Procedure for setting out EVS-type Dry Ice (Carbon Dioxide) baited
Mosquito Light Traps for mosquito monitoring.
Carbon dioxide baited light traps are used to selectively attract mosquitoes, with a minimum of
other insects. To be most effective they should be placed near vegetation, sheltered from windy
conditions and well away from congregations of animals such as in cattle yards, or horse
paddocks. They should be relatively easily seen from the surrounding areas, and not hidden by
tall grass, or up against a building.
1. THE TRAP UNIT.
The basic carbon dioxide baited light trap consists of three sections:
(a) The uppermost section consists of a black bucket with lid. The inside is insulated with
foamed urethane and polyethylene and has four small holes punched through the bottom of the
bucket. The bucket is used to store approximately half a kilogram of dry-ice (solid carbon
dioxide). The carbon dioxide gas that leaks from the bottom of the bucket is the bait or
attractant for the mosquitoes.
(b) The middle section constitutes the trapping mechanism. It consists of a clear plastic
panel with two 'D' sized battery cell holders and a miniature lamp, under which the motor sits,
and a length of PVC tubing which protects a small propeller. An on/off switch is present on
newer models. Switching on/off for older models is accomplished by an alligator clip or by
removal of the batteries.
(c) The bottom section consists of the catch container with an elastic-topped sleeve, the top
of which fits over the bottom end of the PVC tubing. The catch container consists usually of an
ice-cream container with lid. The container is turned upside down, and a hole is cut in the
bottom, to which the bottom of the sleeve is fastened. The lid of the container usually has a
hole which is covered with a fine gauze to allow air flow through the container.
2. POWER SUPPLY.
Standard traps are powered by two 'D' sized alkaline batteries. Usually it is best to replace the
batteries after a single night's operation. The batteries must be placed correctly so as to ensure
the propeller runs in the right direction, drawing insects downward and into the catch container.
With an unobstructed view, the glow from the miniature lamp may be visible for up to 100
metres when operating.
3. TROUBLE SHOOTING.
Check that the trap motor unit is operational before trapping each week. When positioning the
batteries support the backing plate with your fingers to prevent damage to the unit. If the unit is
faulty, first check that the batteries are not depleted, then check that the wiring is not loose. A
common problem with the traps is the wiring near the positive end of the battery housing. The
wires often get pushed up against the metal housing and cause a short circuit in the system.
With regard to the spiralled wire, the loops should not be touching one another. Sometimes the
plugs where the wiring enters the motor get loosened and need to be pushed back into the
contacts.
If, after checking the wiring, the light works but the fan does not, try gently pushing the central
portion of the fan blades upward towards the motor. With the unit switched on but the fan still
not working, try gently turning the blades in the right direction to see if movement starts. If it
does work check if the down draft is strong enough, and also tilt the motor sideways and upside
down to see if it continues working. Switch on and off several times and leave running for
about 30 minutes. If the motor runs for this period of time then trapping should continue (See
also trap maintenance section).
4. TRAP MODIFICATIONS DUE TO WET WEATHER.
If the trap is operated in wet weather, it should have a plastic skirt suspended from the base of
the motor unit, and all the way down and around the catch container to exclude rain. The skirt
should be sealed at the bottom by an elastic band so that mosquitoes are not trapped between
the plastic and the container. A broad rain hood positioned over the top of the dry ice bucket
would also be effective in keeping the trap and catch dry.
5(a). DRY ICE.
When using dry ice it should be collected from the manufacturer in the late afternoon of the
trapping night. If a field trip for a number of nights is planned, the dry ice should be stored in a
well insulated esky sealed with tape. As a rough guide approximately half the weight of the dry
ice will disappear in the esky every two days, and a half a kilogram is needed per trap per
night. The dry ice is placed in the trap bucket just before the trap is set out in the evening.
5(b). DRY ICE ALTERNATIVES.
There are problems associated with obtaining a regular supply of dry ice in some areas and two
alternative systems are available.
It is possible to make blocks of dry ice directly from a gas cylinder. This method can be
difficult to maintain and a lot of CO2 gas can be wasted initially trying to produce a block of
dry ice of the right size and density necessary to last out a night of trapping.
Alternatively the dry-ice can be replaced by carbon dioxide gas from a 10 kg cylinder with a
slow release valve, set to release the gas at a rate of 300ml per minute (range 200 - 400ml/min).
Pressure on the delivery side of the main regulator needs to be set between 140 - 160 kpa. If
less than 140 kpa, then often the gas does not flow through properly, particularly towards the
end of the season when gas pressure in the cylinder is getting low. If used once a week, a 10 kg
cylinder running from 4 pm until 9 am the next day, should last about 20 weeks. The
approximate flow rate can be checked by filling a 250ml sampling bottle with water, inverting
the bottle in water without emptying the bottle, and releasing gas into the bottle for about 50
seconds. By then the gas should have more or less displaced all the water in the bottle.
