Article reproduced from Wing Beats, the bulletin of the
American Mosquito Control Association, produced by the
Florida Mosquito Control Association. Please use the
following citation when referring to this article:
O'Malley, C. 1995. Seven ways to a successful dipping
career. Wing Beats, vol. 6(4): 23-24.
Seven Ways to a Successful Dipping Career
Claudia O'Malley
Introduction: As recently as 1922, members of the
New Jersey mosquito control community were debating the
relative merits of "night collections" as opposed
to larval collection and identification. Some of the past
practices of mosquito control included treating any standing
water encountered, regardless of whether or not mosquito
larvae were present. Guidelines for starting a mosquito
control program included the advice that directors should not
spend an excessive amount of time on surveys. Even now, a few
still feel that larval surveys are only necessary in the
early part of the breeding season; once it is known what
species are present at a site, it can be taken for granted
that the species composition at that site will remain the
same throughout the rest of the season. Most experts,
however, feel that larval surveillance is not only an
important aspect of an effective mosquito surveillance and
control program, but it is an essential component.
Benefits of Larval Surveillance
Larval surveys have many important functions. They are
used to determine the locations and seasons that mosquitoes
use specific aquatic habitats and, when specimens are
identified and counted, the information can be used to
determine species composition and population densities. The
information can be used to determine optimal times for
application of larval control measures, including chemicals,
biologicals, draining or impounding. It can also be used to
help forecast the need for adult mosquito control and to help
assess the effectiveness of both chemical and biological
control measures.
Routine larval surveillance data can be useful in
interpreting adult mosquito surveillance data. For example,
if larval surveys indicate 95- 1 00% control by larvicides
and yet the number of adults does not decline, one can
suspect, in the absence of reinfestation, that an important
larval concentration was missed. A system for the detection
of insecticide resistance is also provided through a larval
surveillance program.
Sampling Larval Mosquitoes
Because mosquito larvae are found in a wide variety of
habitats, a number of different sampling techniques to
determine their presence and density have been developed.
Many, if not all, of the published methods are described in
Mike Service's book, Mosquito Ecology Field Sampling Methods
(Elsevier Applied Science, 1993).. Some methods are complex
mechanical devices, but the most commonly used larval
collection method is the "standard dipper," that
plastic or metal, white or aluminum, solid or screen-bottomed
pint to quart-sized scoop-on-a-handle, that, along with the
"sweep net," defines the Ultimate Inspector. Let's
take a closer look at dipping.
Dipping for mosquito larvae may, at first, seem like a
very simple thing to do. After all, who hasn't dipped water
from a bucket or stream to quench a thirst or cool the top of
one's head? Well, think again. Dipping for mosquito larvae is
not dipping to take a drink. The technique starts long before
the dipper is put into the water. It begins hours or days
before the actual dipping and at least 10 feet away from the
water's edge.
The species of mosquitoes one is looking for and the type
of habitat being sampled will, in part, determine the
sampling method used. Thus, it is important that field
personnel know the preferred breeding habitats and seasonal
occurrence of species known or suspected to be present within
an area.
When searching for mosquito larvae, proceed slowly and
carefully. Approach the area with caution, not to avoid
snakes, although that's a good idea too, but to avoid
disturbing larvae at the water's surface. Vibrations from
heavy footsteps, casting a shadow or moving vegetation that
contacts the water may be enough to cause larvae to dive to
the bottom. Try to approach the water while facing the sun
and with quiet, slow, soft steps, gently move vegetation only
as necessary.
Mosquito larvae of most genera, particularly the common
Culex, Aedes and Anopheles, are usually
found at the water's surface and frequently next to
vegetation or surface debris. In larger pools and ponds, they
are usually near the margins, not in open, deep water.
Dipping should be concentrated around floating debris and
aquatic and emergent vegetation. If there is a strong wind,
dipping should be done on the windward side of the habitat
where larvae and pupae will be most heavily concentrated.
Look for larvae and pupae before beginning to dip, if
possible. If it is raining on the water's surface, get back
in the truck, go have a cup of coffee and wait until the rain
stops.
Each water body may contain a number of different
microhabitats which could contain different mosquito species.
