1. Introduction
Nylanderia fulva (Mayr) (Hymenoptera: Formicidae) is an invasive ant that is a serious pest in the southern United States. Already established as an urban and ecological pest, this ant has the potential to be an agricultural and industrial pest as well. Infestations of
N. fulva may be quite large and in some cases ants are dispersed over entire neighborhoods [
1]. One reported infestation of
N. fulva in Colombia was >10 ha [
2]. LeBrun et al. [
3] reported that introduced
N. fulva may reach numerical abundance greater than 2 orders of magnitude over all other ants. In the southern United States, when occurring at high densities,
N. fulva displaces most of the larger ant species including
Solenopsis invicta Buren.
Nylanderia fulva also significantly lowers species richness and abundance of non-ant arthropods in the invaded area [
3]. Over a period of years,
N. fulva populations decline, even without pest control intervention [
2,
4], but the reasons for population reductions have not yet been elucidated. In the meanwhile, there is a great need for information on management of
N. fulva.
To date there have been few published studies on
N. fulva control. Spraying and baiting are two application methods commonly used to control ants. Barrier treatments are applications of liquid or granular insecticides around the exterior of structures to prevent or restrict ants from entering the structure [
5,
6,
7]. Crack and crevice treatments are indoor applications of liquid insecticides to areas that provide harborages or nesting sites for ants. These structural voids also may be treated with a desiccant dust or a foam formulation of a liquid insecticide.
Both liquid sprays and baits are important to integrated pest management (IPM) strategies; however, efficacy data are often lacking. Applications of some insecticides to the exterior perimeter of a structure are effective at killing thousands of
N. fulva [
1]. However, if the cadavers are not removed, live ants simply use them as a bridge over the treated area. The immense numbers of ants in a typical infestation, homeowner actions to remove dead ants such as washing down patios and sidewalks, intense solar radiation, high humidity, and seasonal daily rainfall present challenges to the residual efficacy of insecticides applied for
N. fulva control.
Data on
N. fulva bait effectiveness also are scant. The use of insecticidal baits instead of broadcast applications of contact insecticides to control invasive species may have a positive impact on the abundance and diversity of native ants [
8]. Another advantage is that baits do not have to be collected by all foraging ants to provide effective control. Bait toxicants are transferred via trophallaxis to nestmates within the colony [
9].
Drees et al. [
10] investigated
N. fulva acceptance of some commercially available granular baits. Advance
® Carpenter Ant Bait (abamectin, BASF, Research Triangle Park, NC, USA) and Maxforce
® Complete Ant Bait (hydramethylnon, BASF, Research Triangle Park, NC, USA) were more accepted by
N. fulva than Amdro
® Ant Block (hydramethylnon, Central Garden and Pet, Atlanta, GA, USA), ProBait
® (hydramethylnon, Zoecon, Shaumburg, IL, USA), Extinguish
® Plus (hydramethylnon and methoprene, Wellmark International, Schaumburg, IL, USA), and Esteem
® (pyriproxyfen, Valent USA Corporation, Walnut Creek, CA, USA). Advance
® Carpenter Ant Bait was tested against
N. fulva in a large field trial in Texas, but it did not adequately suppress ant populations [
11]. Also in Texas, one field application of Esteem
® Ant Bait was not effective against
N. fulva, as ants from nearby untreated areas re-infested the treated area within 14 days [
12].
The objectives of this study were to identify potential products for inclusion in an IPM strategy for suppression of N. fulva by (1) evaluating the repellency and efficacy of commercially available professional contact insecticide products used as sprays and (2) to evaluate the acceptance and efficacy of commercially available ant baits in laboratory assays.
4. Discussion
In this study, we examined the repellency and efficacy of commonly used insecticide products in Florida under experimental conditions that are similar to the way in which
N. fulva would interact with the treated exterior perimeter of a structure. It was not unexpected that products such as Termidor
®, Phantom
®, and Phantom
® aerosol would not repel
N. fulva from traversing a treated surface to forage, as these products contain known non-repellent active ingredients [
15,
16,
17,
18]. However, in this closed-system bioassay, none of the products tested were truly repellent and only Temprid
® and Ortho
® resulted in significantly fewer ants crossing an insecticide treated surface to receive food and water compared to the untreated control.
It was expected that foraging workers would be exposed to the insecticide and die, requiring additional workers to forage. Thus, eventually all workers would succumb to the effects of the insecticide. We expected that without workers to tend the queens, they would eventually die of starvation [
18]. Alternatively, with non-repellent, slow-acting insecticides such as Termidor
® and Phantom
®, the foraging ants would contact the insecticide and transfer the toxicant to non-foraging nestmates resulting in worker and queen mortality [
18,
19].
