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RESEARCH ARTICLE
Year : 2022 | Volume
: 59
| Issue : 1 | Page : 52-56
Laboratory evaluation of the efficacy of boric acid containing toxic sugar baits against Anopheles culicifacies, An. stephensi and Aedes aegypti mosquitoes
Gaurav Kumar, Amit Sharma, Ramesh C Dhiman
ICMR-National Institute of Malaria Research, New Delhi, India
Correspondence Address:
Ramesh C Dhiman
ICMR-National Institute of Malaria Research, Sector-8, Dwarka, New Delhi-110077
India
Source of Support: None, Conflict of Interest: None
CheckDOI: 10.4103/0972-9062.331414
Background & objectives: Attractive toxic sugar baits (ATSB) is a novel tool which employs mosquito sugar feeding behaviour to kill them. The potential of ATSB against mosquito vectors has been demonstrated in limited scope around the world including Israel, some of the African countries and USA. But their efficacy against mosquito vectors of India is yet to be ascertained. Therefore, current study was planned to evaluate the efficacy of TSB (without attractant) against two important malaria vectors Anopheles culicifacies and An. stephensi along with major dengue vector Aedes aegypti.
Methods: TSB solution was prepared by dissolving different concentrations of boric acid in glucose and tested against each of the mosquito species. Another experiment was done by spraying this boric acid sugar solution on to Calendula officinalis plant. It served as a sole source of mosquito feed and mortality of mosquitoes was counted after 24 hours.
Results: The TSB resulted in 100% mortality of Ae. aegypti and An. stephensi at 4% concentration of boric acid while in An. culicifacies 100% mortality was achieved at 3% concentration TSB solution. TSB solution with 2% boric acid, resulted in 99.1% mortality in An. culicifacies while ~95 % mortality of An. stephensi and Ae. aegypti. When TSB solution with 2% boric acid was sprayed on plant, ~89-94% mortality was observed in both An. stephensi and An. culicifacies.
Interpretation & conclusion: Based on the results, boric acid based toxic sugar bait solution could be a promising tool for vector control. Further studies are needed to find out its toxicity of TSB against non-target organisms and residual efficacy in field trials in different ecotypes.
Keywords: attractive toxic sugar baits (ATSB); mosquito vector; boric acid; India; malaria
Malaria and dengue are the two most prominent vector borne diseases prevalent in India. During 2019, 0.33 million cases of malaria with 77 deaths and 0.16 million cases of dengue with 166 deaths were reported in India[1]. There are six primary vectors of malaria in the country of which Anopheles culicifacies and An. stephensi cause more than 70% of the malaria cases[2],[3]. For malaria, larval control through larvicides and indoor residual spraying (IRS) along with distribution of long-lasting insecticidal nets (LLINs) against adult vectors are the operational intervention strategies of the national programme. In India, dengue is mainly transmitted by Aedes aegypti mosquitoes and vector control is the only option to control dengue. The threat of development of resistance against chemical insecticides poses a major hurdle in the control of mosquito vectors. Therefore, it is necessary to employ additional tools which can be used at a community level to control the mosquito vectors.
The first toxic sugar bait was used by Lea (1965) who used different concentrations of malathion with 20% sucrose solution for feeding of Ae. aegypti resulting in 85% mortality of the mosquitoes[4]. Later, various toxicants were used for adult mosquito control which included boric acid, spinosad, chlorfenapyr and dinotefuran[5],[6],[7],[8]. Due to low mammalian toxicity of boric acid, it has been used in attractive toxic sugar baits (ATSB) against different species of mosquitoes[9]. ATSB using boric acid was used to control Culex quinquefasciatus in storm drains of Florida[10]. A field trial of ATSB with plant-spraying method was conducted against the malaria vector An. gambiae s.l. in Mali, West Africa[9]. In both these trials, 85–90% reduction in mosquito populations was reported[9],[10]. Evaluation of boric acid baits against Ae. albopictus in tropical environments of Florida was also done, and nearly 52% mortality of mosquitoes was observed[11].
