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The CDC Trap as a Special Monitoring Tool
Article reproduced from Proceedings of the Seventy-Sixth Annual Meeting of the New Jersey Mosquito Control Association, Inc. 1989, pp 26-33. Please use the following citation when referring to this article:
McNelly, J. R.. 1989. The CDC trap as a speciea monitoring tool. Proceedings of the Seventy-Sixth Annual Meeting of the New Jersey Mosquito Control Association, Inc. pp 26-33.
JAMES R. McNELLY
Cape May County Mosquito Extermination Commission, P.O. Box 66, Cape May Court House, NJ 08210
Abstract: With the addition of carbon dioxide as an attractant in the form of dry ice, the CDC trap becomes a powerful surveillance tool. The CO, baited CDC trap samples a wider range of mosquito species and significantly increases the numbers of mosquitoes captured compared to a trap that utilizes light as the sole attractant. Since the specimens are captured alive, virus assays are possible. A discussion of the history, advantages, applications and guidelines for CDC trap usage are the subjects of this paper.
Introduction: The New Jersey Light Trap (Mulhern 1942), provided the mosquito control community with a mechanical device capable of sampling host seeking mosquitoes. The trap was designed with the hope of maximizing adult surveillance results and minimizing human labor and bias. At present, this trap remains a useful tool in mosquito surveillance but its design places certain restrictions on its use. Conventional usage requires electric current to power a trap that is expected to remain at a location for long periods of time. As a result, the trap proved to be inefficient as a short-term monitor of mosquito populations, particularly in areas where electric current is inaccessible. -
Soon after the creation of the New Jersey Light Trap the search was on for a trap that would more adequately fulfill the needs of those in the mosquito community concerned with arbovirus surveillance. In short, a trap that was portable, capable of the collection of live specimens, and not dependent on electricity as a source of power. Over the years, a variety of trap types concerned with portability and live capture of specimens has been designed (Bellamy and Reeves 1952, Nelson and Chamberlain 1955). In 1962, the CDC miniature light trap (CDC = Centers for Disease Control) was introduced specifically for arbovirus surveillance and other short-term mosquito investigations (Sudia and Chamberlain 1962).
The CDC trap mimicked the New Jersey Light Trap in the principle of attracting mosquitoes with white light and capturing them with the down draft produced by a motor and fan. However, the CDC trap utilized lightweight components, a 6-volt battery and a live capture net. Weighing in at under two lbs, the CDC trap was quickly adopted as a standard trap type in the collection of arbovirus samples.
Equally important to a discussion of CDC traps is the research that was conducted with carbon dioxide as a mosquito attractant. Rudolfs (1922) first suggested the possibility of carbon dioxide being an attractant; Headlee (1934) was the first to explore its potential in conjunction with a mechanical trap. Enhanced catches with carbon dioxide-baited light traps were subsequently noted (Headlee 1941, Reeves and Hammon 1942, Huffaker and Back 1943) and in 1966 Newhouse et al. created the perfect "marriage", combining carbon dioxide in the form of dry ice with the CDC Trap.
Discussion: The overall design of the CDC trap has remained intact since 1962, with only minor differences in the construction. Changes have been made to the wiring harness in relation to different types of battery utilization. The original traps were powered with 6-volt wet cell lead-acid batteries (Sudia and Chamberlain 1962) and were wired to accommodate a dipole battery hookup. Some agencies prefer the original design and CDC traps can still be purchased that will run off current supplied by a 6-volt battery (1). However, a sealed gel cell battery would appear preferable to the old wet cell type since it would eliminate any hazards associated with battery acid. Traps designed to run off current supplied by a series of disposable or rechargeable D cell batteries are also available (1,2). The size and type of battery that is used may be determined by your agency's existing trap inventory. When new traps are being purchased, consideration should be paid to the anticipated applications of those traps and battery selection should be based on the most practical design for the task they will fulfill (Weber 1988).
Other modifications are available to facilitate special needs in surveillance. If live specimens are not a requirement, a kill jar can be substituted for the live collection net. A photoswitch option that automatically turns the trap on and off is also available. An air-actuated gate system should always be used when the trap is operated by a photoswitch. The gate stays open to allow mosquito entrance as long as the trap is running but closes to prevent specimens from escaping when the trap stops running. The gate system is a desirable option whenever a live collection net is used since it offers a measure of safety against any type of trap failure. The air-actuated gate system may be purchased as a separate unit (1) and retrofitted to any of the existing CDC trap models.
