Laboratory and Field Evaluations of Polyacrylamide Hydrogel Baits Against Argentine Ants (Hymenoptera: Formicidae).

The development of effective baits to control the Argentine ant, Linepithema humile (Mayr), has been problematic because foragers prefer sweet liquids, while many toxicants are insoluble in water and liquid baits are generally difficult to deliver. The incorporation of thiamethoxam and sucrose solutions into a water-absorbing polyacrylamide hydrogel provides a unique and novel carrier and method of application for liquid baits. Formulations of thiamethoxam affected the size of the hydrogels, and sucrose solutions containing 0.0003% technical thiamethoxam provided hydrogels as large as those made with 25% sucrose solution or deionized water. Concentrations of thiamethoxam as low as 0.000075% in the hydrogels provided 50% kill of workers within 3 d in a laboratory setting. In small colony studies, baiting with 0.00015 and 0.000075% thiamethoxam hydrogels provided 100% mortality of workers and queens within 8 d. An enzyme-linked immunosorbent assay indicated that thiamethoxam was absorbed into the interior of the polyacrylamide matrix. The water loss rates of the hydrogels were dependent upon the relative humidity. Polyacrylamide hydrogels with >50% water loss were less attractive to ants. Field studies in highly infested areas indicated that concentrations of 0.0006 or 0.0018% thiamethoxam were more effective than 0.00015%. Hydrogels may provide a cost-effective alternative to providing aqueous baits to control Argentine ants.

Ant control efficacy of pyrethroids and fipronil on outdoor concrete surfaces.

BACKGROUND: Urban and residential concrete surfaces are often treated with pesticides to control ants. Run-off from irrigation/rainfall can remove pesticides from concrete and contaminate urban waterways. Recent regulations and mitigations in regions such as California aim to reduce insecticide run-off (e.g. pyrethroids), but are often proposed without adequate consideration of their impact on ant control efficacy. METHODS: We carried out an outdoor study with a side-by-side comparison between pesticide run-off potential and residual ant toxicity after exposing treated concrete to summer conditions and simulated precipitations. RESULTS: Treatments with bifenthrin, permethrin or fipronil all showed fast ant knockdown initially, and over 50% of ants were killed within 16 h after 1-min contact with the treated surfaces. Shorter ant median lethal time (LT50) was observed on concrete treated with over-the-counter (OTC) granule/dust formulations than OTC liquid or professional formulations. However, the treated surfaces rapidly lost ant control efficacy after outdoor exposure and repeated precipitations. Except for OTC solid permethrin treatment, the ant toxicity disappeared after 20 days for all other formulation treatments. In contrast, pesticides were detected in run-off water from simulated precipitations even 89 days after the treatment, with levels above 0.5 µg L(-1) for bifenthrin, 30 µg L(-1) for permethrin and 0.15 µg L(-1) for fipronil. Pyrethroid run-offs from OTC solid formulations were >10 times higher than the other two formulations. CONCLUSIONS: The results suggested viable options for run-off mitigation include decreasing use on concrete such as using crack/spot treatments instead of broadcast applications, limiting pesticide use on hardscapes away from water contact, and avoiding using dust/granule formulations on hardscapes.

Effect of horizontal transfer of barrier insecticides to control Argentine ants (Hymenoptera: Formicidae).

Horizontal transfer of three contact insecticides, bifenthrin, beta-cyfluthrin, and fipronil, was tested in laboratory colonies. Donor ants were exposed for 1 min to insecticide-treated sand substrates and placed with unexposed ant colonies at two different temperatures. Mortality was monitored to compare the ability of donors to transfer lethal doses of these insecticides to untreated individuals. Treated donor insects, live or dead, were added into colonies to determine the importance of donor behavior on lethal transfer. Fipronil was readily transferable between individuals, resulting in high mortality rates. Bifenthrin and beta-cyfluthrin were less transferable, exhibiting moderate-to-low mortality rates similar to the controls. Greater mortality occurred at 27-29 degrees C than at 21-23 degrees C for bifenthrin, but not the other treatments or controls. Colony mortality did not significantly increase when adding live donors, suggesting that necrophoresis was probably an important donor behavior in addition to grooming and trophallaxis on horizontal transfer.

Effect of delayed toxicity of chemical barriers to control Argentine ants (Hymenoptera: Formicidae).

Brief exposures of Argentine ants to four different insecticide treatments, bifenthrin, beta-cyfluthrin, bifenthrin + beta-cyfluthrin, and fipronil, were conducted to determine Kaplan-Meier product limit survivorship percentiles (SPs) at 21-23 and 27-29 degrees C. Bifenthrin, beta-cyfluthrin, and bifenthrin + beta-cyfluthrin provided rapid kill at 21-23 degrees C with SP10 values ranging from 11.2 to 33.7 min. Fipronil provided delayed toxicity at 21-23 degrees C with SP10 values ranging from 270 to 960 min. At 27-29 degrees C, all of the SP10 values significantly decreased. Field tests in which Argentine ants were induced to forage across insecticide-treated surfaces were used to determine the effect that speed of action has on foraging and recruitment ability, and whether these insecticides are repellent. The slower-acting fipronil allowed a greater amount of foraging and consequently a greater fraction of the colony to be exposed, whereas fast-acting bifenthrin, beta-cyfluthrin, and bifenthrin + beta-cyfluthrin inhibited recruitment, resulting in fewer ants being exposed and killed. Implications for controlling ants by using perimeter barrier treatments are discussed.