ATSB is a recently developed mosquito vector tool which utilizes sugar feeding behaviour of mosquitoes to lure and kill them by employing an orally ingested toxin[9]. Initially, it was developed in Israel to control the mosquito An. sergentii[5],[12]. Later, ATSB was used to control An. gambiae s.l. populations in Mali and was found to be successful in reducing vector population of mosquitoes by >90%[9]. Additionally, field trials in both Israel and Mali have shown that ATSB not only reduced the total number of mosquitoes, but also reduced the longevity of older mosquitoes[9],[12],[13]. To our knowledge, ATSB has not been evaluated against Indian malaria and dengue vectors. Therefore, here we have tested the efficacy of TSB (without any attractant) against An. culicifacies, An. stephensi and Ae. aegypti under laboratory conditions using boric acid.
Material & MethodsTwo species of mosquitoes An. stephensi and An. culicifacies were obtained from the central insectary of ICMR-National Institute of Malaria Research (ICMR-NIMR), New Delhi. F1 generation of field caught Ae. aegypti were used for the study. Adult An. stephensi, An. culicifacies and Ae. aegypti (3–5 day old) were starved for six hours and then used for experiments[14]. Toxic sugar bait (TSB) solution was prepared by dissolving various concentrations of boric acid SRL make with 99.5%, (1%, 2%, 3% and 4% w/v) and glucose (10% w/v) in water.
Evaluation of TSB under laboratory settings
Cotton pads soaked in TSB solution in a disposable plastic bowl were placed into inside Barraud’s cage (30x 30x30 cm). Both male and female mosquitoes (20 each in 1:1 ratio) were introduced into cages and allowed to feed on cotton pad [Figure 1]A. After 24 hours, observations on mortality were taken by scoring live or dead. The experiments were done with three replicates with one control without boric acid (using test species of same generation). Only sugar solution in cotton pads was provided to for control. During the experiment room temperature was maintained at 26–28°C with 70–80% relative humidity and 12:12hr light and dark period. To prevent scavenging ants, legs of table were dipped into water containing plastic bowls.
Figure 1: Laboratory experiment showing evaluation of 2% boric acid in Attractive Toxic Sugar Bait, in cotton pad (A.) and sprayed on plant (B.) on malaria vectorsKnock down of mosquitoes due to boric acid TSB ingestion was determined for two doses of boric acid 2% and 3%. Mortality of An. stephensi and An. culicifacies was recorded every hour upto six hours and thereafter at 24h interval. To assess the residual efficacy of TSB, both male and female An. stephensi mosquitoes (20 each in 1:1 ratio) were exposed to 2% boric acid TSB as per above mentioned procedure in triplicate. The plastic bowl containing the cotton pad was filled with TSB solution so as to keep cotton wet for longer duration. Each day, fresh batch of mosquitoes (from new generation) were introduced in new cages up to 8 days period while the cotton pad soaked in TSB solution remain unchanged. The mortality of mosquitoes was recorded after 24 hr period.
Effect of ATSBs sprayed on plants
The methodology used in the experiment with plants was adapted from Xue et al. (2006)[15]. The experiment was performed using large clothed cages (60x60x60 cm). One commonly available plant Calendula officinalis was selected randomly for the experiment as it was present in fair number in flower pots at the premises of ICMR-NIMR, Delhi. The plant was first washed by spraying water to remove any dust. Once the sprayed water was dried, the plant was sprayed with 50 ml of TSB solution with one control plant sprayed with 10% glucose solution (without boric acid) [Figure 1]. Thereafter, the plant was introduced into each cage [Figure 1]B. One hour after spraying on plants, 100 mosquitoes (1:1 sex ratio) of An. culicifacies and An. stephensi were released in each cage. Mortality of the mosquitoes in each treatment was measured after 24 h. When mortality in control was 5–20 percent, Abbott’s formula was applied to correct the test mortality[16].
ResultsThe efficacy of TSB resulted in 100% mortality of Ae. aegypti and An. stephensi at 4% concentration of boric acid while in An. culicifacies at 3% concentration TSB solution 100% mortality was achieved. TSB solution with 2% boric acid, resulted in 99.1% mortality in An. culicifacies while ~95% mortality of An. stephensi and Ae. aegypti. When the concentration of boric acid in TSB solution was lowered to 1%, mortality was still higher in An. culicifacies (86.1%) but lower in case of An. stephensi (16.2%) and Ae. aegypti (33.3%) [Table 1].
Table 1: Impact of Toxic Sugar Bait (TSB) on the survival of mosquito vectorsWhen TSB solution with 2% boric acid was sprayed on plants, 88.9-94.4% mortality was observed in both An. stephensi and An. culicifacies. The mortality of mosquitoes in control (plants sprayed with sugar only) was 10% [Table 2]. Thus, boric acid-based toxic sugar solution was found effective against An. culicifacies, An. stephensi and Ae. aegypti mosquitoes.