Most of the insecticides tested did not provide 100% mortality by the end of the 30-day study, even when worker ants were forced to cross the treated surface for food and water, and queens survived for ~2 days after all the workers died in the Termidor
® and Temprid
® treatments. The wood panels used in the study, even though painted and treated with the highest concentration of insecticide allowed by the label, may not have had sufficient active ingredient bioavailability. Substrate effects on pesticide efficacy have been demonstrated previously. Wagner and Strawn [
20] found that knockdown of
L. humile was less than 90% one day after treating concrete with chlorpyrifos compared to six months on other substrates. A comparison of substrate effect on the efficacy of Termidor
®, Phantom
®, and Talstar
® showed that worker mortality of
Monomorium pharaonis (L.) was less on concrete than hardwood mulch [
21].
Structures in Florida are constructed of a variety of building materials and future studies should include additional commonly used construction materials. However, even if efficacy is improved by choice of a suitable substrate, laboratory bioassays do not account for environmental conditions such as overspray from irrigation, intense UV radiation, and high temperatures that may degrade the active ingredient. Furthermore, this bioassay did not account for insect behaviors that may impact the effectiveness of contact insecticides for
N. fulva control. In this study, at least some foraging ants contacted the insecticide treated surface daily to obtain food and water. Under natural conditions,
N. fulva would likely have access to alternative food resources and could avoid exposure to the insecticide on the exterior perimeter of a structure. In addition, the
N. fulva queens in this study survived without workers for ~2 days. As a polydomous tramp ant species lacking intraspecific aggression, an
N. fulva queen without workers in the field may relocate. In the laboratory, queens from one colony have been placed successfully with workers from another colony, indicating the possibility of colonies accepting new queens in field situations [
22]. The results of this study suggest further work can be conducted on the success of queen adoption by new nests. Additional studies may also investigate mechanisms that may confer reduced susceptibility of
N. fulva to insecticides on treated surfaces.
In our bait studies, the control of 10% sucrose solution proved to be more accepted than almost all other baits except Advion
® Ant Gel and InTice
™ Smart Ant Gel. Although InTice
™ Smart Ant Gel had the highest acceptance score, it did not induce significant mortality. This is not surprising, as the active ingredient, borax (=sodium tetraborate decahydrate), is known to be slow acting [
23]. It was surprising that 381 B Advance
®, a liquid formulation, was not highly accepted. The active ingredient 1.3% borax is also the active ingredient in InTice™ Smart Ant Gel (5% borax), a bait that was highly accepted. While the inert ingredients are proprietary and unknown, the advertising for InTice™ Smart Ant Gel claims that it is “super sweet,” suggesting it may have a higher concentration of sugar, and therefore, induce more feeding. In general, gel baits were more accepted by
N. fulva in this study. However, their utility in an IPM program is limited. The volume of gel bait required to impact a
N. fulva infestation would be costly and aesthetically unappealing. The recent label amendment to Maxforce
® Quantum (imidacloprid, Bayer Environmental Science, Research Triangle Park, NC, USA) allows the gel bait to be mixed into a 25% sucrose solution without compromising efficacy in an effort to satisfy the need for large quantities of bait.
In our study, most granular baits were less accepted than the liquid sucrose control by
N. fulva. However, of the granular baits, Advance
® 375A (spring, fall) and Advance
® Carpenter Ant Bait (summer) had some of the highest acceptance scores. These baits contain both protein and carbohydrate constituents. Stanley [
24] also recommended protein-based baits for some crazy ant species. Advance
® 375A and Advance
® Carpenter Ant Bait both contain the active ingredient abamectin, while baits that generally resulted in the highest mortality 3 DAT contained hydramethylnon. Granular baits containing the active ingredient boric acid were the poorest performing baits based on the criteria defined in this study of high percent mortality by 3 DAT and 100% mortality in 7 days. However, boric acid baits did provide approximately 50% mortality by 3 DAT and if the study had been conducted for a longer period of time, 100% mortality may have eventually occurred. Therefore, boric acid-containing baits may have utility in IPM programs in sensitive environments where the use of other classes of chemical insecticides is limited.
The oil-containing baits, developed to be attractive to red imported fire ants,
S. invicta, show variable acceptance by
N. fulva. During the spring and fall, oil-based baits were less accepted, while during the summer, acceptance was not different than sucrose controls. Stanley and Robinson [
25] showed that the black crazy ant,
Paratrechina longicornis, was not attracted to oil containing baits. However, Zenner-Polania [
2] used a mixture of pork lard, corn bran, fish meal, sugar, proprionic acid, and carbaryl as a
N. fulva bait that “gave good ant control for at least two months.”
Amdro
® Pro, an oil-based bait, was not highly accepted by
N. fulva, yet resulted in a high percent mortality at 3 days (
Table 5). In the small arena, no-choice assay, ants were unable to avoid the bait. A similar phenomenon was reported by Oi [
26] who documented significant
N. fulva brood reduction after exposure to the insect growth regulators pyriproxyfen and (S)-methoprene, but also noted repellency to the active ingredients. Under field conditions where ants have dietary choices, a bait formulated with an unacceptable matrix or repellent active ingredient will not provide an acceptable level of efficacy.