Experimental Control of Invasive Ant Species

Science Center Objects

Islands and atolls throughout the Pacific have been impacted by invasive ant species. Threatening native ants and other arthropods with their aggressive behavior and ability to colonize large geographic areas, invasive ants pose one of the most serious threats to island ecosystems. This project focuses on three areas of the Pacific: American Samoa, Rose Atoll, and Johnston Atoll. 

Brown tern on a branch
Common tern perched on a tree branch on Rose Atoll. Photo: P. Banko

Overview:

National Park of American Samoa: In the National Park of American Samoa (NPSA), invasive ant species may have impacts on native ants and other arthropods and are among the greatest threats to ecosystems due to the aggressive nature of many species and the ability of some to live in high densities over large areas. The relatively small size of island ecosystems in American Samoa makes them particularly vulnerable to the effects of invasive ants, which include several species that are among the world’s worst invasive species: big-headed ant (Pheidole megacephala), yellow crazy ant (Anoplolepis gracilipes), and tropical fire ant (Solenopsis geminata). Invasive ants may also impact some vertebrate species in NPSA,  including ground-nesting seabirds and landbirds, such as the friendly ground-dove (Gallicolumba stairii) and the spotless crake (Porzana tabuensis), both of which are C1 candidates for federal protection due to declining populations and limited ranges. Similar concern exists for several Partulid land snails that are also listed as C1. Ground-nesting, non-colonial bees of the families Halactidae (sweat bees) and Megachilidae (mason bees) are also at risk from invasive ants, which could undermine their role as pollinators of many plants in American Samoa. Managers need information that will help them understand the threats posed by invasive ants to native species and ecosystems under their protection.

Rose Atoll National Wildlife Refuge: Invasive ants have facilitated the rapid decline of stands of Pisonia grandis trees on Rose Atoll, Palmyra Atoll, and some other Pacific islands by protecting damaging scale insects (Homoptera: Pulvenaria urbicola) from natural enemies such as parasitic wasps and predaceous beetles. At least two species of invasive ants, the big-headed ant and Tetramorium bicarinatum, are capable of tending the scale insect on Rose Atoll. Due to the importance of Pisonia as nesting and roosting habitat for seabirds and shorebirds, eradicating the scale and the ants that protect it are considered essential to restoring seabird nesting habitat, a key goal of the Comprehensive Conservation Plan (CCP) of the Refuge. An arthropod survey would identify native and non-native elements and identify the threats that invasive ants pose to native species. A survey will lead to the development of methods to inventory and monitor arthropods on other remote island refuges and is critical for developing strategies to control ants. 

Johnston atoll sign
Photo: R. Peck

Johnston Atoll National Wildlife Refuge: The yellow crazy ant (Anoplolepis gracilipes) has irrupted on Johnston Atoll, threatening the safety of nesting seabirds. In 2010, U.S. Fish and Wildlife Service (USFWS) began a program to eradicate yellow crazy ants from Johnston Atoll using formicidal baits. By January 2012, ant abundance was reduced by >95%, but eradication was not achieved. Through 2014, USFWS tested different baits and treatments to evaluate whether ants can be eradicated, but eradication remains elusive. The succession of key personnel after four years and the re-organization of the FWS work force resulted in a loss of experienced personnel dedicated to advancing the experimental eradication program. USFWS concluded the most prudent course of action was to pursue an interagency agreement with the USGS requesting they provide technical assistance to evaluate the program and help develop a strategy for eradication.

Project Objectives:

NPSA: Objectives are to identify patterns of ant diversity and distribution, determine the composition and richness of the native ant fauna, and locate the fronts of advancing populations. Also important is identifying environmental variables that may influence the distributions of invasive ants to facilitate modeling of potential ranges of ants. Another important objective is to help managers prioritize control and prevention efforts.

Rose Atoll NWR: Objectives are to determine the diversity of terrestrial invertebrates, evaluate threats to native biota from invasive ants, and develop techniques for eliminating ants that pose the most serious threats to native species.

Pulvinaria scales on Pisonia leaves
Pulvenaria scales on a Pisonia tree leaf. Photo: R. Peck

Johnston Atoll NWR: The objective is to provide technical guidance for developing a toxic bait program that can be used to eradicate yellow crazy ants from Johnston Atoll. Technical guidance will include analyzing the results of field trials and, based on those results, suggesting new treatments to test. New treatments may include adjustments to baits currently being tested (e.g. rates or frequency of application) or suggestions for new baits or combinations of baits to test. The process is expected to be one of regular feedback between USFWS and USGS where the application of baits is fine-tuned until a feasible method for eradicating yellow crazy ants from the atoll emerges.

Pisonia grandis tree with seabirds perching
Seabirds perched in a Pisonia tree on Rose Atoll. Photo: R. Peck

Research objectives are aligned with core activities and priorities of the USGS Ecosystems Science Strategy. The research and monitoring activities are intended to benefit DOI managed lands that include high-impact invasive species and federally endangered species. Research is applied to help determine how the abundance and distribution of species may be changing over time to facilitate DOI inventory and monitoring efforts and to inform resource management and agency decision-making. Research objectives also include developing survey and monitoring standards and protocols, developing landscape-scale monitoring in alignment with DOI priorities, and monitoring responses of priority species to environmental change.

Highlights and Key Findings:

National Park of American Samoa: Surveys were completed on Tutuila, Tau, and Ofu. Overall, 19 native and 15 alien species were identified, with 8-18 species found per site. The invasive Paratrechina vaga, Strumigenys rogeri and P. minutula were most widespread, being found at 15, 14 and 13 sites, respectively. Pheidole umbonata was the most widespread native ant, found at 10 sites. Pheidole megacephala was behaviorally dominant, excluding nearly all other species from baits on the 7 sites where it was found; native Pheidole (3 species) were generally co-dominant on the other sites. The mean ratio of native to non-native ants was 2.5 times greater on sites where P. megacephala was absent compared to sites where it was found, suggesting preliminarily that Pheidole megacephala may reduce native ant diversity.

Rose Atoll National Wildlife Refuge: Arthropod surveys on Rose Atoll indicate the presence of seven ant species, two of which seem to be facilitating a scale insect that is killing native Pisonia trees, which are important nesting sites for seabirds. Using the results of the arthropod surveys as a basis for developing methods for reducing or eliminating the impacts of ants on the atoll ecosystem, we tested the efficacy of five formicidal baits in March 2013. We found that a third species interferes with efforts to eliminate the two scale-tending species but that eradication of all three species might be possible using multiple bait types in an integrated strategy.

Johnston Atoll National Wildlife Refuge: We identified concentrations of baits that were most effective at killing queen and worker ants as well as the most effective rate at which these baits should be applied over the landscape on Johnston Atoll. Yellow crazy ant numbers were reduced to near-zero levels by September 2016.

Progress:

Research results are detailed in available technical reports:

Ants of the National Park of American Samoa

Arthropods of Rose Atoll with special reference to ants and Pulvinaria urbicola scales (Hemiptera: Coccidae) on Pisonia grandis trees

Devloping tools to eradicate ecologically destructive ants on Rose Atoll: effectiveness and attractiveness of formicidal baits

Efforts to eradicate yellow crazy ants on Johnston Atoll: results from Crazy Ant Strike Teams X, XI and XII (June 2015–December 2016)

Efforts to eradicate yellow crazy ants on Johnston Atoll: Results from Crazy Ant Strike Team IX, December 2014-June 2015