Traps

An enormous amount of consumer interest has been generated by the marketing of new devices designed to attract, then either trap or kill, mosquitoes. The general idea is to reduce the number of questing mosquitoes that would otherwise be afflicting the homeowner. Many products even claim to significantly reduce or even collapse local mosquito populations by decreasing the number of egg-laying females through their capture.

Power supplies for each type of trap vary. Some are totally self-contained, utilizing propane to provide both power and a source of carbon dioxide as an attractant. These units have the advantage of portability, allowing them to be placed at a considerable distance away from home-sites and electrical outlets. This may be an important consideration on larger properties, i.e. those over an acre in area, by allowing mosquitoes to be intercepted long before they come into the vicinity of human activity. This portability comes at a price, though, for the thermoelectric generator that uses excess heat from the combustion process to generate electricity to run the intake fans is quite expensive. Most units rely upon power cords utilizing AC outlets. This limits them somewhat to smaller areas served by extension cords, but their price is commensurately less than their self-contained counterparts.

All of these traps utilize some form of attractant that lures the host-seeking female mosquitoes to a capture or killing device. In some cases, mosquitoes are captured via an impellor fan that suctions them into a net, where they desiccate while other trapping systems use a sticky surface to which the mosquitoes adhere when they land. Still others utilize an electric grid to electrocute mosquitoes drawn into contact. Attractants used are generally variations on a common theme of imitating the mammalian scents and body heat that provide host cues to questing female mosquitoes. The vast majority of these traps use carbon dioxide, produced either through the combustion of propane or via CO2 cylinder as the primary attractant. The CO2 is released at between 350m (for propane) and 500 ml/min (cylinder with regulator). The plume of CO2 produced mimics human exhalation and thus makes these traps quite specific for capturing blood-feeding insects looking for human hosts. Therefore, non-target insects such as moths and beetles will be largely unaffected. The CO2 is often synergized with 1-Octen-3-ol (a derivative of gasses produced in the rumen of cows) to increase attractiveness by several orders of magnitude. The 1-Octen-3-ol is slow-released at a rate of ca. 0.5 mg/h. In areas where the Asian Tiger Mosquito is a problem, specific lures must be used to effect capture. The BG-Sentinel Trap (http://www.bg-sentinel.com/en/bg-sentinel.html) is proving to be an excellent mechanism for capturing large numbers of this species compared to other traps but whether this translates into a reduction in biting pressure is problematic. These are not set-and-forget devices each requires some level of maintenance, i.e. propane tanks need replacement, capture nets need emptying, adhesive boards require replacement and grids require cleaning to ensure their continued effectiveness, particularly in areas of high catch.

The process of a mosquito questing for a blood meal involves a complex, interconnected cascade of behaviors, each probably having its own cues, be they visual, thermal, or olfactory. The complexity of these questing behaviors may account for the bewildering variations in trapping efficiency noted for certain species of mosquitoes at different times, seasons and places. With 176 species of mosquitoes currently recognized in the United States, this is no small issue and will require many years before research can provide a clarification. There is some anecdotal evidence that these baited traps, indeed, capture more females of some species than others, depending, to some extent, on the concentration of carbon dioxide emitted, the lure used and the mosquito species present. There may also be seasonal and circadian variables that affect mosquito responses to certain attractants. For example, a few years ago the Salt Lake City Mosquito Abatement District ran a comparison test of the Mosquito Magnet with an American Biophysics ABC trap. Each trap was operated for one night and then switched to the other’s location over a two-week period. The Mosquito Magnet captured enormous numbers of Ochlerotatus sierrensis, the western tree hole mosquito, but few Culex pipiens, Culex tarsalis, or Ochlerotatus dorsalis. The ABC trap performed just the opposite, capturing great numbers of Culex pipiens. The reasons for this are not entirely clear, but serve to underscore the need for more research and to point out that each trap may have its own operational use.

Nonetheless, these devices will trap and kill measurable numbers of mosquitoes. Whether this will produce a noticeable reduction in the mosquito population in each case will depend upon a number of factors, e.g. individual tolerance level, absolute mosquito population size, proximity, size and type of breeding habitat producing re-infestation, wind velocity and direction, and species of mosquito present, and others. Depending upon their placement, wind direction, and inherent trapping efficiency, traps may actually draw more mosquitoes into an area than they can possibly catch. Thus, the homeowner must still use repellents and practice source reduction methods as adjuncts to realize any measure of relief. Indeed, the AMCA has received a number of testimonials from buyers who are dissatisfied with these products and the level of support they've received from the manufacturers. Whether these reflect failure to follow placement or baiting directions is unknown. Many districts have used CDC Light Traps baited with dry ice and collected upwards of 65,000 mosquitoes in each trap, each night. A few districts in Florida have even successfully controlled certain mosquito species utilizing defined flyways by placing large numbers of traps to intercept the mosquitoes on foraging flights. Unfortunately, this hasn’t translated into effective control for the backyard use of these systems. Two recent studies bear this out: Collier B.W., Perick M.J., Boquin G.J., Harrington S.R. and Francis M.J. 2006. Field evaluations of mosquito control devices in southern Louisiana. Journal of the American Mosquito Control Association. 22:444-450 and Henderson, J. P., Westwood R., and Galloway T. 2006. An assessment of the effectiveness of the Mosquito Magnet Pro model for suppression of nuisance mosquitoes. Journal of the American Mosquito Control Association. 22(3):401-407. Neither study could demonstrate any meaningful reduction in biting pressure attributable to these devices.

The advertising claims for acre-wide control by these devices appear to be overstated. In most cases they are based upon best-case extrapolations from captures of released mosquitoes made inside screened enclosures. To be sure, it would be very difficult for the manufacturers to conduct controlled studies yielding reliable, statistically significant data with natural occurring mosquito populations due to confounding variables largely beyond the control of the researcher. Meaningful, repeatable data capture requires stability in these factors, generally over a period of years - extremely difficult to obtain in nature. In fact, Mosquito Control Districts using several different types of traps in survey operations often experience large variations in mosquito trap counts among traps as well as by location, trap height, and time of season.

Please be cautioned against putting too much faith in traps as your sole means of control. These traps represent an evolving technology that is a most welcome addition to our mosquito control armamentarium. Their potential is great, but shouldn’t be overestimated. It is unclear whether the traps attract mosquitoes into an area where humans may then provide a stronger source of attraction. In other words, will the bug zapper results showing larger numbers of mosquitoes in yards with a zapper be repeated by the CO2 traps? Time will tell. It’s highly unlikely that these devices, whatever their improvements, will ever fully supplant organized community-wide mosquito control programs, for there is no single silver bullet that will prove to be the ultimate answer to mosquito problems. Effective mosquito management requires integrating a variety of available control strategies i.e. surveillance, source reduction, biological control methods, traps, environmentally friendly larvicides, and, when necessary, application of public health adulticides, into a comprehensive program that exploits known mosquito vulnerabilities. They are the result of almost one hundred years of experience in making mosquito control in the United States the safest and most technically proficient in the world today.