Proceedings of the Eighty-Second Annual Meeting of the New Jersey Mosquito Control Association, Inc. 1995, pp 53-57.
(Please use this citation when referring to this work)
WAYNE J. CRANS
Department of Entomology, Mosquito Research & Control, PO Box 23 1, Cook College, New Brunswick, NJ 08903
Introduction:
Artificial resting structures have been used to sample mosquito populations since the early days of malaria control when it was learned that a number of important malaria vectors congregated in diurnal resting places (Boyd 1930). Most mosquito species are either nocturnal or crepuscular and remain relatively inactive during the daylight hours. Sampling mosquitoes during their period of inactivity is considerably more efficient than focusing on the relatively brief periods when the mosquitoes are actively seeking a host. Natural resting sites for mosquitoes include dense vegetation, animal bur-rows, caves, and tree holes. Basements, stables, chicken coops, and culverts are examples of man-made structures harboring large diurnal populations of resting mosquitoes. Empty nail kegs placed on their sides were one of the first artificial resting structures employed specifically to sample mosquito populations and were widely used by the Tennessee Valley Authority for the sampling of Anopheles quadrimaculatusfor malaria eradication (Anonymous 1939). Goodwin (1942) found that a 1cubic ft wooden box, open at one end and painted either red or black attracted large numbers of An. quadrimaculatus. He also demonstrated that the 1cubic ft box was easier to transport than the nail keg and was considerably more manageable in the field than resting structures with larger dimensions.
Burbutis and Jobbins (1958) used Goodwin's design to sample Culiseta melanura in southern New Jersey and found that 1 cubic ft resting boxes painted with 2 coats of Chinese red enamel attracted large numbers of Cs. melanura as well as Anopheles species when they were placed on the ground in shaded habitats. Gusciora (I961) used the 1 cubic ft. resting box as a sampling tool for the New Jersey State Department of Health Arbovirus Surveillance Program and tried several modifications to improve its efficiency. Crans (1977) adopted a line of 25 resting boxes as a standard to measure Cs. melanurapopulations for eastern equine encephalitis virus (EEEV) surveillance. This paper reviews resting box methodology for mosquito control agencies wishing to incorporate resting boxes into their surveillance programs.
The Standard 1 Cubic Ft. Resting Box:
Resting boxes are rectangular wooden boxes measuring approximately 12" x 12" x 12" and open on one end. Goodwin (I 942) built his boxes with I" x 4" tongue-and-groove pine stock. Today, resting boxes are generally constructed with exterior plywood. If the boxes are made with 1/4" plywood, internal braces are needed to ensure structural integrity. Boxes constructed from 3/8" or 1/2" exterior plywood can be glued and/or nailed without additional bracing for structural strength. Boxes constructed from 1/4" plywood are light in weight and easier to transport to the field. They are less sturdy, however, and must be replaced more frequently than boxes constructed from heavier stock.
The exact dimensions of the finished box depend on the thickness of the plywood that is used. The sides of the box are generally constructed from 4 1-ft square panels and measurements for the back are adjusted to complete the cube. A 2 to 3" diameter hole should be drilled into one of the side panels prior to assembly. The hole will admit an anesthetic into the box during the collection process and should be covered with window screening to prevent mosquito escapes.
The finished boxes should be painted flat black on the outside and either red or rust brown on the inside. The black exterior is highly attractive to mosquitoes searching for a shaded resting site and also renders the boxes as inconspicuous as possible to would-be vandals. Anesthetized mosquitoes stand out clearly against the red interior. The addition of color inside the box does not attract more mosquitoes but does make the collection process easier.
Resting Box Rlacement:
Habitat Resting boxes do not work well in any habitat exposed to sunlight for extended periods. Open fields, suburban yards, salt marshes, and sparse woodland habitats are examples of areas producing very few mosquitoes. A dark, forested habitat with a high canopy appears to yield the highest collections. Hardwood forests with secondary undergrowth can be productive if the boxes are placed in open areas where they are visible to mosquitoes seeking shaded resting places. Resting boxes placed within cedar swamp habitats collect fewer specimens than boxes placed in the surrounding woodlands. Well shaded habitats having little or no secondary ground cover provide the best habitat for resting box collections. Mature stands of Red Maple have the combination of canopy and lack of ground cover that contributes to high mosquito collections. White Pine plantations, where the trees are planted in orderly rows, generally yield the highest collections due to the exclusion of light from the closely planted trees and the humid ground provided by accumulation of packed pine needles.
Position: Resting boxes are generally placed on the ground with the open end facing west to minimize the influence of direct sunlight during the early part of the day. In well shaded habitats, the exact direction of the open end becomes less important. Collectors, however, should be trained to re-position any box that allows direct sunlight to enter during any portion of the morning hours. The panel containing the screened hole should be used as the bottom of the box and placed directly on the ground. This prevents drafts that would disturb the resting mosquitoes during periods of wind and admits ground moisture to maintain humidity inside the box. Avoid placing the open end of the box too close to tree trunks, ferns, dense bushes, or briars that may interfere with the collection process. Any movement of the box prior to collection will disrupt resting mosquitoes and interfere with maximum counts. For surveillance purposes, resting boxes are usually positioned no closer than 10 ft from one another in either a line or grid design. In arbovirus surveillance, 25 boxes are used to make weekly comparisons in the mosquito populations, and the boxes are arranged in 5 rows of 5 boxes each. Most mosquito control agencies use fewer boxes per site but arrange them in straight even lines to make collecting easier.
