Acceptance of fire ant baits by nontarget ants in Florida and California

Red imported fire ants, Solenopsis invicta Buren (Hymenoptera: Formicidae), or fire ants, are an invasive species that can be efficiently controlled with insecticidal baits. However, a major concern with fire ant baiting is the reinfestation of areas previously cleared of fire ants. Fire ants can quickly reinfest a habitat void of ants because their high reproductive rate, extensive queen dispersal, and aggressive behavior, allow them to outcompete other ants, especially in disturbed habitats where native ant abundance and diversity are reduced (King and Tschinkel 2008, 2016; Tschinkel and King 2017). Fire ant baiting can result in a repetitive pattern of suppression and reinfestation. A possible strategy to alter this cycle is to maintain populations of other ant species by only applying baits that are not accepted by nontarget ants (McNaught et al. 2014; Zakharov and Thompson 1998). Selective baits could be applied after mature fire ant populations have been cleared and young fire ant colonies are beginning to recolonize the previously baited area. Selectively eliminating fire ant colonies will improve the opportunity for other ant species to establish and offer biotic resistance to fire ant reinvasion. This strategy is applicable to fire ant eradication programs, vector control, school districts, and other entities responsible for fire ant suppression.

Because fire ant baits typically consist of active ingredients dissolved in a lipid-based food attractant, such as soybean oil (Williams et al. 2001), they are generally not foraged upon by non-oil feeding ants. However, some oil-feeding ant species do accept fire ant baits and are detrimentally impacted, such as bigheaded ants in the genus Pheidole (Hoffmann 2010). In contrast, other oil-feeding ants, e.g. Pharaoh ants, Monomorium pharaonis L. and little fire ants, Wasmannia auropunctata (Roger), do not accept some fire ant baits, presumably due to the active ingredient and/or formulation (Montgomery et al. 2015; Oi et al. 2022; Williams and Vail 1993). The objective of this study was to identify currently available fire ant baits that are not fed upon by nontarget ants.

Nontarget ants found in Gainesville (Alachua County), Florida and in the Coachella Valley (Riverside County) of California were tested to determine their acceptance or nonacceptance of various fire ant baits. Nonacceptance of a bait by several nontarget ant species would indicate which bait(s) exhibited more specificity to fire ants.

Florida bait acceptance tests were conducted in 2022 with laboratory reared colonies (Hymenoptera: Formicidae) of bicolor trailing ants, Monomorium floricola (Jerdon), and field colonies of pyramid ants, Dorymyrmex bureni (Trager); big-headed ants, Pheidole megacephala (Fabricius); and Florida harvester ants, Pogonomyrmex badius (Latreille). Bait acceptance by each species was tested among eight commercial fire ant baits: Advion^® (0.045 % indoxacarb, Syngenta, Greensboro, North Carolina, USA); Amdro^® Pro (0.73 % hydramethylnon, BASF, Research Triangle Park, North Carolina, USA); Antixx^® (0.015 % spinosad, Neudorff, Emmerthal, Germany); Clinch^® (0.011 % abamectin, Syngenta); Esteem^® (0.5 % pyriproxyfen, Valent, Walnut Creek, California); Extinguish^® (0.5 % (S)-methoprene, Wellmark, Schaumburg, Illinois, USA); Extinguish^® Plus (0.365 % hydramethylnon, 0.250 % (S)-methoprene, Wellmark); and Siesta^® (0.063 % metaflumizone, BASF).

Laboratory bait acceptance tests conducted on M. floricola (n = 3) utilized colony fragments (≥2,000 workers plus a small amount of brood) from a single laboratory colony. After a 48-h starvation period, they were given simultaneous access to the eight baits (0.5 g [approximately 1 teaspoon] each), spaced 3.8 cm apart and 7.6 or 30.4 cm from their nest cells. The baits were placed in weigh boats with one side removed to facilitate ant access. For the field acceptance tests with D. bureni (n = 6), P. badius (n = 3) and Ph. megacephala (n = 3), nests were located, and 0.5 g of each bait was placed approximately 0.9 m apart around a circle approximately 1.4 m from a nest entrance. Tests were conducted when weather was conducive to ant activity with temperatures between 26 and 32 °C. Ants feeding on or carrying away bait granules at each 10 min interval up to 60 min were summed to obtain the total count of ants that accepted each bait. If all counts were low after 60 min, sausage lures were placed adjacent to the baits for an additional 10 min to confirm foraging ants were present. Proportions of the total number of ants accepting each bait out of the total across all baits were calculated to compare bait acceptance.

Analyses of variance and Ryan-Einot-Gabriel-Welsch multiple range tests were conducted on the proportions of the total number of ants at each fire ant bait across replicates (analysis of variance procedure, SAS 9.4, 2023 SAS Institute Inc., Cary, North Carolina, USA). Separate analyses were performed for each ant species. If necessary, the logarithmic transformation (log₁₀[x + 0.01] where x = proportion of ants that accepted bait) was used to obtain homogeneous variances. Analyses were conducted only if there was a minimum of three replicates per ant species.

