010. The results will inform management efforts to be with a coordinated, multi-year control effort. Eradication is carried out in 2011 and beyond. realistic because Argentine ant queens are flightless and so 4. Biosecurity efforts to prevent the spread of Argentine – unless transported mechanically, such as by a flood event ants from known infestation sites or new introductions from or by humans – can only disperse and form new colonies by the mainland have been launched. For example, because walking, which limits their rate of spread (Krushelnycky et Argentine ants are known to be present within roughly 100m al. 2004; Silverman and Brightwell 2008). of a nursery for native plants on the island, all plant pots The largest Argentine ant eradication effort recorded to must be submerged in water and determined to be free of date was carried out in Western Australia from the mid- Argentine ants before they are allowed to leave the nursery. 1950s to 1988 (Hoffmann et al. 2010). The project was If Argentine ants are found in the nursery its operations halted when organochlorine pesticides were banned and will cease and all plants in it will be destroyed. We are in no effective alternative was found. The programme did the process of developing a full biosecurity plan for the reduce the area infested from about 18,000 ha to 1458 island. We will consult with biosecurity plans developed by ha but failed to eradicate Argentine ants from the state. the New Zealand Department of Conservation for Tiritiri One reason for this failure was that some infested areas Matangi Island and other islands, as well as the rodent could not be treated due to agricultural and environmental prevention plan developed for the Pribilof and Aleutian concerns. Another contributor to failure was reduced Islands by the Alaska Department of Fish and Game public support as the infested area declined and fewer (Fritts 2007). We are collaborating with graduate students people had direct experience with the ants (Hoffmann et at the UC Santa Barbara to evaluate potential vectors of al. 2010). Argentine ant control and eradication have also new invaders, assess potential education and outreach been attempted on islands, including Maui in the state of measures for different island visitor groups (e.g., campers, Hawaii, Norfolk Island in the southwestern Pacific, and researchers, boaters, day-hikers) and produce a cost-benefit Tiritiri Matangi Island in New Zealand. Efforts to eradicate analysis of different prevention and monitoring measures. ants from Haleakala National Park and adjacent areas on 5. Efforts are underway to identify bait formulations Maui have not been successful and the ants continue to (attractant plus toxicant) and baiting regimes which will spread although efforts to control them around high value kill Argentine ant queens and eliminate colonies and whose areas within the Park continue (Krushelnycky et al. 2005). use on TNC and NPS properties will be permitted by state Two rounds of baiting have been carried out on Norfolk and federal regulatory agencies. In order to eliminate Island but the effects of the second round have not yet been colonies rather than simply reduce the numbers of foragers, assessed and the overall effects of the project are not yet the effects of any toxicant we use must be delayed long known (V. Van Dyk, Flybusters Antiants / FBA Consulting enough to allow foraging workers to share it with workers pers. comm.). The effort on Tiritiri Matangi has been more who can pass it on to the queens they are tending (Rust et al. promising, and conservation managers believe that they 2004). University of California Riverside researchers M. are close to success (Ward 2009; C. Green, NZ DOC pers. Rust and L. Greenberg are conducting laboratory and field comm.). studies to determine which attractants are most preferred Based on experiences in other conservation areas, and which toxicants (and at what concentrations) are most the Expert Working Group recommended a four-pronged effective against Argentine ant queens. The attractants they approach:
111 Island invasives: eradication and management are testing include sucrose and water mixes and several C. Green, D. Holway, M. James, K. Kupfer, L. Laughrin, commercially available gel and liquid formulations (minus T. Matsuda, M. Meyer, M. Rust, L. Serpa, P.V. Smith, N. toxicants). Their toxicant tests build on previous work Tsutsui, J. Van Dyk, V. Van Dyk, V. Vartanian, D. Ward, by Roa (1992), Silverman and Roulston (2001), Klotz and A. Wenner. We hope to have accurately conveyed their et al. (2004a; b) and Rust et al. (2004), and include the recommendations; any errors are our own. We thank Brian toxicants dinotefuran, fipronil, imidacloprid, indoxacarb, Cohen for creating Fig. 1. We also thank D.J. O’Dowd and and thiamethoxam. an anonymous reviewer for helpful review of an earlier The product identification and permit process will take version of this paper. us past the period of the year when the launch of a control effort would be most effective in 2011. In the meantime, REFERENCES boric acid-based baits and botanical oils that kill ants on Atkinson, A.J.; Fisher, R.N.; Rochester, C.J. and Brown, C.W. 2003. contact (thyme oil) are available and do not require permits. Sampling design optimization and establishment of baselines for In situations where our 2010 surveys reveal a need to contain herpetofauna arrays at the Point Loma Ecological Reserve. United the leading edge of a known infestation, or where it may be States Geological Survey, Western Ecological Research Center, San possible to eliminate a small, newly detected infestation, Diego, California, U.S.A. 39 pp. we may choose to use these compounds in 2011. Barthell, J.F.; Randall, J.M.; Thorp, R.W. and Wenner, A.M. 2001. Promotion of seed set in yellow star-thistle by honeybees: evidence of Assuming that research, permitting, and other due an invasive mutualism. 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