Resting Box Collection:
Collection protocol: Personnel making collections from resting boxes should be equipped with catch covers, sprayers, and mechanical aspirators. Catch covers are squares of heavy cloth, similar in design to a shower cap, and are used to cover the boxes and trap the resting mosquitoes during the collection process. The covers are not commercially available and must be hand sewn. Catch covers should be fabricated from cloth heavy enough to resist tearing from snags on broken limbs, thorned bushes and cat briar. We sew a clear vinyl window in the cover as a convenience to view the collection during the anesthetization process.
Collection involves approaching the box from the rear, slipping the catch cover over the open end of the box, and spraying short bursts of anesthetic through the screened hole to anesthetize the mosquitoes trapped inside. We use a Preval Power Unit (1) to introduce the anesthetic. The power units are pressurized and screw into glass bottles that are designed to hold fluids that can be atomized. Each power sprayer will deliver up to 16 oz of any liquid that is added to the spray unit bottle. Under normal conditions, a power unit will deliver enough anesthetic to process 50 to 100 resting boxes.
The amount of anesthetic and the time required to completely anesthetize a collection varies considerably under different environmental conditions. The anesthetic acts as an irritant, causing the mosquitoes to fly about and open their spiracles. Tapping the box during the anesthetization process keeps the specimens flying and hastens the uptake of the anesthetic. On hot, dry days, 3 to 4 short bursts will completely immobilize the collection within I min. When conditions are more humid, considerably more is required, and it may take several minutes before the specimens are anesthetized. During periods of rain, mosquitoes can keep their spiracles closed for extremely long periods making collecting tedious and impractical.
The mosquitoes are collected by removing the catch cover and aspirating the specimens directly from the box with a mechanical aspirator. In most cases, the mosquitoes stand out clearly against the red interior of the box and are easily aspirated out. Experienced field technicians can distinguish female mosquitoes from males, crane flies, and fungus gnats. Some specimens, however, may become entangled in spider webs which can seriously interfere with the collection process. All technicians should be cautioned to keep spiders out of the collection vials to prevent predation and webbing in the vials prior to sorting and processing. Technicians should also be instructed to clear the boxes of as much webbing as possible during each collection to minimize the impact of spiders and their webs on the collection process.
Time of Day: Mosquitoes utilizing resting boxes as diurnal resting sites enter the boxes during the morning hours, remain inactive during late morning and early afternoon, and then exit the boxes later in the day (Crans, unpublished data). During one field study, we monitored a series of boxes at hourly intervals for 26 consecutive hours and found the boxes remained empty all through the night. Mosquitoes did not enter any of the boxes we were monitoring until dawn and did so avidly as soon as it was light enough for us to discern the boxes against the forest background. We found mosquito populations build steadily within the boxes during the morning hours and remain static until midafternoon. On the basis of these observations, we recommend collections be made between the hours of 10:00 AM and 2:00 PM to encounter maximum numbers for surveillance purposes.
Mosquito Species: Table I lists the relative abundance of mosquito species in our resting box collections from southern New Jersey. The data clearly show resting box surveillance is highly biased toward Cs. melanura. Anopheles quadrimaculatus is the only other species appearing in significant numbers. Mosquitoes in the genus Culex appear in relatively small numbers but occur regularly at all of our collection sites. As a result, the technique might be useful as an indicator of Culex abundance in some areas. Data clearly show resting boxes are extremely poor indicators for members of the genus Aedes. Aedescanadensis and Aedes triseriatus were the only aedine species represented in the collections even though Aedes sollicitans, Aedes cantator and Aedes vexans appeared in large numbers in light traps at these same collection sites.
Table 1. Relative proportion of mosquito species collected from resting boxes in coast areas of southern New Jersey
Mosquito Species, Average/year, % Total
- Cs. melanura, 29130, 66.4
- An. quadrimaculatus, 9878, 22.5
- Cx. territans, 1259, 2.9
- Cx. restuans, l032, 2.4
- Cx. salinarius, 912, 2.1
- Cx. pipiens, 474, 1.1
- An. punctipennis, 397, 0.9
- Ae. canadensis, 270, 0.6
- An. bradleyi, 231, 0.5
- Cq. perturbans, 72, 0.2
- Ae. triseriatus, 58, 0.1
- Other (10 species), 166, 3.8
Conclusions:
Resting boxes have a place in the surveillance programs of mosquito control agencies that wish to conduct their own monitor for Cs. melanura. Agencies that wish to include resting boxes in their summer programs should be aware that the technique is highly selective for Cs. melanura. The technique is not a replacement for light traps and resting boxes are not a reliable indicator for most pest species.
(I) Available from Precision Valve Corporation, Yonkers, NY 10703
Acknowledgments:
This is New Jersey Agricultural Experiment Station Publication No. E40101-01-95 supported by state funds and funds from the New Jersey State Mosquito Control Commission.
Literature Cited:
- Anonymous. 1939, Tennessee Valley Authority, Board of Malaria Consultants, Fifth Annual Report.Boyd, M.F. 1930. An Introduction to Malariology. Harvard University Press, 437 pp.
- Burbutis, PP and DM Jobbins. 1958. Studies on the use of a diurnal resting box for collection of Culiseta melanura (Coquillet). Bull Brooklyn Entomol Soc. 53:53-58.
- Crans, W.J. 1981. Eastern encephalitis in New Jersey in 1980. Proc NJ Mosquito Control Assoc. 68:147-154.
- Gusciora, W.R. 1971. The resting box technique for the sampling of Culiseta melanura (Coquillet). Proc NJ Mosquito Exterm Assoc. 48:122-125.
- Goodwin, M.H. 1942. Studies on artificial resting places of Anopheles quadrimaculatus Say. NATL Malaria Soc J 1:93-99.