Use of adulticides
to control adult mosquito populations is generally reserved as a last resort. (J115.13.w2)
- "Mosquito adulticides should be considered the least
desirable method of control and only used when current isolations of virus and/or evidence
of disease has been established." (D72)
Appropriate Use of
Adulticides:
Although
other components of an integrated
mosquito management programme should generally avoid the production of large mosquito
populations, use of adulticides may become necessary due to a wide variety of events
including heavy precipitation, flooding, high tides, environmental constraints,
inaccessible larval habitats, missed breeding sites, human disease outbreaks, budget
shortfalls, absent employees, or equipment failures. (J84.7.w35)
It is recognised that although adulticiding is usually the least efficient mosquito
control technique, nevertheless this form of control, based on surveillance data, is an
extremely important part of any integrated mosquito management program. (D67)
- Emergency vector control (spraying), if is to have an effect in
reducing cases, must be applied at the start of the epidemic, not after
the epidemic is well underway. (P48.4.w14)
- It is important to remember that delaying adulticide applications in areas with
surveillance indicators indicating sustained or intensified virus transmission in an area
and infection with WN virus of potential accessory vector mosquito species (for example,
those demonstrating mammal host ranges), until human cases occur, negates the value and
purpose of the surveillance system. (D67)
- Use of adulticides can be warranted when
other means of control have been ineffective and there is a substantial
risk of disease transmission. (P32.1.w8)
- A high level of protection may be provided in the face of an epidemic, or when an epidemic
threatens, by using insecticides
to eliminate vectors. It is important to recognise that outdoor space treatments (such as
the use of ultra low volume (ULV)
application of insecticides at high concentration, spread by aerosol over a wide area) may
be extremely important for rapid effect to control an epidemic. It is also important to
recognise that their effect is short lived and repeated application is required for
continued control. (D71)
- Area-wide uses of adulticides can often be
avoided; application may be targeted to areas where WNV-infected
mosquitoes have been detected by surveillance efforts and to nearby
border areas. (P32.1.w8)
- For WN virus, dead bird density, plotted geographically, may be used
to assist in directing use of adult mosquito control. (P39.3.w12)
Mechanics of Adulticide Use:
The choice of adulticide and the time of application should be based on the
distribution and behaviour of the target mosquito species. Most Culex
spp. are nocturnal, which may compromise aerial application in urban
areas. (D67)
- The mosquito species to be targeted must be identified. (P48.4.w14)
- Decisions on exactly where to spray should be made on the basis of
local data; in Florida sentinel chicken coops with attached mosquito
traps have been used to provide information on mosquito infection rate
and species involved. (P48.4.w14)
- Meteorological conditions must be
considered for timing of spraying. In general late afternoon, early
evening, night or early morning, when the air is cool and wind
velocity low (less than about six miles an hour) is preferred. (D70)
- For Culex spp. focal area spraying at
night using truck-mounted equipment may be more effective than
application during the afternoon or at dusk. (P32.1.w23)
- It is important to recognise that multiple applications of
adulticides may be required for adequate control of vectors such as Culex
pipiens. (D67)
Adulticides must drift through the habitat in which mosquitoes are
flying in order to provide optimum control. They are typically applied as an
Ultra-Low-Volume (ULV) spray where small amounts of insecticide are dispersed either by
truck-mounted equipment or from fixed-wing or rotary aircraft. Ground or aerial applied
thermal
fog applications of adulticides are used in some areas, but to a
much lesser degree. (D67)
- Thermal fogging reduces visibility but may penetrate
vegetation better than a ULV spray application and the fog is easily tracked downwind to
target areas. (J84.7.w35)
Other uses of adulticides include residual
spraying of the walls of catch basins and, in some circumstances,
vegetation around dwellings or parks, playgrounds etc. (D70)
Considerations in Chemical Control of Adult
Mosquitoes:
- It is recognised that there is serious public concern regarding the use of
insecticides. Public education programs, to distribute information about the nature of
mosquito-borne disease and about the risks and benefits of adulticide use, are vital;
public acceptance is critical for the application of emergency adult mosquito control. (D67)
- Public concern regarding the possibility of becoming ill due to the
use of pesticides may be at a similar level to concern regarding
becoming ill due to infection with WNV infection. (P39.3.w12)
- Application of insecticides over water bodies
should be avoided where there is a low dissolved oxygen content (less
than 2 ppm) as fish deaths are likely to occur under these
circumstances. (P32.1.w23)
Two examples of considerations in decision making regarding chemical control of adult
mosquitoes are given below.
