A Simple CO₂ Generating System Incorporated with CDC Light Trap for Sampling Mosquito Vectors

Traps for capturing mosquitoes and other blood-feeding arthropods are often baited with carbon dioxide (CO₂) as an attractant. Dry ice is popularly used as a CO₂ source due to its high efficiency and ease of use. However, dry ice can be difficult to obtain in many rural and remote areas. The objective of this study was to develop a simple and inexpensive method that could continuously generate CO₂ overnight (about 10 h) while being used with CDC light traps for sampling adult mosquitoes. In principle, CO₂ was produced from the reaction between hydrochloric acid (HCl) (12% w/w) and limestone powder (mainly composed of calcium carbonate, CaCO₃). In laboratory experiments, an average of 256 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 a flow rate of 30 drops/min (about 1.6 mL/min). With this procedure, an average of 55 mL of CO₂ per min was obtained (approximately equivalent to the CO₂ exhaled by two chickens). The performance of this CO₂ generating system incorporated with CDC light traps for sampling 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. 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 (9 nights per village and 27 nights in total). A total of 1620 mosquitoes (97.7% being females) consisting of 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, 46.91%), Cx. bitaeniorhynchus (n = 504, 31.11%) and Cx. tritaeniorhynchus (n = 157, 9.69%). Light traps 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 = 1341) and species (14 spp.). Although the light trap with limestone and acid (Set III) collected fewer mosquitoes (n = 267) and species (9 spp.) than the trap set with dry ice (Set II), it collected all common vector species in the study areas as collected by Set II. The presence of an acid solution had no bias in the collection of mosquitoes with different physiological ages as determined by the parous rate. The present study demonstrated that this CO₂ generating system is reliable, simple and inexpensive, and could be an alternative to dry ice. The system can be modified to increase the amount of CO₂ generated for higher efficacy of mosquito collection. This CO₂ production method can be applied to collect other blood-sucking arthropods as well.