Microhabitats are such places as under tree roots, within
clumps of emergent vegetation, under floating or overhanging
vegetation and in open water. Learn to recognize different
microhabitats within an area and sample as many as possible
in order to obtain an accurate picture of the area's species
composition.
A Choice Of Seven
Now that you've found your way safely to the edge of a
marsh, pond, ditch, swamp or woodland pool, what do you do
with your dipper. Just plunge it in? That's fine if you need
water, but not necessarily if you want to catch mosquitoes.
Believe it or not, there are seven basic ways to dip for
mosquito larvae. Which one or ones you use depend, as we
mentioned earlier, on the genus or genera of mosquitoes you
suspect may be present and on the habitat, microhabitat and
weather conditions.
The first and usually the best method to start with is
the SHALLOW SKIM. The shallow skim consists of submerging the
leading edge of the dipper, tipped about 45 degrees, about an
inch below the surface of the water and quickly, but gently,
moving the dipper along a straight line in open water or in
water with small floating debris. End the stroke just before
the dipper is filled to prevent overflowing. The shallow skim
is particularly effective for Anopheles larvae that
tend to remain at the surface longer than Aedes and
Culex. Anopheles are usually associated with
floating vegetation and debris.
Fig 1 
The second method to try in open water, with or without
floating objects, is the COMPLETE SUBMERSION. Many mosquito
larvae, particularly those of the genera Aedes and
Psorophora, are very active and usually dive below the
surface quickly if disturbed. In this case, a quick plunge of
the dipper below the surface of the water is required,
bringing the dipper back up through the diving larvae. Bring
the dipper up carefully to avoid losing the larvae in the
overflow current.
Fig. 2 
When you need to sample at the edges of emergent
vegetation, try the PARTIAL SUBMERSION technique. To do this,
push the dipper, tilted at about 45 degrees, straight down
adjacent to the vegetation. This causes the water around the
vegetation to flow into the dipper, carrying the larvae with
the flow. There is no need to move the dipper horizontally.
Pull the dipper up before it is full.
Fig. 3 
In very shallow water, try the FLOW-IN method. Larvae can
be collected by pushing the dipper into the substrate of the
pool and letting the shallow surface water, debris and larvae
flow into the dipper. Do not move the dipper
horizontally.
Fig. 4 
To sample for larvae that may be under floating or
emergent vegetation, use the SCRAPING technique. This method
is used in habitats that contain clumps of vegetation such as
tussocks of sedges, floating mats of cattails or water
lettuce or other plants that are too large to get in the
dipper, or clumps of submerged vegetation such as hydrilla or
bladderwort. Dip from the water in towards the vegetation and
end by using the dipper to scrape up against the base or
underside of the vegetation to dislodge larvae. This method
is usually more effective if the bottom of the dipper is
screened and it is often used to sample for
Coquillettidia and Mansonia mosquitoes.
Fig. 5 
The SIMPLE SCOOP is the "dipping to get water"
method that was discouraged earlier. It consists of simply
scooping a dipperful of water. This is probably the most
commonly used method, particularly by new inspectors, and it
is often the method referred to in much of the literature as
"the standard dipping procedure." While it can be
successfully used to collect Culex larvae, it is still
not the method of choice.
Fig. 6 
The dipper can also be used as BACKGROUND. This is
especially useful in woodland pools and other shallow water
or when larvae are disturbed and dive to the bottom. Submerge
the dipper completely to the bottom litter and slowly move it
around. The darker mosquito larvae and pupae will stand out
against the background of a white or aluminum dipper. Once
larvae appear in the dipper, just lift it upward.
Fig. 7 
One or more of these methods, properly used, can
determine the mosquito species composition of most aquatic
habitats, excluding those whose openings are smaller than the
dipper, such as tires, rock pools, treeholes and tree root
systems like those found in cedar and red maple swamps. In
those cases, a smaller container, such as a vial, measuring
spoon or tea strainer can be used in the same seven ways as
the dipper described above. Then there is the tubular dipper,
the chef's poultry baster, for those really hard to get to
places like plant axils, treeholes and tree root holes.
Now that we know how to efficiently collect mosquito
larvae, what do we do with the specimens and the data. That's
the subject of a future article. Until then, happy
dipping.
Claudia O'Malley is with the Burlington County Mosquito
Extermination Commission in New Gretna, N.J.
This page was updated on
10 May 2010
.