The percentages of bait acceptance were not statistically different among the eight fire ant baits presented to each of the four ant species tested in Florida (Table 1). However, while P. badius and D. bureni had relatively fewer ants exhibiting bait acceptance across all baits and M. floricola had large numbers at each bait, Ph. megacephala had low mean percentages and total counts at Extinguish and Siesta baits (mean, 0.7; range, 0–38; and mean, 1.6; range, 14–44; respectively). In contrast, the other baits had a mean of 255 Ph. megacephala, with a total count range of 0–724. Despite low mean percentages for Extinguish and Siesta, maximum ant counts of 38 and 44, respectively, indicated that Ph. megacephala still foraged on the baits in the presence of more accepted baits. These two baits were tested further in California in addition to other fire ant baits that were used by the Coachella Valley Mosquito and Vector Control District and/or had previously exhibited poor acceptance (Oi et al. 2022).

California bait acceptance tests were conducted in 2023 at field sites in Indio, Palm Desert, and La Quinta, California. These Coachella Valley locations were irrigated, urban turfgrass or xeriscape landscapes. Nest entrances were located for field colonies of California harvester ants, Pogonomyrmex californicus Buckley; big-headed ants, Pheidole crassicornis group; a honeypot ant, Myrmecocystus sp.; Forelius pruinosus Roger; and two bicolored pyramid ants, that keyed to Dorymyrmex bicolor Wheeler and were designated as either “large” or “small” in size. The five baits tested in California were: Advion, Esteem, Extinguish, Extinguish Plus, and Siesta.

Bait acceptance was determined by placing 1.2 cc (1/4 teaspoon) of each bait on filter paper (4.25 cm diameter, Whatman™ #1, Cytiva, Buckinghamshire, United Kingdom) positioned in a semi-circle around a nest entrance. Baits were placed equidistant (7–43 cm) from the nest entrance and 5–14 cm apart; distances depended on species. Tests were conducted when ants were active in mornings and very late afternoon, with temperatures between 27 and 35 °C. The number of ants that removed bait granules from the filter paper were recorded during 10 min intervals for 30 min then totaled over time for each bait. Proportions of total ants among baits were averaged across replicates by species to quantify bait acceptance. Tests were replicated at separate nest entrances; there were one to three replicates per species. Because we observed ants antennating bait granules during all tests, it was not necessary to confirm foraging activity with sausage lures. Analyses followed the same procedures as the Florida acceptance tests.

Two nontarget ants readily accepted all five fire ant baits. The California harvester ant removed all the bait granules at least once for each bait. The large bicolored pyramid ants also consistently removed bait granules across all baits with the Siesta bait being completely depleted in one replicate (Table 2). The small and large bicolored pyramid ants keyed to the same species, D. bicolor. However, only zero to seven small D. bicolor removed bait granules; others antennated the bait then cleaned their antennae, suggesting that all tested baits were not readily accepted. The large and small forms were found in separate sites (La Quinta and Indio, respectively) with consistently different bait acceptance, suggesting these Dorymyrmex are separate species (Oberski 2022; J. Oberski personal communication).

Bait removal by F. pruinosus was limited to zero to 10 ants with inconsistent acceptance among the baits. There were several observations of this species feeding in place on Advion and Siesta baits (15 and 25, respectively). They were seen carrying away bait placed on their trail (Extinguish Plus and Extinguish), and they fed in place and removed bait across all products in two of three replicates of the California harvester ant tests.

The big-headed ant in the Ph. crassicornis group had zero to 16 ants removing bait granules across all products. In addition, zero to at least eight ants fed in place on all the baits. Esteem bait exhibited possible selectivity with two ants removing bait which may be due to the active ingredient as reported for W. auropunctata (Oi et al. 2022). The honeypot ant species (Myrmecocystus sp.) antennated all baits but did not remove nor feed in place on any bait.

Evaluation of fire ant bait acceptance by ants in Florida and California revealed that the nontarget ant species tested either readily accepted all bait products (e.g., Pogonomyrmex spp., M. floricola, large D. bicolor), did not accept any baits (Myrmecocystus sp.), or had limited and varying bait acceptance among all baits (small D. bicolor, F. pruinosus, Ph. crassicornis). Thus, none of the fire ant bait products evaluated exhibited obvious non-acceptance by all the nontarget ant species tested. Given the wide variation in bait acceptance among the many bait products and the variety of ant species tested; the nonsignificant results are not surprising. Ant bait acceptance tests commonly evaluate either a limited number of products on various pest ants (Webb 2014), or several products on a single species (Hara et al. 2014), which reduces sources of variation to identify potentially efficacious baits. The purpose of this study was to efficiently identify fire ant bait(s) that were not detrimental to nontarget ants. Based on this study, to conserve nontarget ants, fire ant bait selection will need to consider the nontarget ants present at individual sites and specific bait(s) that are least accepted by these species. Ideally, determining the efficacy of selected baits against site-specific nontarget ants would more clearly define selective baits (Webb 2014) that encourage biotic resistance against fire ants.