One of the most important considerations in the urban pest management industry is achieving effective pest management while minimizing environmental contamination and effects on human health. Based on previous research findings, our low-impact protocol for the Argentine ant control project discussed here was carefully designed to minimize the environmental footprint of the control effort, while improving pest control effectiveness and economic feasibility.
In many urban residential areas of the United States, the Argentine ant (Linepithema humile) is one of the most common nuisance ant species treated by pest management professionals. Contact and residual insecticide sprays are among the most common treatment options for Argentine ant control because of their ease of application and cost effectiveness.
In this study, we used two new approaches (i.e., a pheromone adjuvant and a biodegradable hydrogel bait) to develop a low-impact Integrated Pest Management (IPM) protocol. Hydrogel is a super-absorbent polymer that can absorb and deliver liquid bait in a bead-shaped form. Once the ants find the hydrogel, they ingest the liquid bait from it. They will then return to the nest to share the bait with the entire colony, while creating a trail to the hydrogels that their nestmates will follow.
Boric acid has been shown to be an effective toxicant when it is incorporated in liquid ant baits. Also, it is generally considered as one of “safer” active ingredients to non-targets. Based on our previous research findings on the pheromone adjuvant and hydrogel baiting, a low-impact IPM protocol was developed and field tested. By using an insect pheromone, one can maximize efficiency for the existing treatment options by luring the target species to the areas where the treatments are made. With the new bait delivery method using the hydrogel, insecticidal liquid bait can be easily incorporated into existing service routes and protocols without impacting the economic feasibility.
TREATMENT PROTOCOL. Residences in Riverside, Calif., were used for the experiments. A one-time perimeter treatment with a fipronil spray at the beginning of summer was incorporated (see Fig. 1). Label instruction for California was followed (0.03 percent for perimeter application). For five treatment houses, the initial fipronil treatment contained a pheromone adjuvant — microencapsulated (Z)-9-hexadecenal (Suterra, Bend, Ore.; 25 ml per 3.8 liter of spray). For comparison, another 10 control houses were treated with 0.03 percent fipronil only. About 0.25-0.3 gal of fipronil spray was used per house.
For all the treatment houses, the initial spray application was followed by one maintenance treatment at week four post-treatment. The biodegradable alginate (seaweed-based) hydrogel bait was produced by the method developed by Tay et al. (2017) (see Fig. 2). In short, sodium alginate solution (1 percent) was slowly dispensed dropwise through a modified 8-inch shower head nozzle (1.6 mm diameter). The droplets were immediately collected in a plastic container with 0.5 percent calcium chloride crosslinker solution. After two minutes, the resulting hydrogel beads were filtered out from the crosslinking solution and rinsed with clean water. The rinsed hydrogel beads were “conditioned” by submerging them in a liquid bait containing sucrose and boric acid overnight (24 hours). Concentrations of the sucrose and boric acid in the final hydrogel bait were 25 and 1 percent, respectively. To improve stability of the final hydrogel bait, 0.25 percent sorbic acid potassium salt was incorporated in the final hydrogel bait. A pheromone adjuvant also was mixed with the hydrogel bait immediately before application. About 4-7 liter of hydrogel bait was used per house (about 40-70 g boric acid per house). The hydrogel bait was scattered on the ground using a manual or motorized spreader, mostly on active ant trails, soil or vegetated surfaces within 5 m of the building (see Fig. 3).
Foraging activity level of ants was estimated based on the total amount of sucrose solution consumed over a 24-hour period (average value from 10 monitoring sites). The sites were monitored once before the initial spray treatment, and weeks one, two and four after the treatment. After the maintenance treatment (hydrogel baiting) in week four, the sites were further monitored at weeks five, six and eight. The efficacy of the treatments was investigated by comparing the ant foraging activity levels between treatment and control houses, or among different monitoring time points within a treatment. Data from one untreated house (not replicated) were used to show natural seasonal ant activity for both years, but not used for the statistical analyses.
CONTROL EFFICACY. Infestation levels in the treatment houses were significantly reduced over time. Multiple comparison tests indicated that the initial perimeter fipronil spray mixed with the pheromone adjuvant provided significant reductions in the ant foraging activity level immediately after the spray application (see Fig. 4, blue line). However, the control houses that received the fipronil spray only (without pheromone adjuvant) did not show any significant reduction in ant foraging activity after the treatment (on page 42, orange line). In fact, some control houses experienced a substantial increase in ant activity even after the initial spray application.
As the maintenance treatment, the biodegradable boric acid hydrogel bait (between week four and five) was effective in providing immediate reduction in ant foraging activity (week five), keeping the ant infestation levels low for the treatment houses (see Fig. 5). By week eight, the treatment houses had an overall 80 percent reduction in ant foraging activity level when compared to pre-treatment data (before the initial spray treatment). During the entire study period, the untreated control house did not show any consistent drop in ant activity.
FINAL THOUGHTS. The novel spray and hydrogel baiting protocol demonstrated in the current study could be an effective method in managing Argentine ants without repeated use of sprays. The pheromone adjuvant will increase the numbers of exposed ants, thereby maximizing the efficacy of residual spray products. When used as a stand-alone method in a preliminary study, the biodegradable hydrogel bait with boric acid takes a few weeks to achieve the acceptable levels of control (more than 80 percent reduction) against large populations of Argentine ants. Thus, perimeter treatment with an effective spray material would be useful in providing the initial quick control.
With its relatively low toxicity profile on non-target organisms, boric acid baiting can be an important tool for the follow-up maintenance services. Relatively high cost associated with material and labor has been a drawback for conventional baiting methods. The use of a biodegradable hydrogel matrix as a carrier of liquid bait is an important breakthrough in addressing this challenge when controlling pestiferous sugar-feeding ants. The low-impact IPM protocols demonstrated in this study keeps the ant activity levels low for a longer period of time, which would also reduce the callbacks, substantially saving the cost associated with re-treatments.