Crystal Clear

Current chemical control options to manage urban pest ants rely on residual sprays or baits, each one posing unique disadvantages that limit their usefulness and efficacy. Residual sprays are often highly effective, but many have little long-term impact and re-infestations are common. This is because sprays mostly kill the foraging workers, but have less effect on reproductives and brood, which often are protected within subterranean nests away from treated areas. This necessitates frequent re-applications. Another drawback of residual insecticides is that they kill a significant proportion of non-target, beneficial organisms. Finally, the relatively long residual activity of some sprays is a concern with regards to re-application intervals, environmental pollution and pesticide residues.

An alternative to residual sprays is baits, which exploit the recruitment and food-sharing behavior of ants to maximize efficacy. Baits reduce the need for broadcast sprays and therefore limit the amount of insecticide used. Baits typically are applied as granules or gels, and liquid baits are relatively uncommon. Granular baits are protein- or oil-based and are mostly effective against protein-feeding Myrmicinae ants such as fire ants and big-headed ants, but are only weakly attractive to ants that feed mainly on liquid sugars.

Liquid baits are attractive to pest ants because they mimic natural food sources such as honeydew and nectar, the principal dietary components of many ants. However, the adoption of liquid baits for urban pest management has been rather slow and liquid baits are currently not a popular option for ant management. One disadvantage is their relatively short life span under field conditions. Because of their high sugar content, liquid baits are highly susceptible to water loss by evaporation and contamination by yeast, which leads to fermentation and loss of palatability.

Another major challenge that precludes a wider adoption of liquid baits in ant management is the lack of commercially available bait dispensers that are effective, inexpensive and easy to service. Currently available bait dispensers are labor intensive because they need to be put out in fairly large densities, require periodic cleaning and maintenance, and need to be frequently monitored and refilled.

HYDROGELS. To overcome the problems associated with traditional baits, Purdue University’s Grzegorz Buczkowski and his team tested a novel bait delivery technology that relies on water-storing crystals. Water-storing crystals, also known as hydrogels, are superabsorbent polymers that are widely used in forestry, horticulture and landscaping as a means of conserving water. The crystals are able to absorb about 300 times their weight in water. Dry granules resemble salt crystals and when immersed in water swell up and turn into a gel-like substance that resembles chunks of ice. The hydrated granules slowly release water over time and the process may be repeated indefinitely. The saturated water crystals combine the advantages of both sprays and liquid baits because they are easy to apply, don’t require a dispenser, don’t need to be serviced or refilled, can be applied using traditional broadcast spreaders, are relatively inexpensive and minimize insecticide use because they require a relatively small amount of active ingredient.

RESULTS. Results of laboratory trials showed that bait crystals saturated in 25 percent sucrose solution containing 0.0007 percent thiamethoxam are highly attractive to Argentine ants and effective against all castes and life stages, including workers, queens and brood (Buczkowski et al. 2014). Fresh bait crystals were highly effective and required approximately two days to kill all workers and about six days to achieve complete mortality in queens and brood. Results of bait aging tests showed that the bait crystals lose about 70 percent of moisture in eight hours and the duration of outdoor exposure had a significant effect on bait acceptance and efficacy. A gradual decrease in mortality was observed for all castes and life stages as bait age increased. In general, fresh baits and those aged for less than eight hours retained their efficacy and caused substantial mortality. Baits aged longer than eight hours were substantially less attractive and less effective.

Results from field studies with bait crystals containing 0.0007 percent thiamethoxam corroborate the results of laboratory tests and demonstrate that thiamethoxam is highly effective in suppressing Argentine ant populations (Buczkowski et al. 2015). Bait crystals were scattered directly on the ground within the treated plots and ant activity in the treated plots declined by 94 percent within two weeks.

TRACKING BAIT DISPERSAL. Baits are an excellent tool for managing ant populations, but the efficacy of baits is linked to the understanding of ant foraging patters and ant movement. The challenge is to know how many bait placements are necessary for effective control. Specifically, it is important to determine the optimal bait density to maximize bait efficacy while minimizing material and labor costs.

Previous studies show that incremental increases in bait station density have an increasingly suppressive effect on ant activity. Optimal bait density may depend on many factors including the size of the ant populations, time of year and the presence of alternative food sources. Another important factor is the distance and direction that the ants carry the bait once a bait station is located. The colonies of many pest ants are made up of multiple nests that may cover a relatively large area. The nests may be connected by trails that serve as pathways for exchanging toxic baits among the nests, but the degree to which ant foraging patterns and foraging distance affect bait efficacy are not well understood.

A field study in a commercial plum orchard infested with Argentine ants was designed to shed light on this issue (Buczkowski et al. 2015). The ants were observed to forage mostly along tree rows, following irrigation pipes and trellises, and bait movement was expected to follow a similar pattern. Bait crystals saturated in 25 percent sugar solution and containing a marker were placed at the beginning of foraging trails and the downstream movement of ants (and therefore bait) was estimated by sampling workers from various points along the foraging trails. Results of the marking study showed that the percentage of ants testing positive for the marker declined sharply as distance from the bait station increased. At 24 hours after baiting, 70 percent of ants tested positive for the marker at the bait station and the percentage declined to 33 percent at 8 meters and 8 percent at 17 meters from the bait station.

These results demonstrate that Argentine ants forage locally and distribute bait to nearby nests. This suggests that numerous bait stations may be necessary to reach all nests and achieve satisfactory control and baits should be placed close to foraging trails.

CONCLUSIONS. Liquid bait use has been limited due to the lack of bait dispensers that are effective, inexpensive and easy to service especially when used on a commercial scale. Bait crystals overcome many of these limitations and combined with a toxicant (thiamethoxam) are an effective tool for managing ant infestations. The results also demonstrate that bait crystals might be an attractive economic alternative to traditional bait dispensers.

However, the use of thiamethoxam in combination with hydrogels is not yet registered and was used only for experimental purposes in the study. Future studies will focus on testing the bait crystals against a wider range of urban pest ants, evaluating different active ingredients and feeding attractants in combination with the bait crystals, testing the bait crystals in a variety of urban situations, and evaluating the potential for commercial development.

References

Buczkowski G., Roper E., Chin D., Mothapo N. and Wossler T. (2014a). Hydrogel baits with low-dose thiamethoxam for sustainable Argentine ant management in commercial orchards. Entomologia Experimentalis et Applicata. 153: 183–190.

Buczkowski G., Roper E. and Chin D. (2014b). Polyacrylamide hydrogels: an effective tool for delivering liquid baits to pest ants (Hymenoptera: Formicidae). Journal of Economic Entomology, 107:778–757.