1) The following information regarding decision making in
mosquito adulticiding operations is quoted directly from the CDC
Epidemic/Epizootic West Nile Virus in the United States: Guidelines for
Surveillance, Prevention and Control 3rd Revision (D147):
[Text copied directly]
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Once arbovirus
activity is detected in a jurisdiction and a decision is made to implement or intensify mosquito control by using adulticides, the size of the area to be treated must be determined. In the broadest context, the underlying program objective (i.e., interruption of the enzootic transmission cycle vs. prevention of transmission to humans and domestic animals) should determine the amount of adulticide coverage that is required. For most jurisdictions the objective is the prevention of transmission to humans and domestic animals. There is no simple formula for determining how large an area to treat around a positive surveillance indicator or a suspected or confirmed human case of WNV. Nor is there adequate information to guide decisions about the degree of vector population suppression that must be attained, or for how long this suppression must be maintained to reduce human disease risk. At a minimum, the following factors must be considered when deciding the scope of the adulticiding effort:
1. The general ecology of the area, e.g., key habitat types and the presence of natural
barriers such as large rivers;
2. The population density, distribution, flight range, and age structure (proportion of parous females) of the target mosquito species;
3. The flight range of the avian amplifying host(s);
4. The length of time since birds started dying or became infected in the affected area (typically, there may be a lag of several weeks between recovery of dead birds and confirmation of WNV infection) or since virus-positive mosquito pools were collected;
5. The human population characteristics – spatial distribution and density relative to the positive locality (e.g., urban vs. rural), age demographics;
6. Evidence of persistent WNV activity detected by the surveillance program; and
7. Season of the year and how long WNV activity can be expected to persist until the epizootic/epidemic vector(s) enter
diapause.
Several of these factors will be unknown or poorly understood. Technical assistance from a mosquito control professional, particularly one experienced in mosquito control in the region, is crucial in this process. Practical experience in conducting mosquito control is required to refine control
recommendations. For example, the size of an area selected for control applications may be reduced in response to structures like open areas, bodies of water, major highways, or other barriers that may restrict the distribution of targeted species. Alternatively, adulticide coverage may be expanded to cover large urban or suburban residential neighborhoods with dense human populations.
Hypothetically, in some settings where focal early season enzootic WNV activity has been detected, early season adulticiding may be useful in interrupting virus transmission and lead to lower transmission rates later in the season. However, effective larval control of the principal enzootic mosquito vector is probably a more costeffective way to interrupt early-season virus amplification. |
2) The following Considerations for Adult
Mosquito Control are quoted directly from New York State West Nile Virus Response Plan
- Guidance Document (D72)
Appendix B: [Text copied directly]
| West Nile Virus: Considerations for Adult Mosquito Control The
main objective of mosquito control is to decrease the risk of a human outbreak of West
Nile virus (WNV) infections. This should be primarily accomplished by:
· Continuing to stress reduction in mosquito habitats;
· Larviciding where feasible and practical;
· Using personal mosquito protection measures, especially for the elderly and
immunocompromised.
Adulticiding is supplementary to these measures and is a local decision that
should be based on the considerations listed (in no particular order) below.
