Efforts at control of the Argentine ant in Haleakala National Park, Maui, Hawaii

Abstract

The Argentine ant, Linepithema humile (Mayr), has proven to be a threat to native arthropod species in Haleakala National Park, and is also a potential threat to the park's native flora. As it continues to expand its range, an effort has been undertaken to eradicate it, or at the least, control its spread. During a year-long bait preference test implemented at each of the ant's two infestation sites, the commercially available Maxforce granular ant bait from the Clorox Corporation was found to be the most attractive and feasible bait for large scale control. Subsequently Maxforce, which is formulated with 0.9% hydramethylnon, was used in test plots to determine the efficacy of the bait in the field. Initially, Maxforce was tested at two application rates: broadcast at 2 lbs/acre and 4 lbs/acre. Later, the following treatments were also tested: a Maxforce and honey granule mix, Maxforce with 0.5% hydramethylnon, Maxforce with a different solvent, Maxforce distributed in exposed piles, and Maxforce distributed in covered piles. While there were significant differences in the magnitude of ant reduction among the various treatments, all yielded the same general result. Foraging ant numbers at monitoring bait stations declined an average maximum of 97.0% in the test plots, with no plots achieving 100% reduction. At two months post treatment the average number of foraging ants was still reduced by 92.1%. Nest survival in the plots was impacted to a lesser degree, and was difficult to measure accurately due to the occurrence of nest movement. Nevertheless, data showed no significant differences in the rates of nest survival between the treatments after two months. A second identical application in plots treated with Maxforce at 2 and 4 lbs/acre did not result in eradication. Bait attractiveness and a small window of foraging opportunity were judged to be the main obstacles in achieving total eradication. The next step in Argentine ant investigations at Haleakala should test the effectiveness of treating range margins with Maxforce for preventing or slowing range expansion.