The full potential of a CDC trap cannot be obtained without the addition of dry ice. The trap was designed to use an extremely small light bulb and the light output is much weaker than the 25 watt bulb in a New Jersey light trap. Mosquitoes can be collected with light as the only attractant but the addition of dry ice greatly enhances the trap's capabilities. Newhouse et al. (1966) reported an increase of 400-500% in overall catch when the trap is supplemented with dry ice. These findings correlate well with those of Headlee (1934), the first to use carbon dioxide in conjunction with a mechanical trap. Dry ice also increases the number of species captured by 20-25% (Morris and DeFoliart 1969, Magnarelli 1975, Slaff et al. 1983) and improves the ratio of blooded and parous individuals for arbovirus surveillance (Morris and DeFoliart 1969, Feldlaufer and Crans 1979).
If the CDC trap is used with dry ice, removal of the light bulb will actually improve the collection by eliminating "trash insects", such as beetles and moths that fly readily to light (Carestia and Savage 1967). This eliminates the tedious sorting process that is a prerequisite for identification of most light trap collections. Without the light, the trap is also less noticeable, a consideration in areas where traps may be subject to theft. During the Vietnam War this aspect received especially strong attention for reasons other than theft (Miller et al. 1969, Herbert et al. 1972).
The amount of dry ice as well as the type of container used to hold it will effect the amount of carbon dioxide released over time. In most instances, a five lb. block of dry ice is sufficient to cover the normal dusk to dawn trapping period. This delivers between 400-500 ml of carbon dioxide per min., a rate that is comparable to the amount released by a large mammal (Morris and DeFoliart 1969). Insulated containers are available from the manufacturer' but they are easily constructed. A favorite of the author's is a denim drawstring bag that is large enough to hold 5 lbs of dry ice. The bag is sewn with two layers of denim separated by a layer of insulating material taken from a space blanket. A draw string is then sewn into the top of the bag to allow the opening to be cinched. The bag is hung in a position to allow the carbon dioxide to release directly next to and slightly under the aluminum hood of the CDC trap. When host-seeking mosquitoes enter the stream of gas, they are drawn into the trap by the fan.
In regions where dry ice is difficult to obtain, there are other options. One would be the purchase of a dry ice maker; another requires construction of a cylinder delivery system similar to that described by Parker et al. 1986. In both cases, the economics should be considered in relation to the projected goals and long-term benefits from the surveillance data.
Applications: Personnel involved in the surveillance of adult mosquitoes are normally faced with answering one or more basic questions, typically WHAT SPECIES ARE PRESENT and/or HOW MANY? How best to resolve those questions is dependent on a myriad of variables. These may include the amount of time available to the worker, under what field conditions the surveillance will take place, and what is the information gathered ultimately to be used for? A suitable trap is then selected to do the most efficient job. For the most part, a CDC trap is a surveillance tool that is used in special situations. Unlike the New Jersey light trap that remains stationary in a location for long periods of time, the CDC trap's portable design is intended for short term use in a variety of locations.
Circumstances usually have the surveillance specialist attempting to define an unknown mosquito population in terms of species and numbers. Additional information regarding the extent of an infestation, its disease potential and other particulars may also be desired. A common situation is a complaint of mosquito activity called in by a resident. Two or more CDC traps placed on the property would prove or disprove the validity of the complaint. Regardless of the actual catch, the home owner is usually favorably impressed with the attention that's been paid to his or her complaint. One way of heading off potential problem areas is to survey those areas before people move into them. In Essex County, inspectors routinely survey new housing developments with CDC traps while they are still under construction.
General population checks may be required in the vicinity of proposed project sites, such as an area that is scheduled for an adulticide treatment or one that will undergo water management to determine the extent of the existing problem. In both instances, properly placed CDC traps will provide the necessary data and help direct control efforts. In the case of adulticiding, the success or failure of the control effort will be determined over a relatively short trapping period. In the latter case, surveillance at periodic intervals over the course of an entire mosquito season would help to develop and document a short term history of adult mosquito activity for the area. The success of the project after management would be determined with post management trapping to document the degree of mosquito reduction after the work was completed.