Triggers for Spraying
Adulticing should be considered only when there is evidence of WNV epizootic
activity at a level suggesting high risk of human infection (for example, high dead bird
densities, high mosquito infection rates, multiple positive mosquito species including
bridge vectors, horse or mammal cases indicating escalating epizootic transmission, or a
human case with evidence of epizootic activity) and abundant adult vectors. In general,
the finding of a WNV positive bird or mosquito pool does not by itself constitute evidence
of an imminent threat to human health and warrant mosquito adulticiding.
When to Spray
The goal of spraying is to reduce the risk of human disease by decreasing the
number of vector adult mosquitoes as much as possible. The pesticide is effective only
when it makes contact with a mosquito. This is most likely to happen when mosquitoes are
actively flying. To this end, it is imperative that spraying take place when Culex [Culex
spp.] mosquitoes—the primary target mosquitoes—are most active. This means
spraying between dusk and dawn. It is also important to note when spraying will NOT be
effective. Spraying during inclement weather--rain, fog, high winds--will simply knock the
pesticide mist to the ground, rendering it inactive. In addition, spraying at temperatures
below 55-60 degrees is less effective. At these temperatures, mosquitoes are slower and
are usually resting on the ground in the shrubbery and leaf litter. The pesticide mist
usually can’t reach them there.
Where to Spray
Another consideration is the terrain in the proposed spraying area. If there is
substantial vegetation bordering the roads, ground spraying with trucks may not provide
adequate coverage. Dense vegetation associated with roadside trees, shrubs, or hedges can
interfere with truck-mounted insecticide applications. In situations such as these,
counties that choose to spray may wish to consider application of pesticides using
backpack sprayers, or altering their route so that trucks can more efficiently apply the
spray. Aerial application may be considered when all other methods of application are
inadequate and/or inefficient. Aerial spraying should be limited to the immediate area
where the vector population has been documented to exist through vector surveillance and
to adjacent areas considered at risk for imminent disease transmission. You can contact
your local DEC [Department of Environmental Conservation] office for guidance in
situations involving spraying near water or with applications that might be hindered by
vegetation.
Human Population Density
The population density in an area where there is evidence of intense epizootic
activity should also be taken into consideration. If the area is rural and there are few
people, the cost and potential risks of spraying may not justify its use. If the area is
heavily populated, you have stronger indications for considering adult mosquito control,
since the goal of spraying is to minimize the risk of a human outbreak of West Nile virus
infection.
Mosquito Population
Information from mosquito surveillance can be helpful in determining when to
conduct mosquito control, and in monitoring the effectiveness of control activities. While
all mosquitoes do not need to be tested specifically for the presence of West Nile Virus,
those that are tested can provide valuable information regarding spraying decisions. Be
aware that surveillance efforts to detect virus in birds are much easier to conduct than
similar efforts to detect virus in mosquitoes. What may be more important than testing
mosquitoes for West Nile Virus is knowing the NUMBERS and SPECIES of the vector population
in the locality. The best way to do this is by mosquito trapping. Systematic mosquito
trapping, however, requires specially trained staff and is time intensive. For localities
without this capacity, there are other potential sources of information on mosquito
activity. Staff can visually inspect the area where a positive bird was found, or around
human population centers for habitats likely conducive to mosquito breeding. Staff can
also personally observe mosquito activity.
Lag Time
It is important to look at the dates that the positive surveillance specimens
(mosquitoes, birds, and/or mammals) were collected. In most cases the positive specimens
will have been collected about two weeks before. In the time between the date the specimen
was collected and the date when the test results are complete, circumstances may have
occurred which would alter a decision to spray. For example, a county may have sprayed
since the collection date, a weather event may have adversely affected mosquitoes, or
mosquito habitat may have been modified resulting in a reduced need to spray.
West Nile Surveillance Results Over Time
WNV surveillance information may be monitored by county or even smaller
jurisdictions, such as towns, over time, to determine what is happening with the outbreak.
For example, if there has been a consistently good system for recording dead crow
sightings, and the number of dead crow sightings drops for several weeks in a row after
spraying, that may indicate that the previous spraying has killed off a large enough
number of mosquitoes that transmission to crows is not continuing. Such analyses should
not be graphed by day (because of day-to-day instability in reporting), but analysis by
week should be helpful.