To understand how fast the mortality of mosquitoes occurs due to consumption of consumption of TSB containing boric acid, we recorded hourly knock down of mosquitoes for six hours and thereafter mortality was recorded after 24 hr. Results of the test indicated that mortality is very slow (0–5% in An. stephensi and 0–17% in An. culicifacies) initially up to six hours and boric acid ingestion takes time to kill mosquitoes [Figure 2]. There was no significant difference on knock down of mosquitoes with increase of boric acid concentration from 2% to 3% in ATSB used.
Figure 2: Knockdown effect of boric acid ATSB against Indian malaria vectors An. culicifacies and An. stephensiExperiment on residual efficacy of boric acid containing TSB revealed that boric acid based TSB is effective in controlling An. stephensi mosquitoes for eight days with mortality rate of 91% after 1 day to 59% after 8 days [Figure 3]. Mortality in males (56–92%) was always higher than females (50–91%) but the difference was not significant.
Figure 3: Residual efficacy of ATSB containing 2% boric acid against An. stephensi mosquito Discussion In the present study, 94–100% mortality in An. culicifacies, An. stephensi and Ae. aegypti mosquitoes was observed with 2–4% boric acid in sugar solution in 24 hours, under laboratory conditions. When sprayed on plants, boric acid based TSB solution also showed good results with ~90% mortality of the mosquitoes after 24 hours. Muller et al. (2010)[9] and Beier et al. (2012)[12] found 90% and 95–100% mortality of An. gambiae and An. sergentii in field studies undertaken in Mali (West Africa) and Israel respectively. Our results are in conformity with studies undertaken on ATSB using 2% boric acid against malaria vectors of West Africa and Israel. The mortality against Aedes and Culex species has been reported lesser than anophelines[11],[15],[17],[18],[19]. In our laboratory study using TSB on plant, 10% mortality was found in An. culicifacies and An. stephensi in control group also, it might be due to innate property of plant or any unidentified natural conditions which may be investigated further. As we use control of the same plant the effect is independent of plant property in test cage and therefore, corrected mortality was used.
Biologically, the male mosquitoes depend exclusively on plant sugars for their survival while female mosquitoes also visit plants for sugar meal. The requirement of Ae. aegypti (being container breeder) for visiting plants for sugar meal appears to be less than An. culicifacies which is a rural malaria vector[20],[21]. It is reflected by high mortality (86.1%) of An. culicifacies in 1% TSB as compared to 33.3 and 16.2% mortality in Ae. aegypti and An. stephensi respectively. As An. culicifacies is outdoor breeder, the mosquitoes are totally dependent on surrounding vegetation. Ae. aegypti and An. stephensi are highly adapted for urban system and indoor container breeder. Therefore, these are less dependent on surrounding vegetation. It cannot be ascertained whether Aedes mosquitoes took less boric acid, which is a matter of further research. The duration of residual efficacy of sugar pad with boric acid was determined for one week period and it effectively killed the mosquitoes with ~60% mortality till 8 days. Therefore, it can be used as an efficient tool to control vector population indoors along with LLIN.
After taking blood meal, the female mosquitoes have to take rest mainly indoors. After the digestion of blood meal, TSB may be taken by female mosquitoes searching sugar source. Therefore, the feasibility of using ATSB indoors, appears more fruitful for controlling female mosquitoes which transmit pathogens. The current study paves the way for further use of ATSB by adding attractant under field conditions in different ecotypes in India. ATSB has the potential for personal protection from mosquito bites, particularly in areas with less vegetation where mosquitoes find scarce source of plants as source of sugar. The ATSB technology is cost effective and can be adopted by the communities for getting relief from mosquito bites and thus reduction in malaria or dengue. Therefore, based on the results, boric acid based toxic sugar bait solution could be a promising tool for vector control and to ascertain that, further studies are needed to find out its toxicity against non-target organisms and its residual efficacy in field trials in different ecotypes.
AcknowledgementsThe authors are thankful to laboratory staff particularly Mr. M.C. Sharma and Mr. V.P. Singh for providing necessary help in conducting laboratory experiments. Intramural funds of ICMR-NIMR, Delhi funded the study.
Conflict of interest: None
References
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