Protocols for Argentine ant eradication in conservation areas

The Argentine ant (Linepithema humile) is a widespread, abundant and ecologically disruptive invader that is present throughout major portions of coastal California and on half of the California Channel Islands. On Santa Cruz Island, the Argentine ant had invaded about 2% of the island's area in four distinct locations as of 2012. Given the negative ecological effects resulting from Argentine ant invasions, we sought to develop a cost‐effective method of eradication. Here, we describe the results of large‐scale, field‐tested methods for Argentine ant eradication and post‐treatment detection. Our eradication protocol employs a novel toxicant‐delivery system: an aqueous solution of sucrose and 6 ppm of thiamethoxam mixed with hydrating polyacrylamide beads. Ants feed on the solution present on the bead's surface for about 24 h after which time bead dehydration prevents feeding. We distributed hydrated beads by helicopter over 74 ha of infested areas plus a 50‐m buffer on 14 occasions between June 2013 and September 2014. Treatments reduced Argentine ant activity to subdetectable levels within four months. In 2014, we conducted a high‐intensity detection protocol using lures (n = 55 363) in areas treated in 2013. This effort did not detect Argentine ants. In 2015, we conducted a medium‐intensity detection protocol using lures (n = 2250) in areas treated in 2013 and 2014 but not searched in 2014; this sampling effort did not detect Argentine ant activity except for a single remnant infestation (c. 0.3 ha in area), which was retreated in 2015. The cost of treatments was approximately $1400 per ha; this cost is comparable to other ant eradication efforts. The cost of our preferred detection method, which used lures spaced every 10 m, was $500 per ha. These results demonstrate sufficient protocol efficacy to justify expansion of treatments to other infested areas in ecologically sensitive areas.