A phone survey of New Jersey's Mosquito Control Agencies and Commissions revealed that CDC traps were especially useful in areas where virus activity is suspected. This normally involves a farm where an equine death due to eastern equine encephalitis is under investigation. Collections are made by the county commission or agency and transferred to the Agricultural Experiment Station for virus assay. Sudia and Chamberlain (1967) provide a review of the proper protocol for handling collections for virus isolation attempts. They also provide a diagram of a site involving a horse death and the recommended method of CDC trap surveillance. The diagram gives a good representation of trap placement that is of value not only in the context given, but also to any situation where an unknown adult population is being surveyed. Traps in the horse scenario are placed in likely areas of mosquito-horse contact, near stables, in pastures, and in "transition" zones along the edge of the dominant vegetation ecosystems. By trapping in the edges of transition zones, the trap is more likely to attract a wider range of species and not exclude mosquitoes that host seek only in one ecosystem. With New Jersey's wide diversity in mosquito fauna, behavioral differences should be considered in any investigation trapping program.
The height at which the trap is suspended can influence the species composition of the collection. Normally, traps are hung 5-6 ft off the ground, the height at which the New Jersey light trap operates. This height is satisfactory for the majority of species encountered in routine surveillance i.e. Aedes and Culex sp., but will not adequately sample species like Culiseta melanura, which host seeks in the canopy layer (Main et al. 1966, Crans et al. in prep.). Likewise, mosquitoes that do not host-seek between dusk and dawn will either be missed or underrepresented. A dry ice baited CDC trap would be of limited value for Aedes albopictus surveillance if the trap were operated at night because Ae. albopictus is a daytime feeder (Herbert 1972). Diurnally feeding adults can be trapped by simply adjusting the trap hours of operation to include a representative portion of daylight collection time (Newhouse et al. 1966).
The amount of carbon dioxide that is released could affect the collection by excluding species that feed on hosts with lower respiration rates than the 4-5 lbs of dry ice exudes (Morris and DeFoliart 1969). These same authors also report that dry ice attracted several species of males which showed a positive correlation with the overall mosquito density. Huffaker (1943), however, felt that carbon dioxide repulsed male mosquitoes.
Guidelines for CDC Trapping: The following guidelines are offered to minimize variability in the use of CDC traps for mosquito surveillance:
Conclusions: The control of adult mosquitoes begins with proper surveillance. For special surveillance of short duration, the dry ice baited CDC trap is an efficient, reliable surveillance tool for the surveillance specialist. This trap can be used to assess a homeowner's complaint, check the success of an adulticide or gather virus information. The CDC trap's portability, battery power, and efficiency add versatility to the surveillance program.
(1)The John W. Hock Co., P.O. Box 12852, Gainesville, FL 32604
(2) Hausherr's Machine Works, Old Freehold Road, Toms River, NJ 08753
Acknowledgments: The author gratefully acknowledges the input and advice that was provided by the members of the surveillance symposium.
- Bellamy, R. E. and W. C. Reeves. 1952.
DISCUSSION FOLLOWING THE PRESENTATION
QUESTION: I would like to make a few remarks in defense of the New Jersey light traps. We used CO, with New Jersey light traps with the bulbs removed and got similar percentage increases. I agree, however, that you cannot operate in some locations because you are limited by electricity. We also were able to use bottled C02 as an alternate to dry ice and it worked fairly well.
McNELLY: I chose to ignore the possibility of converting a New Jersey light trap although there are at least 2 counties in the state that use the New Jersey light trap in much the same fashion as you would a CDC trap. The New Jersey light trap is heavy and it really defeats the purpose of portability. It is much easier to carry several CDC traps around than it is a single, converted New Jersey light trap. It is possible to bait New Jersey light traps with CO, and get essentially the same results as you would with a CDC trap. Compressed gas is an option if you do not have access to dry ice. Marc Slaff has a paper on a gas delivery system that he used in conjunction with a CDC trap.
QUESTION: I do not think there is much difference in the cost of compressed gas vs. dry ice but you do have to lug a bottle around. We used the big bottles and they lasted about 4 nights.
McNELLY: There are machines available that you can make your own dry ice with. Essex County uses this type of system in their surveillance program. Again, it's a matter of what the commission can afford and the cost effectiveness of the investment.
QUESTION: Do you know of any analysis that will allow you to export data from one trapping device and make it comparable to the other? We have 25 years of data from the New Jersey light trap. CDC information would be more useful if we could make the comparison.
McNELLY: I haven't done that and I am not aware of any direct comparison between the New Jersey and the CDC trap. The literature frequently mentions the differences in the drawing powers of the two traps. The New Jersey trap has a much more powerful fan but I do not know how this would translate into comparable collection potential.
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