In the laboratory assessment on efficacy of the traps based on ant mortality, the highest mortality rate was observed in T5 (fipronil 0.3 GR, rava and sugar (0.3:1:1)), followed by T3 (boric acid (0.5%) and sugar (25%) and T4 (Thiamethoxam 25 WG (0.0001%) and sugar (25%), which were statistically comparable. These were followed by T2 (soap and vinegar), T1 (baking soda and sugar (1:1) and T6 (talc based formulation of
B. bassiana and sugar (1:2). Accordingly, the average number of ants died in each trap on day 19 were 1.38, 1.43, 6.51, 5.29, 10 and 1.95 respectively.
Cent per cent mortality was recorded in [(fipronil, rava and sugar (0.3:1:1)] 24 hours after placement of traps. The insecticide, fipronil which belongs to the group phenyl pyrazoles, exerts its effect on the chloride channels blocking the activation of GABA
(Akhtar et al., 2021; IRAC, 2022). It has contact and stomach action and achieves control of a wide range of insect pests, however the environmental and human health implications of fipronil is a matter of concern
(Tingle et al., 2003). Though fipronil based trap has recorded the highest ant mortality, its possible threats to human and animal health cannot be neglected. High risk and toxicity of fipronil was studied by scientists in water, soil and food samples. Toxicity to bees also has been reported. Due to high toxicity, countries like USA, France, Uruguay restricted or prohibited the usage of fipronil and it is permitted for seed treatment alone in China (
GOMAC, 2008).
Against this backdrop, the use of other traps with least toxicity and greater efficiency would be more advisable in households and homesteads as there are chances of insecticide contamination in foods as ants are pests of nuisance at homes as well.
Following T6, greater ant mortality was recorded in T3 (boric acid 0.5% and sugar 25%) and T4 [thiamethoxam (0.0001%) and sugar (25%)] which were significantly on par on the nineteenth day after placement. The traps T3 and T4 showed a gradual increase in ant mortality indicating delayed toxicity of the active ingredient. To effectively manage ant populations with baits, the toxicant should have delayed action, be easily transferable between ants through trophallaxis and kill the recipient without repelling foraging ants (
Nyamukondiwa, 2008). Hence, an ideal toxicant is one that does not start killing ants for several hours, allowing it to spread completely within the entire colony (
Davis and Van Schagen, 1993).
In T3 (boric acid 0.5% and sugar 25%) 65.11 per cent mortality of
M. bicolor was recorded on the nineteenth day after treatment. Boric acid acts as a stomach poison and has been used for ant control over the past several decades. It interferes with water regulation in ants, leading to greater consumption of bait to offset dehydration
(Klotz et al., 1996). At minimal concentrations, it is highly effective in eliminating laboratory colonies of
Tapinoma melanocephalum, M. pharaonis, Solenopsis invicta and
L. humile (
Ulloa-Chacon and Jaramillo, 2003).
In treatment, thiamethoxam (0.0001%) and sugar (25%), 52.85 per cent mortality was recorded on the nineteenth day. New generation insecticides with unique modes of action are highly effective at lower doses against target pests while remaining safe for natural enemies
(Seetharamu et al., 2020). Thiamethoxam, which falls under the novel neonicotinoid class of insecticides commonly used for controlling sucking pests, has gained traction over the past few years in the insecticide industry
(Giri et al., 2018). They have specific toxicity on insect nervous system over mammalian system chiefly exerting its effect by interfering with the nicotinic acetylcholine receptor. This distinct mechanism of action makes them effective for controlling insects that have developed resistance to conventional insecticides such as the carbamates, organophosphates and pyrethroids. It has minimal impact on beneficial insects, low toxicity towards mammals and are not reported to be associated with any developmental malformations or unprecedented mutations (
Shobhana and Farid, 2008).
Thiamethoxam was selected as the active ingredient for baiting due to its relatively high water solubility of 4.10 g L
-1 at 25
oC. High water solubility is considered an essential property for developing an effective aqueous bait
(Rust et al., 1997). Alginate hydrogel beads soaked in a sucrose solution containing 1 mg L-1 thiamethoxam resulted in complete control of all castes of Argentine ants,
Linepithema humile (Mayr), within 14 days in the laboratory. Additionally, the field trial showed a 79 percent reduction in ant activity after 8 weeks
(Tay et al., 2017). However, in the present study only 52.85 per cent mortality was recorded in thiamethoxam based trap, this could be probably due to difference in ant species tested. The ant species tested (
M. bicolor) was found to have greater hardiness and hence greater longevity under artificial conditions owing to the presence of a thicker cuticle.
Peeters and Ito (2015) reported that various formicoids have a thick cuticle as an adaptation for specific lifestyles
viz.,
Calyptomyrmex,
Cataulacus,
Cephalotes,
Meranoplus (Myrmicinae),
Echinopla and
Polyrhachis (Formicinae). Cuticle is an expensive resource at colony-level. As against thinner cuticle, thicker cuticle offers protection, helps reduce desiccation losses etc. This indicated the need for administering greater dose of the toxicant for increased mortality of the test ant species.
