Comparison of the efficacy of light traps with carbon dioxide generated from dry ice and limestone reacting with acid for sampling of mosquitoes

Abstract

Traps for capturing mosquitoes and other blood-feeding arthropods are commonly baited with carbon dioxide (CO2) as an attractant. Dry ice is popularly used as a CO2 source due to its high efficiency and convenience. In many rural or remote areas, however, dry ice may be difficult to obtain. Several alternative CO2 production methods have been reported such as fermentation of sugar and yeast, reaction of weak acids and carbonate sources, and burning of petroleum gases. The problems of these methods include low or discontinuous output of CO2 or requirement of expensive tools. The objective of this study was to develop a simple and inexpensive method that can continuously generate CO2 overnight (about 10 h) to incorporate with CDC light traps for sampling of adult mosquitoes. In principle, CO2 was produced from the reaction between an aqueous solution of strong acid, hydrochloric acid (HCl) (12% w/w or about 4M), and limestone powder (mainly composed of calcium carbonate, CaCO3). In laboratory experiments, it was found that an average of 247 ml of CO2 was produced from 1 g of limestone. For continuous production of CO2, an intravenous drip infusion set, as commonly used in hospitals, was modified for dripping the acid solution (1 L in a normal saline bag) onto limestone powder (800 g in a 1.5 L bottle) at the flow rate of 30 drops/min (about 1.6 ml/min). According to this procedure, an average of 57 ml of CO2 per min was obtained, which is approximately equivalent to CO2 exhaled by two chickens. The performance of this CO2 generating system incorporated with CDC light traps for sampling of mosquitoes was evaluated in three rural villages of Sanpatong District, Chiang Mai Province, Thailand. Three trap sets were used, i.e., Set I, light trap alone; Set II, light trap with dry ice (1 kg); and Set III, light trap with limestone and acid. A Latin square design was applied to reduce biases. In each village, mosquitoes were collected at three fixed sites, each with one of the three trap sets. They were rotated daily for three rounds (nine nights per village and 27 nights in total). A total of 1,624 mosquitoes (97.7% being females) consisting of ten common and six rare species. Six genera, Aedes, Anopheles, Armigeres, Coquilletidia, Culex and Mansonia, were captured across three different sampling sets from all villages. The predominant species collected were Culex vishnui (n = 760, 46.80%), Cx. bitaeniorhynchus (n = 504, 31.03%) and Cx. tritaeniorhynchus (n = 157, 9.67%). Light trap alone (Set I) collected very low numbers of mosquitoes (n = 12) and species (6 spp.), whereas light traps with dry ice (Set II) collected the highest numbers of mosquitoes (n = 1,345) and species (16 spp.). Although light trap with limestone and acid (Set III) collected lower number of mosquitoes (n = 267) and species (9 spp.) than the trap set with dry ice, it collected all important vector species of Culex, Coquillettidia and Mansonia in the study areas as collected by Set II. The parous rates of the three predominant Culex species collected by the different trap sets were not statistically different, indicating that the presence of hydrochloric acid vapour had no bias on the collection of mosquitoes with different physiological ages. The present study demonstrated that this CO2 generating system is reliable and inexpensive and could be incorporated with CDC light trap for sampling common mosquito vector species when dry ice is not available. Modification is allowed to increase the amount of generated CO2 for higher efficacy of mosquito collection. This CO2 production method can be applied for collection of other blood sucking arthropods.