6. SITE SELECTION.
Select the trap site with care, keeping in mind the purpose of the trap. Usually the trap should
be positioned between a residential area and the mosquito breeding area, or at the edge of a
mosquito breeding area. The site should be near vegetation, out of strong winds, away from
competing lights, and away from congregations of animals. The trap should be hung at
approximately chest height from a tree or similar object.
7. SETTING THE TRAP.
Install the batteries and check to ensure that the polarity is the same, and that the fan is blowing
air in the right direction. The CO2 exit holes in the bottom of the bucket should be checked for
blockages by pushing the lid of the container in and out, noting whether CO2 vapour is visibly
flowing out the holes. If it is not, then open the holes with a thin wire or strong twig.
8. LABELLING TRAP.
Number or label the traps and collection/catch containers in some way so as to not mix up traps
from different sites, and place the trap out about 1 - 2 hours before sunset. Make sure no
branches or leaves are touching the trap. Double check that no ants are in the tree and, if
necessary, coat approximately 10 cm of the chain with petroleum jelly for protection.
It is very important that traps be set at the same sites each week, and ideally for a number of
years, to obtain meaningful comparative population data. Moving the trap to different sites
means that collections cannot be compared between trapping nights at weekly, monthly or
yearly intervals.
9. RECOMMENDED TRAPPING NIGHTS.
Traps are best set out on Sunday, Monday, Tuesday, or Wednesday nights. Do not trap after
Wednesday night as any delay in the courier service will result in the specimens arriving too
late for processing. Preferably traps should be set as early as possible in the week so that if
there is bad weather trapping can still be carried out by Wednesday.
10(a). COLLECTING THE TRAP.
Collect the trap the following morning within 2 hours of sunrise, and preferably earlier during
hot sunny weather. This is to prevent the mosquitoes dying from heat stress or dehydration.
While mosquitoes should be sent in alive - as live mosquitoes are needed for virus isolation dead mosquitoes are sorted into species to at least obtain continuous trapping data for each site.
10(b). CAUTION.
Before removing the catch container and switching off the motor, inspect the trap catch
container for mosquitoes. Note that if the mosquitoes are capable of flying up into the sleeve or
into the PVC tubing, then the fan is not operating fast enough.
11. INSPECT TRAP FAN AND LIGHT OPERATION.
The trap light and fan should still be running in the morning. If not, or if the fan is running too
slowly, then label the trap for servicing or repair (see Point 3).
12. IF TRAP FAILURE OCCURS.
If a trap's motor or batteries have failed during the night, then another trap should be run at the
site the following night.
13. REMOVING MOSQUITO COLLECTION CONTAINER.
Leave the mosquitoes in the collection containers with the lid still on. To remove the catch
container choke off the top of the sleeve with one hand and remove the top of the sleeve from
the PVC tube. Tie off and seal the top of the sleeve to prevent mosquitoes escaping. Check the
label on the container is correct and that the lid on the catch container is not likely to come off
(tape on if necessary).
14. TRAP MAINTENANCE.
Switch off the motor unit and remove the batteries after each night's trapping. Store
batteries separately from motor and propeller to prevent damage to motor unit. The
motors should be oiled once a fortnight, rinsed in 100% kerosene or turpentine once a
month or sprayed with WD-40, re-oiled lightly and then stored in a dry place. Trap
collection buckets should be wiped over with a tissue or rag soaked in at least 70%
alcohol each week before use, and the lid mesh checked for holes. Due to general wear
and tear, and perishing of the sleeve and mesh, collection lids and buckets should be
replaced every few years.
15. POST TRAPPING CARE OF MOSQUITOES.
Ensure that the catch container is kept out of the direct sun and cool (preferably stored in
an esky) during transport back to base. Esky should also be kept shaded/cool when ever
possible.
Place some newspaper then 1 or 2 freezer packs (blocks) followed by more newspaper
into the bottom of the esky. Tape lids onto trap containers to prevent lids coming off
during transit and put some moistened (but not wet) paper towel over the containers to
prevent the mosquitoes dehydrating. Pack the containers securely into esky with some
crumpled newspaper, to prevent containers rolling around.
Fill in Arbovirus Monitor's Report Sheet and enclose within the esky. Securely tape up
the esky. Keep esky in a cool area until picked up by courier service.
16. OVERNIGHT COURIER SERVICE.
Send esky with mosquitoes and monitor report sheet via overnight courier to :
DEPT. OF MEDICAL ENTOMOLOGY
Rm 3202 Level 3
ICPMR Building
Westmead Hospital
WESTMEAD NSW 2145
Courier Details as of 17/Mar/2005:
Courier:
TNT (Australia Wide Pickup: 13 11 50)
Account no.:
90438773
Contact phone number:
Stephen Doggett
John Clancy
(02) 9845 7265
fax: 02 9893 8659
Email: stephend@icpmr.wsahs.nsw.gov.au
Email : johnc@icpmr.wsahs.nsw.gov.au
Eskies and containers will normally be returned the same day that they arrive.