Local Perspectives on Spraying
Different communities have varying perspectives on the benefits of mosquito
control. These should be taken into account in the decision whether or not to spray. This
can be difficult, as people can have strong opinions on both sides of the issue. The US
Environmental Protection Agency has approved these chemicals for use and they must go
through rigorous testing in order to get that approval.
For further information about toxicity of the common pesticides used for adult mosquito
control, please consult your local DEC regional office.
Whether or not you choose to spray, it is recommended that you contact your regional
DEC office and start to get some of the paperwork together regarding pesticide application
permits, etc. You may also want to call some of your colleagues at other counties for
advice—what did they do in the same situation, how did their bidding process for
pesticide applicators go, etc.
The decision regarding spraying is basically a risk assessment: whether or not you, as
a county, believe the risk of contracting West Nile Virus is greater than the risk from
applying pesticides for mosquito control. It is also a cost assessment where you must take
into account medical costs, life years lost (for WNV fatalities), costs of spraying
campaigns, etc. As with any decision about access to health prevention and care, many
factors must be considered. Hopefully, the above list of factors will assist in your local
decision making process.
If you have any questions, and wish to discuss your situation further, please feel free
to call the Arthropod-Borne Disease Program at 518-474-4568. |
Barrier and Local Use of
Adulticides:
- Within houses, mosquitoes may be killed using appropriate adulticides
such as synergised pyrethrum or synthetic pyrethroids
sprayed using a household aerosol space spray (used according to label directions). (W175.Nov01.WNV2)
- In some areas of the USA barrier treatments, which are typically applied
as high volume liquids with hand-held spray equipment using compounds with residual
characteristics, are commonly applied. The use of barrier treatments may be particularly
attractive to individual homeowners living near mosquito producing habitats where residual
chemicals applied along a property border can provide some control benefits. (D67)
- Local reductions of mosquitoes around houses may involve the use of
adulticides, particularly products containing allethrin, Malathion or
carbaryl, sprayed with a
compressed air sprayer onto vegetation, walls and other potential mosquito resting areas
(used according to label directions). (W175.Nov01.WNV2)
- Residual insecticides may repel or kill
mosquitoes. Microencapsulated formulations are the most durable.
Pyrethroids such as lambda cyhalothrin, detamethrin and permethrin
are the most effective repellent insecticides. (N11.30.w1)
Control of Adult Mosquitoes Overwintering in
Hibernacula:
- There is a need to investigate effective methods to control
overwintering mosquitoes (Culex spp. and Anopheles
spp. mosquitoes overwinter as adults) in hibernacula, and, if appropriate, apply these
methods judiciously in established adult mosquito resting areas. (D72)
Evaluation of Adult Mosquito
Control:
- The effectiveness of adulticiding programmes should be monitored.
- Evaluation of adult mosquito control should include as a minimum,
surveillance to measure pre- and post-spray mosquito densities inside and outside
the control
area using CO2-baited traps and gravid traps and vector mosquito infection rates pre-
and post- spray inside and outside the control area, as well as weather conditions during
application (temperature, wind speed, wind direction). (D67,
D147)
- Small areas can be selected for assessment of effectiveness, using
landing counts, before and after treatment. Typically treatment is
expected to reduce the landing count by greater than 85%. (D73)
- Additional desirable data includes the population age structure of key
mosquito species (e.g. Culex pipiens), data on the performance of individual pieces
of application equipment (such as droplet size of ULV sprays, and flow rate) and
monitoring of the spray track by GPS
if equipment is available on aircraft.(D67,
D147)
(D67,
D73, D147)
Evaluation of
effect of Adulticides on non-target Organisms:
- The local effects of adulticide treatments on
non-target organisms such as fish and lobsters should be evaluated. (P32.1.w23) See
section below: Effects on Non-target Organisms
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